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+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
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+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
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+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
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+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
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+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
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+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
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+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
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+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
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+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
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+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
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+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
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+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
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+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
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+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
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+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
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+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
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+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
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+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable Universe contains an estimated 1×1024 stars,[1][2] but most are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.
+
+For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.
+
+A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[3] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.[4]  In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[5] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.
+
+Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[6] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
+
+Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[7] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[9] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
+
+The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC.[10] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (c. 1531–1155 BC).[11]
+
+The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[12] The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue.[13] Hipparchus is known for the discovery of the first recorded nova (new star).[14] Many of the constellations and star names in use today derive from Greek astronomy.
+
+In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[15] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[16] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[17] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[18][19][20]
+
+Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[21] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[22] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[23]
+
+According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[24] 
+Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[25] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[26] and by medieval Islamic cosmologists[27] such as Fakhr al-Din al-Razi.[28] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[29]
+
+The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.[25]
+
+William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[30] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
+
+The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.[31] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
+
+The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[25] Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[32]
+The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.[33]
+
+Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[34] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[35]
+
+With the exception of supernovae, individual stars have primarily been observed in the Local Group,[36] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our
+galaxy).[37] But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[38]
+In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Group[39] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years[40]—ten times further than the most distant star cluster previously observed.
+
+In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed—about 180 million years after the Big Bang.[41]
+
+In April, 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years away from Earth.[42][43]
+
+In May 2018, astronomers reported the detection of the most distant oxygen ever detected in the Universe—and the most distant galaxy ever observed by Atacama Large Millimeter Array or the Very Large Telescope—with the team inferring that the signal was emitted 13.3 billion years ago (or 500 million years after the Big Bang). They found that the observed brightness of the galaxy is well-explained by a model where the onset of star formation corresponds to only 250 million years after the Universe began, corresponding to a redshift of about 15.[44]
+
+The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[45] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
+
+As well as certain constellations and the Sun itself, individual stars have their own myths.[46] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[46] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
+
+Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[47][48]
+
+The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[49] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[50] which catalogs and standardizes proper names for stars. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[51][52] The IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU, professional astronomers, nor the amateur astronomy community.[53] One such star-naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,[54][55][56][57] and the New York City Department of Consumer and Worker Protection issued a violation against ISR for engaging in a deceptive trade practice.[58][59]
+
+Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:
+
+The solar mass M⊙ was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass
+together (GM⊙) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be:
+
+However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.
+
+Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m.[60]
+
+Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions—known as molecular clouds—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[61] Most stars form in groups of dozens to hundreds of thousands of stars.[62]
+Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.
+
+All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[63]
+
+The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy).[65][66] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[67]
+
+As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[68] These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.
+
+Early stars of less than 2 M☉ are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[69][70]
+These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[71]
+
+Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
+
+Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.[72] A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.
+
+Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[73]
+The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[74]
+
+Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M☉ every year,[75] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M☉ each year, significantly affecting their evolution.[76] Stars that begin with more than 50 M☉ can lose over half their total mass while on the main sequence.[77]
+
+The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M☉, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 M☉) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.[64] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M☉[78] are expected to have moved off the main sequence.
+
+Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,[79] which affects the strength of its stellar wind.[80] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.
+
+As stars of at least 0.4 M☉[4] exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly as they form a red giant. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.[74][81]
+
+As the hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M☉, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.[6]
+
+After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate.
+
+During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.
+
+When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[82]
+
+The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.[83]
+
+As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place.[84] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.
+
+In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M☉) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.[85]
+
+A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula.[85] The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M☉.[86] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole, the matter is in a state that is not currently understood.
+
+The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[85]
+
+The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.
+
+Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 2 trillion (1012) galaxies.[87] Overall, there are as many as an estimated 1×1024 stars[1][2] (more stars than all the grains of sand on planet Earth).[88][89][90] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[91]
+
+A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.[92] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy.
+
+It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the stars are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[93]
+
+The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to arrive.[94] This is typical of stellar separations in galactic discs.[95] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
+
+Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[96] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.[97]
+
+Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.
+
+Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old.[98] (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13.799 ± 0.021).[98][99]
+
+The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.[100][101]
+
+When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[103] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.[104]
+
+The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[105] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[106] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[107] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.[108]
+
+Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[109]
+
+The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[110]
+
+Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of our sun.[111][112] Betelgeuse, however, has a much lower density than the Sun.[113]
+
+The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
+
+Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[115]
+
+When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[116] A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations.[117]
+
+The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[118]
+
+Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.[119] During
+the Maunder Minimum, for example, the Sun underwent a
+70-year period with almost no sunspot activity.
+
+One of the most massive stars known is Eta Carinae,[120] which,
+with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M☉ as an upper limit for stars in the current era of the universe.[121] This
+represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses,[122] but
+it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M☉ limit on massive star formation.[123]
+
+The first stars to form after the Big Bang may have been larger, up to 300 M☉,[124] due
+to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.[125][126]
+
+With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core.[127] For
+stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ.[128][129] When the metallicity is very low, however, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[129][130] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.
+
+The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[35]
+
+The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart.[131] By contrast, the Sun rotates once every 25–35 days depending on latitude,[132] with an equatorial velocity of 1.93 km/s.[133] A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.[134]
+
+Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[135] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[136] The rotation rate of the pulsar will gradually slow due to the emission of radiation.[137]
+
+The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.[138] The temperature is normally given in terms of an effective temperature,  which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core.[139] The temperature in the core region of a star is several million kelvins.[140]
+
+The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[35]
+
+Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area.[141]
+
+The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[142] which
+streams from the outer layers as electrically charged protons and alpha and beta particles. Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.
+
+The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
+
+The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[143] Besides
+visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
+
+Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.[144]) With these parameters, astronomers can also estimate the age of the star.[145]
+
+The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.[146]
+
+Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.  Giant stars have much larger, more obvious starspots,[147] and
+they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[148] Red
+dwarf flare stars such as UV Ceti may also possess prominent starspot features.[149]
+
+The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).
+
+Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[151] (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
+
+On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
+
+Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[151] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
+
+The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
+
+As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[152] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[153]
+
+The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[155] It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.[156]
+
+Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.[157]
+
+In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[157] The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.
+
+Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[157]
+
+White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.[158]
+
+Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
+
+During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira.[159]
+
+Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[159] This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.
+
+Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[6] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[160] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[159]
+
+Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[159] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.[161]
+
+The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[162][163]
+
+As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M☉, fusion occurs in a slowly expanding shell around the degenerate helium core.[164]
+
+In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
+
+The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiatative heat transfer inefficient) as in the outer envelope.[163]
+
+The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[165] Red dwarf stars with less than 0.4 M☉ are convective throughout, which prevents the accumulation of a helium core.[4] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[163]
+
+The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.
+
+Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[166] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[165] Despite its high temperature, and the corona emits very little light, due to its low gas density. The corona region of the Sun is normally only visible during a solar eclipse.
+
+From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.[167]
+
+A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship E = mc2.[3]
+
+The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[140]
+
+In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction:[168]
+
+These reactions result in the overall reaction:
+
+where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.
+
+In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.[168]
+
+In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[168]
+
+For an overall reaction of:
+
+In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56.[168] Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.
+
+The table at the left shows the amount of time required for a star of 20 M☉ to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.[171]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable Universe contains an estimated 1×1024 stars,[1][2] but most are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.
+
+For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.
+
+A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[3] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.[4]  In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[5] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.
+
+Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[6] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
+
+Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[7] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[9] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
+
+The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC.[10] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (c. 1531–1155 BC).[11]
+
+The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[12] The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue.[13] Hipparchus is known for the discovery of the first recorded nova (new star).[14] Many of the constellations and star names in use today derive from Greek astronomy.
+
+In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[15] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[16] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[17] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[18][19][20]
+
+Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[21] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[22] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[23]
+
+According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[24] 
+Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[25] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[26] and by medieval Islamic cosmologists[27] such as Fakhr al-Din al-Razi.[28] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[29]
+
+The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.[25]
+
+William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[30] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
+
+The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.[31] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
+
+The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[25] Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[32]
+The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.[33]
+
+Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[34] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[35]
+
+With the exception of supernovae, individual stars have primarily been observed in the Local Group,[36] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our
+galaxy).[37] But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[38]
+In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Group[39] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years[40]—ten times further than the most distant star cluster previously observed.
+
+In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed—about 180 million years after the Big Bang.[41]
+
+In April, 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years away from Earth.[42][43]
+
+In May 2018, astronomers reported the detection of the most distant oxygen ever detected in the Universe—and the most distant galaxy ever observed by Atacama Large Millimeter Array or the Very Large Telescope—with the team inferring that the signal was emitted 13.3 billion years ago (or 500 million years after the Big Bang). They found that the observed brightness of the galaxy is well-explained by a model where the onset of star formation corresponds to only 250 million years after the Universe began, corresponding to a redshift of about 15.[44]
+
+The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[45] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
+
+As well as certain constellations and the Sun itself, individual stars have their own myths.[46] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[46] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
+
+Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[47][48]
+
+The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[49] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[50] which catalogs and standardizes proper names for stars. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[51][52] The IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU, professional astronomers, nor the amateur astronomy community.[53] One such star-naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,[54][55][56][57] and the New York City Department of Consumer and Worker Protection issued a violation against ISR for engaging in a deceptive trade practice.[58][59]
+
+Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:
+
+The solar mass M⊙ was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass
+together (GM⊙) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be:
+
+However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.
+
+Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m.[60]
+
+Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions—known as molecular clouds—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[61] Most stars form in groups of dozens to hundreds of thousands of stars.[62]
+Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.
+
+All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[63]
+
+The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy).[65][66] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[67]
+
+As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[68] These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.
+
+Early stars of less than 2 M☉ are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[69][70]
+These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[71]
+
+Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
+
+Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.[72] A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.
+
+Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[73]
+The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[74]
+
+Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M☉ every year,[75] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M☉ each year, significantly affecting their evolution.[76] Stars that begin with more than 50 M☉ can lose over half their total mass while on the main sequence.[77]
+
+The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M☉, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 M☉) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.[64] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M☉[78] are expected to have moved off the main sequence.
+
+Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,[79] which affects the strength of its stellar wind.[80] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.
+
+As stars of at least 0.4 M☉[4] exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly as they form a red giant. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.[74][81]
+
+As the hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M☉, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.[6]
+
+After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate.
+
+During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.
+
+When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[82]
+
+The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.[83]
+
+As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place.[84] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.
+
+In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M☉) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.[85]
+
+A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula.[85] The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M☉.[86] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole, the matter is in a state that is not currently understood.
+
+The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[85]
+
+The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.
+
+Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 2 trillion (1012) galaxies.[87] Overall, there are as many as an estimated 1×1024 stars[1][2] (more stars than all the grains of sand on planet Earth).[88][89][90] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[91]
+
+A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.[92] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy.
+
+It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the stars are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[93]
+
+The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to arrive.[94] This is typical of stellar separations in galactic discs.[95] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
+
+Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[96] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.[97]
+
+Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.
+
+Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old.[98] (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13.799 ± 0.021).[98][99]
+
+The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.[100][101]
+
+When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[103] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.[104]
+
+The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[105] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[106] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[107] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.[108]
+
+Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[109]
+
+The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[110]
+
+Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of our sun.[111][112] Betelgeuse, however, has a much lower density than the Sun.[113]
+
+The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
+
+Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[115]
+
+When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[116] A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations.[117]
+
+The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[118]
+
+Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.[119] During
+the Maunder Minimum, for example, the Sun underwent a
+70-year period with almost no sunspot activity.
+
+One of the most massive stars known is Eta Carinae,[120] which,
+with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M☉ as an upper limit for stars in the current era of the universe.[121] This
+represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses,[122] but
+it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M☉ limit on massive star formation.[123]
+
+The first stars to form after the Big Bang may have been larger, up to 300 M☉,[124] due
+to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.[125][126]
+
+With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core.[127] For
+stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ.[128][129] When the metallicity is very low, however, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[129][130] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.
+
+The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[35]
+
+The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart.[131] By contrast, the Sun rotates once every 25–35 days depending on latitude,[132] with an equatorial velocity of 1.93 km/s.[133] A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.[134]
+
+Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[135] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[136] The rotation rate of the pulsar will gradually slow due to the emission of radiation.[137]
+
+The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.[138] The temperature is normally given in terms of an effective temperature,  which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core.[139] The temperature in the core region of a star is several million kelvins.[140]
+
+The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[35]
+
+Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area.[141]
+
+The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[142] which
+streams from the outer layers as electrically charged protons and alpha and beta particles. Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.
+
+The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
+
+The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[143] Besides
+visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
+
+Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.[144]) With these parameters, astronomers can also estimate the age of the star.[145]
+
+The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.[146]
+
+Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.  Giant stars have much larger, more obvious starspots,[147] and
+they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[148] Red
+dwarf flare stars such as UV Ceti may also possess prominent starspot features.[149]
+
+The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).
+
+Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[151] (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
+
+On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
+
+Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[151] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
+
+The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
+
+As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[152] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[153]
+
+The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[155] It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.[156]
+
+Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.[157]
+
+In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[157] The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.
+
+Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[157]
+
+White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.[158]
+
+Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
+
+During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira.[159]
+
+Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[159] This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.
+
+Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[6] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[160] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[159]
+
+Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[159] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.[161]
+
+The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[162][163]
+
+As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M☉, fusion occurs in a slowly expanding shell around the degenerate helium core.[164]
+
+In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
+
+The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiatative heat transfer inefficient) as in the outer envelope.[163]
+
+The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[165] Red dwarf stars with less than 0.4 M☉ are convective throughout, which prevents the accumulation of a helium core.[4] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[163]
+
+The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.
+
+Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[166] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[165] Despite its high temperature, and the corona emits very little light, due to its low gas density. The corona region of the Sun is normally only visible during a solar eclipse.
+
+From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.[167]
+
+A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship E = mc2.[3]
+
+The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[140]
+
+In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction:[168]
+
+These reactions result in the overall reaction:
+
+where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.
+
+In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.[168]
+
+In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[168]
+
+For an overall reaction of:
+
+In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56.[168] Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.
+
+The table at the left shows the amount of time required for a star of 20 M☉ to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.[171]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+
+
+A year is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hours of daylight, and, consequently, vegetation and soil fertility.
+In temperate and subpolar regions around the planet, four seasons are generally recognized: spring, summer, autumn and winter. In tropical and subtropical regions, several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked.
+
+A calendar year is an approximation of the number of days of the Earth's orbital period, as counted in a given calendar. The Gregorian calendar, or modern calendar, presents its calendar year to be either a common year of 365 days or a leap year of 366 days, as do the Julian calendars; see below. For the Gregorian calendar, the average length of the calendar year (the mean year) across the complete leap cycle of 400 years is 365.2425 days. The ISO standard ISO 80000-3, Annex C, supports the symbol a (for Latin annus) to represent a year of either 365 or 366 days. In English, the abbreviations y and yr are commonly used.
+
+In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.[1]
+
+The word year is also used for periods loosely associated with, but not identical to, the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. Similarly, year can mean the orbital period of any planet; for example, a Martian year and a Venusian year are examples of the time  a planet takes to    transit one complete orbit. The term can also be used in reference to any long period or cycle, such as the Great Year.[2]
+
+English year (via West Saxon ġēar (/jɛar/), Anglian ġēr) continues Proto-Germanic *jǣran (*jē₁ran).
+Cognates are German Jahr, Old High German jār, Old Norse ár and Gothic jer, from the Proto-Indo-European noun *yeh₁r-om "year, season". Cognates also descended from the same Proto-Indo-European noun (with variation in suffix ablaut) are Avestan yārǝ "year", Greek ὥρα (hṓra) "year, season, period of time" (whence "hour"), Old Church Slavonic jarŭ, and Latin hornus "of this year".
+
+Latin annus (a 2nd declension masculine noun; annum is the accusative singular; annī is genitive singular and nominative plural; annō the dative and ablative singular) is from a PIE noun *h₂et-no-, which also yielded Gothic aþn "year" (only the dative plural aþnam is attested).
+
+Although most languages treat the word as thematic *yeh₁r-o-, there is evidence for an original derivation with an *-r/n suffix, *yeh₁-ro-. Both Indo-European words for year, *yeh₁-ro- and *h₂et-no-, would then be derived from verbal roots meaning "to go, move", *h₁ey- and *h₂et-, respectively  (compare Vedic Sanskrit éti "goes", atasi "thou goest, wanderest").
+A number of English words are derived from Latin annus, such as annual, annuity, anniversary, etc.; per annum means "each year", anno Domini means "in the year of the Lord".
+
+The Greek word for "year", ἔτος, is cognate with Latin vetus "old", from the PIE word *wetos- "year", also preserved in this meaning in Sanskrit vat-sa-ras "year" and vat-sa- "yearling (calf)", the latter also reflected in Latin vitulus "bull calf",  English wether "ram" (Old English weðer, Gothic wiþrus "lamb").
+
+In some languages, it is common to count years by referencing to one season, as in "summers", or "winters", or "harvests". Examples include Chinese 年 "year", originally 秂, an ideographic compound of a person carrying a bundle of wheat denoting "harvest". Slavic besides godŭ "time period; year" uses lěto "summer; year".
+
+In the International System of Quantities (ISO 80000-3), the year (symbol, a) is defined as either 365 days or 366 days.
+
+Astronomical years do not have an integer number of days or lunar months. Any calendar that follows an astronomical year must have a system of intercalation such as leap years.
+
+In the Julian calendar, the average (mean) length of a year is 365.25 days. In a non-leap year, there are 365 days, in a leap year there are 366 days. A leap year occurs every fourth year, or leap year, during which a leap day is intercalated into the month of February. The name "Leap Day" is applied to the added day.
+
+The Revised Julian calendar, proposed in 1923 and used in some Eastern Orthodox Churches,
+has 218 leap years every 900 years, for  the average (mean) year length of 365.2422222 days, close to the length of the mean tropical year, 365.24219 days (relative error of 9·10−8).
+In the year 2800 CE, the Gregorian and Revised Julian calendars will begin to differ by one calendar day.[3]
+
+The Gregorian calendar attempts to cause the northward equinox to fall on or shortly before March 21 and hence it follows the northward equinox year, or tropical year.[4] Because 97 out of 400 years are leap years, the mean length of the Gregorian calendar year is 365.2425 days; with a relative error below one ppm (8·10−7)  relative to the current length of the mean tropical year (365.24219 days) and even closer to the current March equinox year of 365.242374 days that it aims to match.   It is estimated that by the year 4000 CE, the northward equinox will fall back by one day in the Gregorian calendar, not because of this difference, but due to the slowing of the Earth's rotation and the associated lengthening of the day.
+
+Historically, lunisolar calendars intercalated entire leap months on an observational basis.
+Lunisolar calendars have mostly fallen out of use except for liturgical reasons (Hebrew calendar, various Hindu calendars).
+
+A modern adaptation of the historical Jalali calendar, known as the Solar Hijri calendar (1925), is a purely solar calendar with an irregular pattern of leap days based on observation (or astronomical computation), aiming to place new year (Nowruz) on the day of vernal equinox (for the time zone of Tehran), as opposed to using an algorithmic system of leap years.
+
+A calendar era assigns a cardinal number to each sequential  year, using a reference point in the past as the beginning of the era.
+
+The worldwide standard is the Anno Domini, although some prefer the term Common Era because it has no explicit reference to Christianity. It was introduced in the 6th century and was intended to count years from the nativity of Jesus.[5]
+
+The Anno Domini era is given the Latin abbreviation AD (for Anno Domini "in the year of the Lord"), or alternatively CE for "Common Era". Years before AD 1 are abbreviated BC for Before Christ or alternatively BCE for Before the Common Era.
+Year numbers are based on inclusive counting, so that there is no "year zero".
+In the modern alternative reckoning of Astronomical year numbering, positive numbers indicate years AD, the number 0 designates 1 BC, −1 designates 2 BC, and so on.
+
+Financial and scientific calculations often use a 365-day calendar to simplify daily rates.
+
+A fiscal year or financial year is a 12-month period used for calculating annual financial statements in businesses and other organizations. In many jurisdictions, regulations regarding accounting require such reports once per twelve months, but do not require that the twelve months constitute a calendar year.
+
+For example, in Canada and India the fiscal year runs from April 1; in the United Kingdom it runs from April 1 for purposes of corporation tax and government financial statements, but from April 6 for purposes of personal taxation and payment of state benefits;  in Australia it runs from July 1; while in the United States the fiscal year of the federal government runs from October 1.
+
+An academic year is the annual period during which a student attends an educational institution. The academic year may be divided into academic terms, such as semesters or quarters. The school year in many countries starts in August or September and ends in May, June or July.  In Israel the academic year begins around October or November, aligned with the second month of the Hebrew Calendar.
+
+Some schools in the UK and USA divide the academic year into three roughly equal-length terms (called trimesters or quarters in the USA), roughly coinciding with autumn, winter, and spring. At some, a shortened summer session, sometimes considered part of the regular academic year, is attended by students on a voluntary or elective basis.  Other schools break the year into two main semesters, a first (typically August through December) and a second semester (January through May). Each of these main semesters may be split in half by mid-term exams, and each of the halves is referred to as a quarter (or term in some countries). There may also be a voluntary summer session and/or a short January session.
+
+Some other schools, including some in the United States, have four marking periods.  Some schools in the United States, notably Boston Latin School, may divide the year into five or more marking periods. Some state in defense of this that there is perhaps a positive correlation between report frequency and academic achievement.
+
+There are typically 180 days of teaching each year in schools in the US, excluding weekends and breaks, while there are 190 days for pupils in state schools in Canada, New Zealand and the United Kingdom, and 200 for pupils in Australia.
+
+In India the academic year normally starts from June 1 and ends on May 31. Though schools start closing from mid-March, the actual academic closure is on May 31 and in Nepal it starts from July 15.[citation needed]
+
+Schools and universities in Australia typically have academic years that roughly align with the calendar year (i.e., starting in February or March and ending in October to December), as the southern hemisphere experiences summer from December to February.
+
+The Julian year, as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days. This is the normal meaning of the unit "year" (symbol "a" from the Latin annus) used in various scientific contexts. The Julian century of 36525 days and the Julian millennium of 365250 days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify how many days (not how many "real" years), for long time intervals where stating the number of days would be unwieldy and unintuitive. By convention, the Julian year is used in the computation of the distance covered by a light-year.
+
+In the Unified Code for Units of Measure, the symbol, a (without subscript), always refers to the Julian year, aj, of exactly 31557600 seconds.
+
+The SI multiplier prefixes may be applied to it to form ka (kiloannus), Ma (megaannus), etc.
+
+Each of these three years can be loosely called an astronomical year.
+
+The sidereal year is the time taken for the Earth to complete one revolution of its orbit, as measured against a fixed frame of reference (such as the fixed stars, Latin sidera, singular sidus).  Its average duration is 365.256363004 days (365 d 6 h 9 min 9.76 s) (at the epoch J2000.0 = January 1, 2000, 12:00:00 TT).[6]
+
+Today the mean tropical year is defined as the period of time for the mean ecliptic longitude of the Sun to increase by 360 degrees.[7] Since the Sun's ecliptic longitude is measured with respect to the equinox,[8] the tropical year comprises a complete cycle of the seasons; because of the biological and socio-economic importance of the seasons, the tropical year is the basis of most calendars. The modern definition of mean tropical year differs from the actual time between passages of, e.g., the northward equinox for several reasons explained below. Because of the Earth's axial precession, this year is about 20 minutes shorter than the sidereal year.  The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds, using the modern definition.[9] (= 365.24219 days of 86400 SI seconds)
+
+The anomalistic year is the time taken for the Earth to complete one revolution with respect to its apsides. The orbit of the Earth is elliptical; the extreme points, called apsides, are the perihelion, where the Earth is closest to the Sun (January 5, 07:48 UT in 2020), and the aphelion, where the Earth is farthest from the Sun (July 4 11:35 UT in 2020). The anomalistic year is usually defined as the time between perihelion passages.  Its average duration is 365.259636 days (365 d 6 h 13 min 52.6 s) (at the epoch J2011.0).[10]
+
+The draconic year, draconitic year, eclipse year, or ecliptic year is the time taken for the Sun (as seen from the Earth) to complete one revolution with respect to the same lunar node (a point where the Moon's orbit intersects the ecliptic). The year is associated with eclipses: these occur only when both the Sun and the Moon are near these nodes; so eclipses occur within about a month of every half eclipse year. Hence there are two eclipse seasons every eclipse year. The average duration of the eclipse year is
+
+This term is sometimes erroneously used for the draconic or nodal period of lunar precession, that is the period of a complete revolution of the Moon's ascending node around the ecliptic: 18.612815932 Julian years (6798.331019 days; at the epoch J2000.0).
+
+The full moon cycle is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the perigee of the Moon's orbit. This period is associated with the apparent size of the full moon, and also with the varying duration of the synodic month. The duration of one full moon cycle is:
+
+The lunar year comprises twelve full cycles of the phases of the Moon, as seen from Earth. It has a duration of approximately 354.37 days. Muslims use this for celebrating their Eids and for marking the start of the fasting month of Ramadan.
+A Muslim calendar year is based on the lunar cycle.
+
+The vague year, from annus vagus or wandering year, is an integral approximation to the year equaling 365 days, which wanders in relation to more exact years.  Typically the vague year is divided into 12 schematic months of 30 days each plus 5 epagomenal days.  The vague year was used in the calendars of Ethiopia, Ancient Egypt, Iran, Armenia and in Mesoamerica among the Aztecs and Maya.[11]   It is still used by many Zoroastrian communities.
+
+A heliacal year is the interval between the heliacal risings of a star. It differs from the sidereal year for stars away from the ecliptic due mainly to the precession of the equinoxes.
+
+The Sothic year is the interval between heliacal risings of the star Sirius. It is currently less than the sidereal year and its duration is very close to the Julian year of 365.25 days.
+
+The Gaussian year is the sidereal year for a planet of negligible mass (relative to the Sun) and unperturbed by other planets that is governed by the Gaussian gravitational constant. Such a planet would be slightly closer to the Sun than Earth's mean distance. Its length is:
+
+The Besselian year is a tropical year that starts when the (fictitious) mean Sun reaches an ecliptic longitude of 280°. This is currently on or close to January 1. It is named after the 19th-century German astronomer and mathematician Friedrich Bessel. The following equation can be used to compute the current Besselian epoch (in years):[12]
+
+The TT subscript indicates that for this formula, the Julian date should use the Terrestrial Time scale, or its predecessor, ephemeris time.
+
+The exact length of an astronomical year changes over time.
+
+Mean year lengths in this section are calculated for 2000, and differences in year lengths, compared to 2000, are given for past and future years. In the tables a day is 86,400 SI seconds long.[13][14][15][16]
+
+An average Gregorian year is 365.2425 days (52.1775 weeks, 8765.82 hours, 525949.2 minutes or 31556952 seconds).  For this calendar, a common year is 365 days (8760 hours, 525600 minutes or 31536000 seconds), and a leap year is 366 days (8784 hours, 527040 minutes or 31622400 seconds).  The 400-year cycle of the Gregorian calendar has 146097 days and hence exactly 20871 weeks.
+
+The Great Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,700 years.
+
+The Galactic year is the time it takes Earth's Solar System to revolve once around the galactic center. It comprises roughly 230 million Earth years.[17]
+
+A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than a month from year to year.
+
+In the International System of Quantities the symbol for the year as a unit of time is a, taken from the Latin word annus.[18]
+
+In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature, but also specifically in geology and paleontology, where "kyr, myr, byr" (thousands, millions, and billions of years, respectively) and similar abbreviations are used to denote intervals of time remote from the present.[18][19][20]
+
+NIST SP811[21] and ISO 80000-3:2006[22] support the symbol a as the unit of time for a year. In English, the abbreviations y and yr are also used.[18][19][20]
+
+The Unified Code for Units of Measure[23] disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50[24] by using:
+
+where:
+
+The International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Geological Sciences have jointly recommended defining the annus, with symbol a, as the length of the tropical year in the year 2000:
+
+This differs from the above definition of 365.25 days by about 20 parts per million. The joint document says that definitions such as the Julian year "bear an inherent, pre-programmed obsolescence because of the variability of Earth’s orbital movement", but then proposes using the length of the tropical year as of 2000 AD (specified down to the millisecond), which suffers from the same problem.[25][26] (The tropical year oscillates with time by more than a minute.)
+
+The notation has proved controversial as it conflicts with an earlier convention among geoscientists to use a specifically for years ago, and y or yr for a one-year time period.[26]
+
+For the following, there are alternative forms which elide the consecutive vowels, such as kilannus, megannus, etc. The exponents and exponential notations are typically used for calculating and in displaying calculations, and for conserving space, as in tables of data.
+
+
+
+In astronomy, geology, and paleontology, the abbreviation yr for years and ya for years ago are sometimes used, combined with prefixes for thousand, million, or billion.[19][29] They are not SI units, using y to abbreviate the English "year", but following ambiguous international recommendations, use either the standard English first letters as prefixes (t, m, and b) or metric prefixes (k, M, and G) or variations on metric prefixes (k, m, g). In archaeology, dealing with more recent periods, normally expressed dates, e.g. "22,000 years ago" may be used as a more accessible equivalent of a Before Present ("BP") date.
+
+These abbreviations include:
+
+Use of mya and bya is deprecated in modern geophysics, the recommended usage being Ma and Ga for dates Before Present, but "m.y." for the duration of epochs.[19][20] This ad hoc distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America.[31]
+
+Note that on graphs, using ya units on the horizontal axis time flows from right to left, which may seem counter-intuitive. If the ya units are on the vertical axis, time flows from top to bottom which is probably easier to understand than conventional notation.[clarification needed]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable Universe contains an estimated 1×1024 stars,[1][2] but most are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.
+
+For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.
+
+A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[3] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.[4]  In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[5] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.
+
+Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[6] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
+
+Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[7] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[9] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
+
+The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC.[10] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (c. 1531–1155 BC).[11]
+
+The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[12] The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue.[13] Hipparchus is known for the discovery of the first recorded nova (new star).[14] Many of the constellations and star names in use today derive from Greek astronomy.
+
+In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[15] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[16] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[17] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[18][19][20]
+
+Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[21] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[22] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[23]
+
+According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[24] 
+Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[25] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[26] and by medieval Islamic cosmologists[27] such as Fakhr al-Din al-Razi.[28] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[29]
+
+The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.[25]
+
+William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[30] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
+
+The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.[31] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
+
+The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[25] Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[32]
+The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.[33]
+
+Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[34] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[35]
+
+With the exception of supernovae, individual stars have primarily been observed in the Local Group,[36] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our
+galaxy).[37] But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[38]
+In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Group[39] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years[40]—ten times further than the most distant star cluster previously observed.
+
+In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed—about 180 million years after the Big Bang.[41]
+
+In April, 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years away from Earth.[42][43]
+
+In May 2018, astronomers reported the detection of the most distant oxygen ever detected in the Universe—and the most distant galaxy ever observed by Atacama Large Millimeter Array or the Very Large Telescope—with the team inferring that the signal was emitted 13.3 billion years ago (or 500 million years after the Big Bang). They found that the observed brightness of the galaxy is well-explained by a model where the onset of star formation corresponds to only 250 million years after the Universe began, corresponding to a redshift of about 15.[44]
+
+The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[45] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
+
+As well as certain constellations and the Sun itself, individual stars have their own myths.[46] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[46] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
+
+Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[47][48]
+
+The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[49] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[50] which catalogs and standardizes proper names for stars. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[51][52] The IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU, professional astronomers, nor the amateur astronomy community.[53] One such star-naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,[54][55][56][57] and the New York City Department of Consumer and Worker Protection issued a violation against ISR for engaging in a deceptive trade practice.[58][59]
+
+Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:
+
+The solar mass M⊙ was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass
+together (GM⊙) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be:
+
+However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.
+
+Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m.[60]
+
+Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions—known as molecular clouds—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[61] Most stars form in groups of dozens to hundreds of thousands of stars.[62]
+Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.
+
+All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[63]
+
+The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy).[65][66] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[67]
+
+As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[68] These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.
+
+Early stars of less than 2 M☉ are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[69][70]
+These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[71]
+
+Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
+
+Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.[72] A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.
+
+Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[73]
+The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[74]
+
+Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M☉ every year,[75] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M☉ each year, significantly affecting their evolution.[76] Stars that begin with more than 50 M☉ can lose over half their total mass while on the main sequence.[77]
+
+The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M☉, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 M☉) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.[64] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M☉[78] are expected to have moved off the main sequence.
+
+Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,[79] which affects the strength of its stellar wind.[80] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.
+
+As stars of at least 0.4 M☉[4] exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly as they form a red giant. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.[74][81]
+
+As the hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M☉, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.[6]
+
+After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate.
+
+During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.
+
+When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[82]
+
+The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.[83]
+
+As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place.[84] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.
+
+In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M☉) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.[85]
+
+A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula.[85] The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M☉.[86] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole, the matter is in a state that is not currently understood.
+
+The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[85]
+
+The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.
+
+Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 2 trillion (1012) galaxies.[87] Overall, there are as many as an estimated 1×1024 stars[1][2] (more stars than all the grains of sand on planet Earth).[88][89][90] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[91]
+
+A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.[92] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy.
+
+It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the stars are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[93]
+
+The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to arrive.[94] This is typical of stellar separations in galactic discs.[95] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
+
+Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[96] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.[97]
+
+Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.
+
+Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old.[98] (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13.799 ± 0.021).[98][99]
+
+The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.[100][101]
+
+When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[103] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.[104]
+
+The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[105] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[106] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[107] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.[108]
+
+Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[109]
+
+The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[110]
+
+Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of our sun.[111][112] Betelgeuse, however, has a much lower density than the Sun.[113]
+
+The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
+
+Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[115]
+
+When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[116] A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations.[117]
+
+The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[118]
+
+Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.[119] During
+the Maunder Minimum, for example, the Sun underwent a
+70-year period with almost no sunspot activity.
+
+One of the most massive stars known is Eta Carinae,[120] which,
+with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M☉ as an upper limit for stars in the current era of the universe.[121] This
+represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses,[122] but
+it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M☉ limit on massive star formation.[123]
+
+The first stars to form after the Big Bang may have been larger, up to 300 M☉,[124] due
+to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.[125][126]
+
+With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core.[127] For
+stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ.[128][129] When the metallicity is very low, however, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[129][130] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.
+
+The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[35]
+
+The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart.[131] By contrast, the Sun rotates once every 25–35 days depending on latitude,[132] with an equatorial velocity of 1.93 km/s.[133] A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.[134]
+
+Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[135] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[136] The rotation rate of the pulsar will gradually slow due to the emission of radiation.[137]
+
+The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.[138] The temperature is normally given in terms of an effective temperature,  which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core.[139] The temperature in the core region of a star is several million kelvins.[140]
+
+The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[35]
+
+Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area.[141]
+
+The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[142] which
+streams from the outer layers as electrically charged protons and alpha and beta particles. Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.
+
+The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
+
+The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[143] Besides
+visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
+
+Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.[144]) With these parameters, astronomers can also estimate the age of the star.[145]
+
+The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.[146]
+
+Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.  Giant stars have much larger, more obvious starspots,[147] and
+they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[148] Red
+dwarf flare stars such as UV Ceti may also possess prominent starspot features.[149]
+
+The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).
+
+Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[151] (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
+
+On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
+
+Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[151] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
+
+The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
+
+As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[152] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[153]
+
+The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[155] It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.[156]
+
+Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.[157]
+
+In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[157] The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.
+
+Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[157]
+
+White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.[158]
+
+Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
+
+During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira.[159]
+
+Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[159] This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.
+
+Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[6] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[160] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[159]
+
+Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[159] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.[161]
+
+The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[162][163]
+
+As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M☉, fusion occurs in a slowly expanding shell around the degenerate helium core.[164]
+
+In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
+
+The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiatative heat transfer inefficient) as in the outer envelope.[163]
+
+The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[165] Red dwarf stars with less than 0.4 M☉ are convective throughout, which prevents the accumulation of a helium core.[4] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[163]
+
+The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.
+
+Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[166] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[165] Despite its high temperature, and the corona emits very little light, due to its low gas density. The corona region of the Sun is normally only visible during a solar eclipse.
+
+From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.[167]
+
+A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship E = mc2.[3]
+
+The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[140]
+
+In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction:[168]
+
+These reactions result in the overall reaction:
+
+where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.
+
+In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.[168]
+
+In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[168]
+
+For an overall reaction of:
+
+In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56.[168] Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.
+
+The table at the left shows the amount of time required for a star of 20 M☉ to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.[171]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable Universe contains an estimated 1×1024 stars,[1][2] but most are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.
+
+For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.
+
+A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[3] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.[4]  In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[5] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.
+
+Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[6] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
+
+Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[7] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[9] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
+
+The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC.[10] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (c. 1531–1155 BC).[11]
+
+The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[12] The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue.[13] Hipparchus is known for the discovery of the first recorded nova (new star).[14] Many of the constellations and star names in use today derive from Greek astronomy.
+
+In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[15] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[16] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[17] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[18][19][20]
+
+Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[21] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[22] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[23]
+
+According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[24] 
+Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[25] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[26] and by medieval Islamic cosmologists[27] such as Fakhr al-Din al-Razi.[28] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[29]
+
+The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.[25]
+
+William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[30] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
+
+The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.[31] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
+
+The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[25] Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[32]
+The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.[33]
+
+Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[34] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[35]
+
+With the exception of supernovae, individual stars have primarily been observed in the Local Group,[36] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our
+galaxy).[37] But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[38]
+In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Group[39] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years[40]—ten times further than the most distant star cluster previously observed.
+
+In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed—about 180 million years after the Big Bang.[41]
+
+In April, 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years away from Earth.[42][43]
+
+In May 2018, astronomers reported the detection of the most distant oxygen ever detected in the Universe—and the most distant galaxy ever observed by Atacama Large Millimeter Array or the Very Large Telescope—with the team inferring that the signal was emitted 13.3 billion years ago (or 500 million years after the Big Bang). They found that the observed brightness of the galaxy is well-explained by a model where the onset of star formation corresponds to only 250 million years after the Universe began, corresponding to a redshift of about 15.[44]
+
+The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[45] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
+
+As well as certain constellations and the Sun itself, individual stars have their own myths.[46] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[46] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
+
+Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[47][48]
+
+The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[49] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[50] which catalogs and standardizes proper names for stars. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[51][52] The IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU, professional astronomers, nor the amateur astronomy community.[53] One such star-naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,[54][55][56][57] and the New York City Department of Consumer and Worker Protection issued a violation against ISR for engaging in a deceptive trade practice.[58][59]
+
+Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:
+
+The solar mass M⊙ was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass
+together (GM⊙) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be:
+
+However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.
+
+Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m.[60]
+
+Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions—known as molecular clouds—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[61] Most stars form in groups of dozens to hundreds of thousands of stars.[62]
+Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.
+
+All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[63]
+
+The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy).[65][66] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[67]
+
+As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[68] These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.
+
+Early stars of less than 2 M☉ are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[69][70]
+These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[71]
+
+Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
+
+Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.[72] A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.
+
+Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[73]
+The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[74]
+
+Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M☉ every year,[75] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M☉ each year, significantly affecting their evolution.[76] Stars that begin with more than 50 M☉ can lose over half their total mass while on the main sequence.[77]
+
+The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M☉, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 M☉) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.[64] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M☉[78] are expected to have moved off the main sequence.
+
+Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,[79] which affects the strength of its stellar wind.[80] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.
+
+As stars of at least 0.4 M☉[4] exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly as they form a red giant. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.[74][81]
+
+As the hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M☉, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.[6]
+
+After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate.
+
+During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.
+
+When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[82]
+
+The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.[83]
+
+As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place.[84] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.
+
+In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M☉) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.[85]
+
+A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula.[85] The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M☉.[86] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole, the matter is in a state that is not currently understood.
+
+The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[85]
+
+The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.
+
+Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 2 trillion (1012) galaxies.[87] Overall, there are as many as an estimated 1×1024 stars[1][2] (more stars than all the grains of sand on planet Earth).[88][89][90] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[91]
+
+A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.[92] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy.
+
+It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the stars are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[93]
+
+The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to arrive.[94] This is typical of stellar separations in galactic discs.[95] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
+
+Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[96] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.[97]
+
+Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.
+
+Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old.[98] (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13.799 ± 0.021).[98][99]
+
+The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.[100][101]
+
+When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[103] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.[104]
+
+The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[105] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[106] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[107] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.[108]
+
+Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[109]
+
+The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[110]
+
+Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of our sun.[111][112] Betelgeuse, however, has a much lower density than the Sun.[113]
+
+The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
+
+Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[115]
+
+When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[116] A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations.[117]
+
+The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[118]
+
+Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.[119] During
+the Maunder Minimum, for example, the Sun underwent a
+70-year period with almost no sunspot activity.
+
+One of the most massive stars known is Eta Carinae,[120] which,
+with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M☉ as an upper limit for stars in the current era of the universe.[121] This
+represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses,[122] but
+it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M☉ limit on massive star formation.[123]
+
+The first stars to form after the Big Bang may have been larger, up to 300 M☉,[124] due
+to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.[125][126]
+
+With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core.[127] For
+stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ.[128][129] When the metallicity is very low, however, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[129][130] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.
+
+The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[35]
+
+The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart.[131] By contrast, the Sun rotates once every 25–35 days depending on latitude,[132] with an equatorial velocity of 1.93 km/s.[133] A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.[134]
+
+Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[135] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[136] The rotation rate of the pulsar will gradually slow due to the emission of radiation.[137]
+
+The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.[138] The temperature is normally given in terms of an effective temperature,  which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core.[139] The temperature in the core region of a star is several million kelvins.[140]
+
+The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[35]
+
+Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area.[141]
+
+The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[142] which
+streams from the outer layers as electrically charged protons and alpha and beta particles. Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.
+
+The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
+
+The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[143] Besides
+visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
+
+Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.[144]) With these parameters, astronomers can also estimate the age of the star.[145]
+
+The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.[146]
+
+Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.  Giant stars have much larger, more obvious starspots,[147] and
+they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[148] Red
+dwarf flare stars such as UV Ceti may also possess prominent starspot features.[149]
+
+The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).
+
+Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[151] (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
+
+On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
+
+Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[151] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
+
+The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
+
+As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[152] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[153]
+
+The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[155] It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.[156]
+
+Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.[157]
+
+In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[157] The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.
+
+Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[157]
+
+White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.[158]
+
+Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
+
+During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira.[159]
+
+Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[159] This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.
+
+Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[6] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[160] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[159]
+
+Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[159] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.[161]
+
+The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[162][163]
+
+As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M☉, fusion occurs in a slowly expanding shell around the degenerate helium core.[164]
+
+In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
+
+The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiatative heat transfer inefficient) as in the outer envelope.[163]
+
+The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[165] Red dwarf stars with less than 0.4 M☉ are convective throughout, which prevents the accumulation of a helium core.[4] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[163]
+
+The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.
+
+Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[166] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[165] Despite its high temperature, and the corona emits very little light, due to its low gas density. The corona region of the Sun is normally only visible during a solar eclipse.
+
+From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.[167]
+
+A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship E = mc2.[3]
+
+The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[140]
+
+In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction:[168]
+
+These reactions result in the overall reaction:
+
+where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.
+
+In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.[168]
+
+In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[168]
+
+For an overall reaction of:
+
+In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56.[168] Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.
+
+The table at the left shows the amount of time required for a star of 20 M☉ to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.[171]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+Elasmosaurus was a large marine reptile in the order Plesiosauria. The genus lived about 80.5 million years ago, during the Late Cretaceous. The first specimen was sent to the American paleontologist Edward Drinker Cope after its discovery in 1867 near Fort Wallace, Kansas. Only one incomplete skeleton is definitely known, consisting of a fragmentary skull, the spine, and the pectoral and pelvic girdles, and a single species, E. platyurus, is recognized today. Measuring 10.3 meters (34 ft) long, the genus had a streamlined body with paddle-like limbs or flippers, a short tail, and a small, slender, triangular head. With a neck around 7.1 meters (23 ft) long, Elasmosaurus was one of the longest-necked animals to have lived, with the largest number of neck vertebrae known, 72. It probably ate small fish and marine invertebrates, seizing them with long teeth. Elasmosaurus is known from the Pierre Shale formation, which represents marine deposits from the Western Interior Seaway. (Full article...)
+
+July 28
+
+Cirsium eriophorum, the woolly thistle, is a large herbaceous biennial plant in the daisy family, Asteraceae. It is native to Central and Western Europe, where it grows in grassland and open scrubland. Several parts of the plant are edible; the young leaves can be eaten raw, the young stems can be peeled and boiled, and the flower buds can be consumed in a similar way to artichokes. This picture shows a C. eriophorum flower head photographed in Kozara National Park, in Republika Srpska, Bosnia and Herzegovina.
+
+Photograph credit: Petar Milošević
+
+Wikipedia is hosted by the Wikimedia Foundation, a non-profit organization that also hosts a range of other projects:
+
+This Wikipedia is written in English. Started in 2001 (2001), it currently contains 6,130,118 articles. 
+Many other Wikipedias are available; some of the largest are listed below.

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+
+
+A pet, or companion animal, is an animal kept primarily for a person's company or entertainment rather than as a working animal, livestock or a laboratory animal. Popular pets are often considered to have attractive appearances, intelligence and relatable personalities, but some pets may be taken in on an altruistic basis (such as a stray animal) and accepted by the owner regardless of these characteristics.
+
+Two of the most popular pets are dogs and cats; the technical term for a cat lover is an ailurophile and a dog lover a cynophile. Other animals commonly kept include: rabbits; ferrets; pigs; rodents, such as gerbils, hamsters, chinchillas, rats, mice, and guinea pigs; avian pets, such as parrots, passerines and fowls; reptile pets, such as turtles, alligators, crocodiles, lizards, and snakes; aquatic pets, such as fish, freshwater and saltwater snails, amphibians like frogs and salamanders; and arthropod pets, such as tarantulas and hermit crabs. Small pets may be grouped together as pocket pets, while the equine and bovine group include the largest companion animals.
+
+Pets provide their owners (or "guardians")[1] both physical and emotional benefits. Walking a dog can provide both the human and the dog with exercise, fresh air and social interaction. Pets can give companionship to people who are living alone or elderly adults who do not have adequate social interaction with other people. There is a medically approved class of therapy animals, mostly dogs or cats, that are brought to visit confined humans, such as children in hospitals or elders in nursing homes. Pet therapy utilizes trained animals and handlers to achieve specific physical, social, cognitive or emotional goals with patients.
+
+People most commonly get pets for companionship, to protect a home or property or because of the perceived beauty or attractiveness of the animals.[2] A 1994 Canadian study found that the most common reasons for not owning a pet were lack of ability to care for the pet when traveling (34.6%), lack of time (28.6%) and lack of suitable housing (28.3%), with dislike of pets being less common (19.6%).[2] Some scholars, ethicists and animal rights organizations have raised concerns over keeping pets because of the lack of autonomy and the objectification of non-human animals.[3]
+
+In China, spending on domestic animals has grown from an estimated $3.12 billion in 2010 to $25 billion in 2018. The Chinese people own 51 million dogs and 41 million cats, with pet owners often preferring to source pet food internationally.[4] There are a total of 755 million pets, increased from 389 million in 2013.[5]
+
+According to a survey promoted by Italian family associations in 2009, it is estimated that there are approximately 45 million pets in Italy. This includes 7 million dogs, 7.5 million cats, 16 million fish, 12 million birds, and 10 million snakes.[6]
+
+A 2007 survey by the University of Bristol found that 26% of UK households owned cats and 31% owned dogs, estimating total domestic populations of approximately 10.3 million cats and 10.5 million dogs in 2006.[7] The survey also found that 47.2% of households with a cat had at least one person educated to degree level, compared with 38.4% of homes with dogs.[8]
+
+Sixty-eight percent of U.S. households, or about 85 million families, own a pet, according to the 2017-2018 National Pet Owners Survey conducted by the American Pet Products Association (APPA). This is up from 56 percent of U.S. households in 1988, the first year the survey was conducted.[9]There are approximately 86.4 million pet cats and approximately 78.2 million pet dogs in the United States,[10][11] and a United States 2007–2008 survey showed that dog-owning households outnumbered those owning cats, but that the total number of pet cats was higher than that of dogs. The same was true for 2011.[12] In 2013, pets outnumbered children four to one in the United States.[13]
+
+Keeping animals as pets may be detrimental to their health if certain requirements are not met. An important issue is inappropriate feeding, which may produce clinical effects. The consumption of chocolate or grapes by dogs, for example, may prove fatal.
+
+Certain species of houseplants can also prove toxic if consumed by pets. Examples include philodendrons and Easter lilies (which can cause severe kidney damage to cats)[16][17] and poinsettias, begonia, and aloe vera (which are mildly toxic to dogs).[18][19]
+
+Housepets, particularly dogs and cats in industrialized societies, are also highly susceptible to obesity. Overweight pets have been shown to be at a higher risk of developing diabetes, liver problems, joint pain, kidney failure, and cancer. Lack of exercise and high-caloric diets are considered to be the primary contributors to pet obesity.[20][21][22]
+
+It is widely believed among the public, and among many scientists, that pets probably bring mental and physical health benefits to their owners;[23] a 1987 NIH statement cautiously argued that existing data was "suggestive" of a significant benefit.[24] A recent dissent comes from a 2017 RAND study, which found that at least in the case of children, having a pet per se failed to improve physical or mental health by a statistically significant amount; instead, the study found children who were already prone to being healthy were more likely to get pets in the first place.[23][25][26] Unfortunately, conducting long-term randomized trials to settle the issue would be costly or infeasible.[24][26]
+
+Pets might have the ability to stimulate their caregivers, in particular the elderly, giving people someone to take care of, someone to exercise with, and someone to help them heal from a physically or psychologically troubled past.[24][27][28] Animal company can also help people to preserve acceptable levels of happiness despite the presence of mood symptoms like anxiety or depression.[29] Having a pet may also help people achieve health goals, such as lowered blood pressure, or mental goals, such as decreased stress.[30][31][32][33][34][35] There is evidence that having a pet can help a person lead a longer, healthier life. In a 1986 study of 92 people hospitalized for coronary ailments, within a year, 11 of the 29 patients without pets had died, compared to only 3 of the 52 patients who had pets.[28] Having pet(s) was shown to significantly reduce triglycerides, and thus heart disease risk, in the elderly.[36] A study by the National Institute of Health found that people who owned dogs were less likely to die as a result of a heart attack than those who did not own one.[37] There is some evidence that pets may have a therapeutic effect in dementia cases.[38] Other studies have shown that for the elderly, good health may be a requirement for having a pet, and not a result.[39] Dogs trained to be guide dogs can help people with vision impairment. Dogs trained in the field of Animal-Assisted Therapy (AAT) can also benefit people with other disabilities.[24][40]
+
+People residing in a long-term care facility, such as a hospice or nursing home, may experience health benefits from pets. Pets help them to cope with the emotional issues related to their illness. They also offer physical contact with another living creature, something that is often missing in an elder's life.[10][41] Pets for nursing homes are chosen based on the size of the pet, the amount of care that the breed needs, and the population and size of the care institution.[28] Appropriate pets go through a screening process and, if it is a dog, additional training programs to become a therapy dog.[42] There are three types of therapy dogs: facility therapy dogs, animal-assisted therapy dogs, and therapeutic visitation dogs. The most common therapy dogs are therapeutic visitation dogs. These dogs are household pets whose handlers take time to visit hospitals, nursing homes, detention facilities, and rehabilitation facilities.[27] Different pets require varying amounts of attention and care; for example, cats may have lower maintenance requirements than dogs.[43]
+
+In addition to providing health benefits for their owners, pets also impact the social lives of their owners and their connection to their community. There is some evidence that pets can facilitate social interaction.[44] Assistant Professor of Sociology at the University of Colorado at Boulder, Leslie Irvine has focused her attention on pets of the homeless population. Her studies of pet ownership among the homeless found that many modify their life activities for fear of losing their pets. Pet ownership prompts them to act responsibly, with many making a deliberate choice not to drink or use drugs, and to avoid contact with substance abusers or those involved in any criminal activity for fear of being separated from their pet. Additionally, many refuse to house in shelters if their pet is not allowed to stay with them.[45]
+
+Health risks that are associated with pets include:
+
+The European Convention for the Protection of Pet Animals is a 1987 treaty of the Council of Europe – but accession to the treaty is open to all states in the world – to promote the welfare of pet animals and ensure minimum standards for their treatment and protection. It went into effect on 1 May 1992, and as of June 2020, it has been ratified by 24 states.[47]
+
+States, cities, and towns in Western nations commonly enact local ordinances to limit the number or kind of pets a person may keep personally or for business purposes. Prohibited pets may be specific to certain breeds (such as pit bulls or Rottweilers), they may apply to general categories of animals (such as livestock, exotic animals, wild animals, and canid or felid hybrids), or they may simply be based on the animal's size. Additional or different maintenance rules and regulations may also apply. Condominium associations and owners of rental properties also commonly limit or forbid tenants' keeping of pets.[citation needed]
+
+The keeping of animals as pets can cause concerns with regard to animal rights and welfare.[48][49][50] Pets have commonly been considered private property, owned by individual persons. However, many legal protections have existed (historically and today) with the intention of safeguarding pets' (and other animals') well-being.[51][52][53][54] Since the year 2000, a small but increasing number of jurisdictions in North America have enacted laws redefining pet's owners as guardians. Intentions have been characterized as simply changing attitudes and perceptions (but not legal consequences) to working toward legal personhood for pets themselves. Some veterinarians and breeders have opposed these moves. The question of pets' legal status can arise with concern to purchase or adoption, custody, divorce, estate and inheritance, injury, damage, and veterinary malpractice.[55][56][57][58]
+
+In Belgium and the Netherlands, the government publishes white lists and black lists (called 'positive' and 'negative lists') with animal species that are designated to be appropriate to be kept as pets (positive) or not (negative). The Dutch Ministry of Economic Affairs and Climate Policy originally established its first positive list (positieflijst) per 1 February 2015 for a set of 100 mammals (including cats, dogs and production animals) deemed appropriate as pets on the recommendations of Wageningen University.[59] Parliamentary debates about such a pet list date back to the 1980s, with continuous disagreements about which species should be included and how the law should be enforced.[60] In January 2017, the white list was expanded to 123 species, while the black list that had been set up was expanded (with animals like the brown bear and two great kangaroo species) to contain 153 species unfit for petting, such as the armadillo, the sloth, the European hare and the wild boar.[61]
+
+Pets have a considerable environmental impact, especially in countries where they are common or held in high densities. For instance, the 163 million dogs and cats kept in the United States consume about 20% of the amount of dietary energy that humans do and an estimated 33% of the animal-derived energy.[62] They produce about 30% ± 13%, by mass, as much feces as Americans, and through their diet, constitute about 25–30% of the environmental impacts from animal production in terms of the use of land, water, fossil fuel, phosphate, and biocides. Dog and cat animal product consumption is responsible for the release of up to 64 ± 16 million tons CO2-equivalent methane and nitrous oxide, two powerful greenhouse gasses. Americans are the largest pet owners in the world, but pet ownership in the US has considerable environmental costs.[62]
+
+While many people have kept many different species of animals in captivity over the course of human history, only a relative few have been kept long enough to be considered domesticated. Other types of animals, notably monkeys, have never been domesticated but are still sold and kept as pets. There are also inanimate objects that have been kept as "pets", either as a form of a game or humorously (e.g. the Pet Rock or Chia Pet). Some wild animals are kept as pets, such as tigers, even though this is illegal. There is a market for illegal pets.
+
+Domesticated pets are most common. A domesticated animal is a species that has been made fit for a human environment[63] by being consistently kept in captivity and selectively bred over a long enough period of time that it exhibits marked differences in behavior and appearance from its wild relatives. Domestication contrasts with taming, which is simply when an un-domesticated, wild animal has become tolerant of human presence, and perhaps, even enjoys it.
+
+Wild animals are kept as pets. The term “wild” in this context specifically applies to any species of animal which has not undergone a fundamental change in behavior to facilitate a close co-existence with humans. Some species may have been bred in captivity for a considerable length of time, but are still not recognized as domesticated.
+
+Generally, wild animals are recognized as not suitable to keep as pets, and this practice is completely banned in many places. In other areas, certain species are allowed to be kept, and it is usually required for the owner to obtain a permit. It is considered animal cruelty by some, as most often, wild animals require precise and constant care that is very difficult to meet in captive conditions. Many large and instinctively aggressive animals are extremely dangerous, and numerous times have they killed their handlers.
+
+Archaeology suggests that human ownership of dogs as pets may date back to at least 12,000 years ago.[64]
+
+Ancient Greeks and Romans would openly grieve for the loss of a dog, evidenced by inscriptions left on tombstones commemorating their loss.[65] The surviving epitaphs dedicated to horses are more likely to reference a gratitude for the companionship that had come from war horses rather than race horses. The latter may have chiefly been commemorated as a way to further the owner's fame and glory.[66] In Ancient Egypt, dogs and baboons were kept as pets and buried with their owners. Dogs were given names, which is significant as Egyptians considered names to have magical properties. [67]
+
+Throughout the seventeenth and eighteenth-century pet keeping in the modern sense gradually became accepted throughout Britain. Initially, aristocrats kept dogs for both companionship and hunting. Thus, pet keeping was a sign of elitism within society. By the nineteenth century, the rise of the middle class stimulated the development of pet keeping and it became inscribed within the bourgeois culture.[68]
+
+As the popularity of pet-keeping in the modern sense rose during the Victorian era, animals became a fixture within urban culture as commodities and decorative objects.[69] Pet keeping generated a commercial opportunity for entrepreneurs. By the mid-nineteenth century, nearly twenty thousand street vendors in London dealt with live animals.[70] Also, the popularity of animals developed a demand for animal goods such as accessories and guides for pet keeping. Pet care developed into a big business by the end of the nineteenth century.[71]
+
+Profiteers also sought out pet stealing as a means for economic gain. Utilizing the affection that owners had for their pets, professional dog stealers would capture animals and hold them for ransom.[72] The development of dog stealing reflects the increased value of pets. Pets gradually became defined as the property of their owners. Laws were created that punished offenders for their burglary.[73]
+
+Pets and animals also had social and cultural implications throughout the nineteenth century. The categorization of dogs by their breeds reflected the hierarchical, social order of the Victorian era. The pedigree of a dog represented the high status and lineage of their owners and reinforced social stratification.[74] Middle-class owners, however, valued the ability to associate with the upper-class through ownership of their pets. The ability to care for a pet signified respectability and the capability to be self-sufficient.[75] According to Harriet Ritvo, the identification of “elite animal and elite owner was not a confirmation of the owner’s status but a way of redefining it.”[76]
+
+The popularity of dog and pet keeping generated animal fancy. Dog fanciers showed enthusiasm for owning pets, breeding dogs, and showing dogs in various shows. The first dog show took place on 28 June 1859 in Newcastle and focused mostly on sporting and hunting dogs.[77] However, pet owners produced an eagerness to demonstrate their pets as well as have an outlet to compete.[78] Thus, pet animals gradually were included within dog shows. The first large show, which would host one thousand entries, took place in Chelsea in 1863.[79] The Kennel Club was created in 1873 to ensure fairness and organization within dog shows. The development of the Stud Book by the Kennel Club defined policies, presented a national registry system of purebred dogs, and essentially institutionalized dog shows.[80]
+
+Pet ownership by animals in the wild, as an analogue to the human phenomenon, has not been observed and is likely non-existent in nature.[81][82] One group of capuchin monkeys was observed appearing to care for a marmoset, a fellow New World monkey species, however observations of chimpanzees apparently "playing" with small animals like hyraxes have ended with the chimpanzees killing the animals and tossing the corpses around.[83]
+
+A 2010 study states that human relationships with animals have an exclusive human cognitive component and that pet-keeping is a fundamental and ancient attribute of the human species. Anthropomorphism, or the projection of human feelings, thoughts and attributes on to animals, is a defining feature of human pet-keeping. The study identifies it as the same trait in evolution responsible for domestication and concern for animal welfare. It is estimated to have arisen at least 100,000 years before present (ybp) in Homo sapiens sapiens.[82]
+
+It is debated whether this redirection of human nurturing behaviour towards non-human animals, in the form of pet-keeping, was maladaptive, due to being biologically costly, or whether it was positively selected for.[84][85][82] Two studies suggest that the human ability to domesticate and keep pets came from the same fundamental evolutionary trait and that this trait provided a material benefit in the form of domestication that was sufficiently adaptive to be positively selected for.[82][85]:300 A 2011 study suggests that the practical functions that some pets provide, such as assisting hunting or removing pests, could've resulted in enough evolutionary advantage to allow for the persistence of this behaviour in humans and outweigh the economic burden held by pets kept as playthings for immediate emotional rewards.[86] Two other studies suggest that the behaviour constitutes an error, side effect or misapplication of the evolved mechanisms responsible for human empathy and theory of mind to cover non-human animals which has not sufficiently impacted its evolutionary advantage in the long run.[85]:300
+
+Animals in captivity, with the help of caretakers, have been considered to have owned "pets". Examples of this include Koko the gorilla and several pet cats, Tonda the orangutan and a pet cat and Tarra the elephant and a dog named Bella.[83]
+
+Katharine of Aragon with a monkey
+
+The Girl with the Marmot by Jean-Honoré Fragonard
+
+- Young Lady with parrot by Édouard Manet 1866
+
+Antoinette Metayer (1732–88) and her pet dog
+
+The Lady with an Ermine
+
+Sir Henry Raeburn - Boy and Rabbit
+
+Eos, A Favorite Greyhound of Prince Albert
+
+A Neapolitan Woman
+
+Signal, a Grey Arab, with a Groom in the Desert
+
+Eduardo Leon Garrido. An Elegant Lady with her Dog
+
+The Fireplace depicting a Pug, James Tissot
+
+Rosa Bonheur - Portrait of William F. Cody
+
+Hunt
+
+
+
+
+
+
+

en/2500.html.txt

@@ -0,0 +1,252 @@
+
+
+A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable Universe contains an estimated 1×1024 stars,[1][2] but most are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.
+
+For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.
+
+A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[3] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.[4]  In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[5] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.
+
+Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[6] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
+
+Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[7] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[9] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
+
+The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC.[10] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (c. 1531–1155 BC).[11]
+
+The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[12] The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue.[13] Hipparchus is known for the discovery of the first recorded nova (new star).[14] Many of the constellations and star names in use today derive from Greek astronomy.
+
+In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[15] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[16] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[17] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[18][19][20]
+
+Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[21] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[22] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[23]
+
+According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[24] 
+Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[25] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[26] and by medieval Islamic cosmologists[27] such as Fakhr al-Din al-Razi.[28] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[29]
+
+The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.[25]
+
+William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[30] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
+
+The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.[31] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
+
+The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[25] Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[32]
+The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.[33]
+
+Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[34] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[35]
+
+With the exception of supernovae, individual stars have primarily been observed in the Local Group,[36] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our
+galaxy).[37] But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[38]
+In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Group[39] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years[40]—ten times further than the most distant star cluster previously observed.
+
+In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed—about 180 million years after the Big Bang.[41]
+
+In April, 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years away from Earth.[42][43]
+
+In May 2018, astronomers reported the detection of the most distant oxygen ever detected in the Universe—and the most distant galaxy ever observed by Atacama Large Millimeter Array or the Very Large Telescope—with the team inferring that the signal was emitted 13.3 billion years ago (or 500 million years after the Big Bang). They found that the observed brightness of the galaxy is well-explained by a model where the onset of star formation corresponds to only 250 million years after the Universe began, corresponding to a redshift of about 15.[44]
+
+The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[45] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
+
+As well as certain constellations and the Sun itself, individual stars have their own myths.[46] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[46] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
+
+Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[47][48]
+
+The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[49] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[50] which catalogs and standardizes proper names for stars. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[51][52] The IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU, professional astronomers, nor the amateur astronomy community.[53] One such star-naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,[54][55][56][57] and the New York City Department of Consumer and Worker Protection issued a violation against ISR for engaging in a deceptive trade practice.[58][59]
+
+Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:
+
+The solar mass M⊙ was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass
+together (GM⊙) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be:
+
+However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.
+
+Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m.[60]
+
+Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions—known as molecular clouds—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[61] Most stars form in groups of dozens to hundreds of thousands of stars.[62]
+Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.
+
+All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[63]
+
+The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy).[65][66] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[67]
+
+As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[68] These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.
+
+Early stars of less than 2 M☉ are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[69][70]
+These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[71]
+
+Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
+
+Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.[72] A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.
+
+Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[73]
+The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[74]
+
+Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M☉ every year,[75] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M☉ each year, significantly affecting their evolution.[76] Stars that begin with more than 50 M☉ can lose over half their total mass while on the main sequence.[77]
+
+The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M☉, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 M☉) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.[64] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M☉[78] are expected to have moved off the main sequence.
+
+Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,[79] which affects the strength of its stellar wind.[80] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.
+
+As stars of at least 0.4 M☉[4] exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly as they form a red giant. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.[74][81]
+
+As the hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M☉, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.[6]
+
+After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate.
+
+During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.
+
+When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[82]
+
+The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.[83]
+
+As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place.[84] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.
+
+In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M☉) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.[85]
+
+A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula.[85] The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M☉.[86] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole, the matter is in a state that is not currently understood.
+
+The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[85]
+
+The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.
+
+Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 2 trillion (1012) galaxies.[87] Overall, there are as many as an estimated 1×1024 stars[1][2] (more stars than all the grains of sand on planet Earth).[88][89][90] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[91]
+
+A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.[92] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy.
+
+It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the stars are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[93]
+
+The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to arrive.[94] This is typical of stellar separations in galactic discs.[95] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
+
+Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[96] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.[97]
+
+Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.
+
+Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old.[98] (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13.799 ± 0.021).[98][99]
+
+The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.[100][101]
+
+When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[103] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.[104]
+
+The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[105] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[106] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[107] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.[108]
+
+Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[109]
+
+The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[110]
+
+Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of our sun.[111][112] Betelgeuse, however, has a much lower density than the Sun.[113]
+
+The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
+
+Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[115]
+
+When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[116] A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations.[117]
+
+The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[118]
+
+Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.[119] During
+the Maunder Minimum, for example, the Sun underwent a
+70-year period with almost no sunspot activity.
+
+One of the most massive stars known is Eta Carinae,[120] which,
+with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M☉ as an upper limit for stars in the current era of the universe.[121] This
+represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses,[122] but
+it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M☉ limit on massive star formation.[123]
+
+The first stars to form after the Big Bang may have been larger, up to 300 M☉,[124] due
+to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.[125][126]
+
+With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core.[127] For
+stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ.[128][129] When the metallicity is very low, however, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[129][130] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.
+
+The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[35]
+
+The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart.[131] By contrast, the Sun rotates once every 25–35 days depending on latitude,[132] with an equatorial velocity of 1.93 km/s.[133] A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.[134]
+
+Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[135] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[136] The rotation rate of the pulsar will gradually slow due to the emission of radiation.[137]
+
+The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.[138] The temperature is normally given in terms of an effective temperature,  which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core.[139] The temperature in the core region of a star is several million kelvins.[140]
+
+The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[35]
+
+Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area.[141]
+
+The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[142] which
+streams from the outer layers as electrically charged protons and alpha and beta particles. Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.
+
+The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
+
+The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[143] Besides
+visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
+
+Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.[144]) With these parameters, astronomers can also estimate the age of the star.[145]
+
+The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.[146]
+
+Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.  Giant stars have much larger, more obvious starspots,[147] and
+they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[148] Red
+dwarf flare stars such as UV Ceti may also possess prominent starspot features.[149]
+
+The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).
+
+Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[151] (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
+
+On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
+
+Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[151] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
+
+The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
+
+As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[152] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[153]
+
+The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[155] It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.[156]
+
+Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.[157]
+
+In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[157] The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.
+
+Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[157]
+
+White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.[158]
+
+Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
+
+During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira.[159]
+
+Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[159] This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.
+
+Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[6] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[160] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[159]
+
+Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[159] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.[161]
+
+The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[162][163]
+
+As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M☉, fusion occurs in a slowly expanding shell around the degenerate helium core.[164]
+
+In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
+
+The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiatative heat transfer inefficient) as in the outer envelope.[163]
+
+The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[165] Red dwarf stars with less than 0.4 M☉ are convective throughout, which prevents the accumulation of a helium core.[4] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[163]
+
+The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.
+
+Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[166] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[165] Despite its high temperature, and the corona emits very little light, due to its low gas density. The corona region of the Sun is normally only visible during a solar eclipse.
+
+From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.[167]
+
+A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship E = mc2.[3]
+
+The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[140]
+
+In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction:[168]
+
+These reactions result in the overall reaction:
+
+where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.
+
+In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.[168]
+
+In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[168]
+
+For an overall reaction of:
+
+In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56.[168] Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.
+
+The table at the left shows the amount of time required for a star of 20 M☉ to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.[171]

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable Universe contains an estimated 1×1024 stars,[1][2] but most are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.
+
+For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.
+
+A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[3] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.[4]  In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[5] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.
+
+Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[6] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
+
+Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[7] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[9] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
+
+The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC.[10] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (c. 1531–1155 BC).[11]
+
+The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[12] The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue.[13] Hipparchus is known for the discovery of the first recorded nova (new star).[14] Many of the constellations and star names in use today derive from Greek astronomy.
+
+In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[15] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[16] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[17] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[18][19][20]
+
+Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[21] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[22] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[23]
+
+According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[24] 
+Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[25] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[26] and by medieval Islamic cosmologists[27] such as Fakhr al-Din al-Razi.[28] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[29]
+
+The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.[25]
+
+William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[30] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
+
+The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.[31] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
+
+The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[25] Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[32]
+The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.[33]
+
+Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[34] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[35]
+
+With the exception of supernovae, individual stars have primarily been observed in the Local Group,[36] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our
+galaxy).[37] But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[38]
+In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Group[39] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years[40]—ten times further than the most distant star cluster previously observed.
+
+In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed—about 180 million years after the Big Bang.[41]
+
+In April, 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years away from Earth.[42][43]
+
+In May 2018, astronomers reported the detection of the most distant oxygen ever detected in the Universe—and the most distant galaxy ever observed by Atacama Large Millimeter Array or the Very Large Telescope—with the team inferring that the signal was emitted 13.3 billion years ago (or 500 million years after the Big Bang). They found that the observed brightness of the galaxy is well-explained by a model where the onset of star formation corresponds to only 250 million years after the Universe began, corresponding to a redshift of about 15.[44]
+
+The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[45] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
+
+As well as certain constellations and the Sun itself, individual stars have their own myths.[46] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[46] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
+
+Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[47][48]
+
+The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[49] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[50] which catalogs and standardizes proper names for stars. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[51][52] The IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU, professional astronomers, nor the amateur astronomy community.[53] One such star-naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,[54][55][56][57] and the New York City Department of Consumer and Worker Protection issued a violation against ISR for engaging in a deceptive trade practice.[58][59]
+
+Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:
+
+The solar mass M⊙ was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass
+together (GM⊙) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be:
+
+However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.
+
+Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m.[60]
+
+Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions—known as molecular clouds—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[61] Most stars form in groups of dozens to hundreds of thousands of stars.[62]
+Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.
+
+All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[63]
+
+The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy).[65][66] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[67]
+
+As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[68] These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.
+
+Early stars of less than 2 M☉ are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[69][70]
+These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[71]
+
+Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
+
+Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.[72] A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.
+
+Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[73]
+The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[74]
+
+Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M☉ every year,[75] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M☉ each year, significantly affecting their evolution.[76] Stars that begin with more than 50 M☉ can lose over half their total mass while on the main sequence.[77]
+
+The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M☉, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 M☉) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.[64] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M☉[78] are expected to have moved off the main sequence.
+
+Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,[79] which affects the strength of its stellar wind.[80] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.
+
+As stars of at least 0.4 M☉[4] exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly as they form a red giant. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.[74][81]
+
+As the hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M☉, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.[6]
+
+After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate.
+
+During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.
+
+When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[82]
+
+The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.[83]
+
+As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place.[84] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.
+
+In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M☉) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.[85]
+
+A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula.[85] The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M☉.[86] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole, the matter is in a state that is not currently understood.
+
+The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[85]
+
+The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.
+
+Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 2 trillion (1012) galaxies.[87] Overall, there are as many as an estimated 1×1024 stars[1][2] (more stars than all the grains of sand on planet Earth).[88][89][90] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[91]
+
+A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.[92] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy.
+
+It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the stars are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[93]
+
+The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to arrive.[94] This is typical of stellar separations in galactic discs.[95] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
+
+Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[96] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.[97]
+
+Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.
+
+Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old.[98] (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13.799 ± 0.021).[98][99]
+
+The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.[100][101]
+
+When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[103] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.[104]
+
+The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[105] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[106] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[107] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.[108]
+
+Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[109]
+
+The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[110]
+
+Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of our sun.[111][112] Betelgeuse, however, has a much lower density than the Sun.[113]
+
+The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
+
+Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[115]
+
+When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[116] A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations.[117]
+
+The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[118]
+
+Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.[119] During
+the Maunder Minimum, for example, the Sun underwent a
+70-year period with almost no sunspot activity.
+
+One of the most massive stars known is Eta Carinae,[120] which,
+with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M☉ as an upper limit for stars in the current era of the universe.[121] This
+represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses,[122] but
+it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M☉ limit on massive star formation.[123]
+
+The first stars to form after the Big Bang may have been larger, up to 300 M☉,[124] due
+to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.[125][126]
+
+With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core.[127] For
+stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ.[128][129] When the metallicity is very low, however, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[129][130] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.
+
+The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[35]
+
+The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart.[131] By contrast, the Sun rotates once every 25–35 days depending on latitude,[132] with an equatorial velocity of 1.93 km/s.[133] A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.[134]
+
+Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[135] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[136] The rotation rate of the pulsar will gradually slow due to the emission of radiation.[137]
+
+The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.[138] The temperature is normally given in terms of an effective temperature,  which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core.[139] The temperature in the core region of a star is several million kelvins.[140]
+
+The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[35]
+
+Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area.[141]
+
+The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[142] which
+streams from the outer layers as electrically charged protons and alpha and beta particles. Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.
+
+The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
+
+The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[143] Besides
+visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
+
+Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.[144]) With these parameters, astronomers can also estimate the age of the star.[145]
+
+The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.[146]
+
+Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.  Giant stars have much larger, more obvious starspots,[147] and
+they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[148] Red
+dwarf flare stars such as UV Ceti may also possess prominent starspot features.[149]
+
+The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).
+
+Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[151] (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
+
+On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
+
+Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[151] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
+
+The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
+
+As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[152] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[153]
+
+The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[155] It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.[156]
+
+Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.[157]
+
+In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[157] The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.
+
+Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[157]
+
+White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.[158]
+
+Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
+
+During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira.[159]
+
+Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[159] This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.
+
+Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[6] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[160] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[159]
+
+Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[159] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.[161]
+
+The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[162][163]
+
+As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M☉, fusion occurs in a slowly expanding shell around the degenerate helium core.[164]
+
+In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
+
+The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiatative heat transfer inefficient) as in the outer envelope.[163]
+
+The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[165] Red dwarf stars with less than 0.4 M☉ are convective throughout, which prevents the accumulation of a helium core.[4] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[163]
+
+The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.
+
+Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[166] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[165] Despite its high temperature, and the corona emits very little light, due to its low gas density. The corona region of the Sun is normally only visible during a solar eclipse.
+
+From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.[167]
+
+A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship E = mc2.[3]
+
+The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[140]
+
+In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction:[168]
+
+These reactions result in the overall reaction:
+
+where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.
+
+In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.[168]
+
+In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[168]
+
+For an overall reaction of:
+
+In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56.[168] Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.
+
+The table at the left shows the amount of time required for a star of 20 M☉ to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.[171]

en/2503.html.txt

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+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable Universe contains an estimated 1×1024 stars,[1][2] but most are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.
+
+For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.
+
+A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[3] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.[4]  In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[5] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.
+
+Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[6] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
+
+Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[7] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[9] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
+
+The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC.[10] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (c. 1531–1155 BC).[11]
+
+The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[12] The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue.[13] Hipparchus is known for the discovery of the first recorded nova (new star).[14] Many of the constellations and star names in use today derive from Greek astronomy.
+
+In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[15] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[16] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[17] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[18][19][20]
+
+Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[21] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[22] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[23]
+
+According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[24] 
+Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[25] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[26] and by medieval Islamic cosmologists[27] such as Fakhr al-Din al-Razi.[28] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[29]
+
+The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.[25]
+
+William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[30] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
+
+The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.[31] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
+
+The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[25] Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[32]
+The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.[33]
+
+Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[34] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[35]
+
+With the exception of supernovae, individual stars have primarily been observed in the Local Group,[36] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our
+galaxy).[37] But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[38]
+In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Group[39] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years[40]—ten times further than the most distant star cluster previously observed.
+
+In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed—about 180 million years after the Big Bang.[41]
+
+In April, 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years away from Earth.[42][43]
+
+In May 2018, astronomers reported the detection of the most distant oxygen ever detected in the Universe—and the most distant galaxy ever observed by Atacama Large Millimeter Array or the Very Large Telescope—with the team inferring that the signal was emitted 13.3 billion years ago (or 500 million years after the Big Bang). They found that the observed brightness of the galaxy is well-explained by a model where the onset of star formation corresponds to only 250 million years after the Universe began, corresponding to a redshift of about 15.[44]
+
+The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[45] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
+
+As well as certain constellations and the Sun itself, individual stars have their own myths.[46] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[46] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
+
+Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[47][48]
+
+The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[49] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[50] which catalogs and standardizes proper names for stars. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[51][52] The IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU, professional astronomers, nor the amateur astronomy community.[53] One such star-naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,[54][55][56][57] and the New York City Department of Consumer and Worker Protection issued a violation against ISR for engaging in a deceptive trade practice.[58][59]
+
+Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:
+
+The solar mass M⊙ was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass
+together (GM⊙) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be:
+
+However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.
+
+Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m.[60]
+
+Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions—known as molecular clouds—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[61] Most stars form in groups of dozens to hundreds of thousands of stars.[62]
+Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.
+
+All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[63]
+
+The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy).[65][66] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[67]
+
+As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[68] These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.
+
+Early stars of less than 2 M☉ are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[69][70]
+These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[71]
+
+Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
+
+Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.[72] A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.
+
+Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[73]
+The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[74]
+
+Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M☉ every year,[75] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M☉ each year, significantly affecting their evolution.[76] Stars that begin with more than 50 M☉ can lose over half their total mass while on the main sequence.[77]
+
+The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M☉, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 M☉) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.[64] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M☉[78] are expected to have moved off the main sequence.
+
+Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,[79] which affects the strength of its stellar wind.[80] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.
+
+As stars of at least 0.4 M☉[4] exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly as they form a red giant. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.[74][81]
+
+As the hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M☉, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.[6]
+
+After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate.
+
+During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.
+
+When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[82]
+
+The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.[83]
+
+As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place.[84] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.
+
+In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M☉) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.[85]
+
+A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula.[85] The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M☉.[86] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole, the matter is in a state that is not currently understood.
+
+The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[85]
+
+The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.
+
+Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 2 trillion (1012) galaxies.[87] Overall, there are as many as an estimated 1×1024 stars[1][2] (more stars than all the grains of sand on planet Earth).[88][89][90] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[91]
+
+A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.[92] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy.
+
+It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the stars are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[93]
+
+The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to arrive.[94] This is typical of stellar separations in galactic discs.[95] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
+
+Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[96] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.[97]
+
+Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.
+
+Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old.[98] (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13.799 ± 0.021).[98][99]
+
+The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.[100][101]
+
+When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[103] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.[104]
+
+The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[105] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[106] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[107] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.[108]
+
+Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[109]
+
+The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[110]
+
+Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of our sun.[111][112] Betelgeuse, however, has a much lower density than the Sun.[113]
+
+The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
+
+Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[115]
+
+When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[116] A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations.[117]
+
+The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[118]
+
+Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.[119] During
+the Maunder Minimum, for example, the Sun underwent a
+70-year period with almost no sunspot activity.
+
+One of the most massive stars known is Eta Carinae,[120] which,
+with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M☉ as an upper limit for stars in the current era of the universe.[121] This
+represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses,[122] but
+it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M☉ limit on massive star formation.[123]
+
+The first stars to form after the Big Bang may have been larger, up to 300 M☉,[124] due
+to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.[125][126]
+
+With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core.[127] For
+stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ.[128][129] When the metallicity is very low, however, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[129][130] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.
+
+The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[35]
+
+The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart.[131] By contrast, the Sun rotates once every 25–35 days depending on latitude,[132] with an equatorial velocity of 1.93 km/s.[133] A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.[134]
+
+Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[135] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[136] The rotation rate of the pulsar will gradually slow due to the emission of radiation.[137]
+
+The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.[138] The temperature is normally given in terms of an effective temperature,  which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core.[139] The temperature in the core region of a star is several million kelvins.[140]
+
+The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[35]
+
+Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area.[141]
+
+The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[142] which
+streams from the outer layers as electrically charged protons and alpha and beta particles. Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.
+
+The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
+
+The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[143] Besides
+visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
+
+Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.[144]) With these parameters, astronomers can also estimate the age of the star.[145]
+
+The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.[146]
+
+Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.  Giant stars have much larger, more obvious starspots,[147] and
+they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[148] Red
+dwarf flare stars such as UV Ceti may also possess prominent starspot features.[149]
+
+The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).
+
+Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[151] (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
+
+On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
+
+Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[151] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
+
+The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
+
+As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[152] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[153]
+
+The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[155] It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.[156]
+
+Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.[157]
+
+In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[157] The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.
+
+Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[157]
+
+White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.[158]
+
+Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
+
+During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira.[159]
+
+Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[159] This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.
+
+Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[6] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[160] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[159]
+
+Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[159] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.[161]
+
+The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[162][163]
+
+As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M☉, fusion occurs in a slowly expanding shell around the degenerate helium core.[164]
+
+In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
+
+The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiatative heat transfer inefficient) as in the outer envelope.[163]
+
+The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[165] Red dwarf stars with less than 0.4 M☉ are convective throughout, which prevents the accumulation of a helium core.[4] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[163]
+
+The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.
+
+Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[166] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[165] Despite its high temperature, and the corona emits very little light, due to its low gas density. The corona region of the Sun is normally only visible during a solar eclipse.
+
+From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.[167]
+
+A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship E = mc2.[3]
+
+The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[140]
+
+In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction:[168]
+
+These reactions result in the overall reaction:
+
+where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.
+
+In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.[168]
+
+In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[168]
+
+For an overall reaction of:
+
+In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56.[168] Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.
+
+The table at the left shows the amount of time required for a star of 20 M☉ to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.[171]

en/2506.html.txt

@@ -0,0 +1,116 @@
+
+
+An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.[4] The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 July 2020, there are 4,281 confirmed exoplanets in 3,163 systems, with 701 systems having more than one planet.[5]
+
+There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[6] In several cases, multiple planets have been observed around a star.[7] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][8][9] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[10]
+
+The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12] about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium[13]), it is too massive to be a planet and may be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. There is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[14][15] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[16]
+
+The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[17]
+
+Rogue planets do not orbit any star. Such objects are considered as a separate category of planet, especially if they are gas giants, which are often counted as sub-brown dwarfs.[18] The rogue planets in the Milky Way possibly number in the billions or more.[19][20]
+
+The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[22] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
+
+For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
+
+The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[4]
+
+The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[23] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[24]
+
+This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[26]
+
+In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[27]
+
+Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[28] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[29] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[30] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[31] Astronomers now generally regard all the early reports of detection as erroneous.[32]
+
+In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[33] The claim briefly received intense attention, but Lyne and his team soon retracted it.[34]
+
+As of 1 July 2020, a total of 4,281 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[5]  The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[35] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[36] but subsequent work in 1992 again raised serious doubts.[37] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[38]
+
+On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[23] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[39] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
+
+On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[40][41] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
+
+Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[42] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[43]
+
+On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[44][45][46] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[44]
+
+On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[47]
+
+On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[48] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[48] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[48]
+
+In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[49]
+
+As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[50] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[51][52][53]
+
+About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[57] Recently the techniques of singular optics have been applied in the search for exoplanets.[58]
+
+Planets may form within a few to tens (or more) of millions of years of their star forming.[59][60][61][62][63] 
+The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[64] to planetary systems of over 10 Gyr old.[65] When planets form in a gaseous protoplanetary disk,[66] they accrete hydrogen/helium envelopes.[67][68] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[66] This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]
+
+There is at least one planet on average per star.[7]
+About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[74]
+
+Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[75][76] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.
+
+Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[77]
+
+Some planets orbit one member of a binary star system,[78] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[79] and one in the quadruple system Kepler-64.
+
+In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[80][81] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[82] and Kappa Andromedae b, which if seen up close would appear reddish in color.[83]
+Helium planets are expected to be white or grey in appearance.[84]
+
+The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[85]
+
+The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[86][87][88]
+
+For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[89]
+
+There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[89]
+
+Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[89]
+
+In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[90][91]
+
+Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[92][93] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[94]
+
+Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[95][96]
+
+Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[97]
+
+In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[98][99]
+
+Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system.  The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[100]
+
+In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[101][102]
+
+In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[103][104] with one team saying that plate tectonics would be episodic or stagnant[105] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[106]
+
+If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[107][108]
+
+Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[109][110]
+
+The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[111][112]
+
+The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[113]
+
+The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[114]
+
+In December 2013 a candidate exomoon of a rogue planet was announced.[115] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[116]
+
+Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[118]
+
+In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[119][120] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
+
+KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[121] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[122]
+
+In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[123]
+
+Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[124] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[125] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[126]
+
+As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[57] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[57] For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[127] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[128][129]

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+
+
+A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable Universe contains an estimated 1×1024 stars,[1][2] but most are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.
+
+For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.
+
+A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[3] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.[4]  In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[5] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.
+
+Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[6] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.
+
+Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[7] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[9] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
+
+The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC.[10] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (c. 1531–1155 BC).[11]
+
+The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[12] The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue.[13] Hipparchus is known for the discovery of the first recorded nova (new star).[14] Many of the constellations and star names in use today derive from Greek astronomy.
+
+In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[15] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[16] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[17] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[18][19][20]
+
+Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[21] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[22] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[23]
+
+According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[24] 
+Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[25] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[26] and by medieval Islamic cosmologists[27] such as Fakhr al-Din al-Razi.[28] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[29]
+
+The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.[25]
+
+William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[30] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
+
+The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.[31] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
+
+The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[25] Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[32]
+The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.[33]
+
+Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[34] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[35]
+
+With the exception of supernovae, individual stars have primarily been observed in the Local Group,[36] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our
+galaxy).[37] But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[38]
+In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Group[39] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years[40]—ten times further than the most distant star cluster previously observed.
+
+In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed—about 180 million years after the Big Bang.[41]
+
+In April, 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years away from Earth.[42][43]
+
+In May 2018, astronomers reported the detection of the most distant oxygen ever detected in the Universe—and the most distant galaxy ever observed by Atacama Large Millimeter Array or the Very Large Telescope—with the team inferring that the signal was emitted 13.3 billion years ago (or 500 million years after the Big Bang). They found that the observed brightness of the galaxy is well-explained by a model where the onset of star formation corresponds to only 250 million years after the Universe began, corresponding to a redshift of about 15.[44]
+
+The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[45] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
+
+As well as certain constellations and the Sun itself, individual stars have their own myths.[46] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[46] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
+
+Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[47][48]
+
+The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[49] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[50] which catalogs and standardizes proper names for stars. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[51][52] The IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU, professional astronomers, nor the amateur astronomy community.[53] One such star-naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,[54][55][56][57] and the New York City Department of Consumer and Worker Protection issued a violation against ISR for engaging in a deceptive trade practice.[58][59]
+
+Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:
+
+The solar mass M⊙ was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass
+together (GM⊙) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be:
+
+However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.
+
+Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m.[60]
+
+Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions—known as molecular clouds—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[61] Most stars form in groups of dozens to hundreds of thousands of stars.[62]
+Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.
+
+All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[63]
+
+The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy).[65][66] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[67]
+
+As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[68] These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.
+
+Early stars of less than 2 M☉ are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[69][70]
+These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[71]
+
+Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
+
+Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.[72] A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.
+
+Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[73]
+The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[74]
+
+Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M☉ every year,[75] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M☉ each year, significantly affecting their evolution.[76] Stars that begin with more than 50 M☉ can lose over half their total mass while on the main sequence.[77]
+
+The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M☉, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 M☉) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.[64] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M☉[78] are expected to have moved off the main sequence.
+
+Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,[79] which affects the strength of its stellar wind.[80] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.
+
+As stars of at least 0.4 M☉[4] exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly as they form a red giant. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.[74][81]
+
+As the hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M☉, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.[6]
+
+After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate.
+
+During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.
+
+When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[82]
+
+The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.[83]
+
+As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place.[84] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.
+
+In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M☉) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.[85]
+
+A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula.[85] The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M☉.[86] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole, the matter is in a state that is not currently understood.
+
+The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[85]
+
+The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.
+
+Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 2 trillion (1012) galaxies.[87] Overall, there are as many as an estimated 1×1024 stars[1][2] (more stars than all the grains of sand on planet Earth).[88][89][90] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[91]
+
+A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.[92] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy.
+
+It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the stars are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[93]
+
+The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to arrive.[94] This is typical of stellar separations in galactic discs.[95] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
+
+Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[96] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.[97]
+
+Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.
+
+Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old.[98] (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13.799 ± 0.021).[98][99]
+
+The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.[100][101]
+
+When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[103] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.[104]
+
+The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[105] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[106] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[107] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.[108]
+
+Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[109]
+
+The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[110]
+
+Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of our sun.[111][112] Betelgeuse, however, has a much lower density than the Sun.[113]
+
+The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
+
+Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[115]
+
+When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[116] A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations.[117]
+
+The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[118]
+
+Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.[119] During
+the Maunder Minimum, for example, the Sun underwent a
+70-year period with almost no sunspot activity.
+
+One of the most massive stars known is Eta Carinae,[120] which,
+with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M☉ as an upper limit for stars in the current era of the universe.[121] This
+represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses,[122] but
+it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M☉ limit on massive star formation.[123]
+
+The first stars to form after the Big Bang may have been larger, up to 300 M☉,[124] due
+to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.[125][126]
+
+With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core.[127] For
+stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ.[128][129] When the metallicity is very low, however, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[129][130] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.
+
+The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[35]
+
+The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart.[131] By contrast, the Sun rotates once every 25–35 days depending on latitude,[132] with an equatorial velocity of 1.93 km/s.[133] A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.[134]
+
+Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[135] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[136] The rotation rate of the pulsar will gradually slow due to the emission of radiation.[137]
+
+The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.[138] The temperature is normally given in terms of an effective temperature,  which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core.[139] The temperature in the core region of a star is several million kelvins.[140]
+
+The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[35]
+
+Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area.[141]
+
+The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[142] which
+streams from the outer layers as electrically charged protons and alpha and beta particles. Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.
+
+The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
+
+The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[143] Besides
+visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
+
+Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.[144]) With these parameters, astronomers can also estimate the age of the star.[145]
+
+The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.[146]
+
+Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.  Giant stars have much larger, more obvious starspots,[147] and
+they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[148] Red
+dwarf flare stars such as UV Ceti may also possess prominent starspot features.[149]
+
+The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).
+
+Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[151] (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
+
+On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
+
+Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[151] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
+
+The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
+
+As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[152] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[153]
+
+The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[155] It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.[156]
+
+Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.[157]
+
+In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[157] The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.
+
+Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[157]
+
+White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.[158]
+
+Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
+
+During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira.[159]
+
+Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[159] This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.
+
+Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[6] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[160] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[159]
+
+Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[159] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.[161]
+
+The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[162][163]
+
+As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M☉, fusion occurs in a slowly expanding shell around the degenerate helium core.[164]
+
+In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
+
+The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiatative heat transfer inefficient) as in the outer envelope.[163]
+
+The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[165] Red dwarf stars with less than 0.4 M☉ are convective throughout, which prevents the accumulation of a helium core.[4] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[163]
+
+The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.
+
+Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[166] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[165] Despite its high temperature, and the corona emits very little light, due to its low gas density. The corona region of the Sun is normally only visible during a solar eclipse.
+
+From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere.[167]
+
+A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship E = mc2.[3]
+
+The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[140]
+
+In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction:[168]
+
+These reactions result in the overall reaction:
+
+where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.
+
+In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.[168]
+
+In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[168]
+
+For an overall reaction of:
+
+In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56.[168] Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.
+
+The table at the left shows the amount of time required for a star of 20 M☉ to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.[171]

en/2508.html.txt

@@ -0,0 +1,134 @@
+
+
+A week is a time unit equal to seven days. It is the standard time period used for cycles of rest days in most parts of the world, mostly alongside—although not strictly part of—the Gregorian calendar.
+
+In many languages, the days of the week are named after classical planets or gods of a pantheon. In English, the names are Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday. Such a week may be called a planetary week.[citation needed] This arrangement is similar to a week in the Bible in which the seven days are simply numbered with the first day being a Christian day of worship and the seventh day being a sabbath day. The traditional Biblical sabbath is aligned with Saturday.
+
+While, for example, the United States, Canada, Brazil, Japan and other countries consider Sunday as the first day of the week, and while the week begins with Saturday in much of the Middle East, the international ISO 8601 standard[a] has Monday as the first day of the week. The ISO standard includes the ISO week date system, a numbering system for weeks within a given year, where each week starting on a Monday is associated with the year that contains that week's Thursday (so that if a year starts in a long weekend Friday–Sunday, week number one of the year will start after that). ISO 8601 assigns numbers to the days of the week, running from 1 to 7 for Monday through to Sunday.
+
+The term "week" is sometimes expanded to refer to other time units comprising a few days, such as the nundinal cycle of the ancient Roman calendar, the "work week", or "school week" referring only to the days spent on those activities.
+
+
+
+The English word week comes from the Old English wice, ultimately from a Common Germanic *wikōn-, from a root *wik- "turn, move, change". The Germanic word probably had a wider meaning prior to the adoption of the Roman calendar, perhaps "succession series", as suggested by Gothic wikō translating taxis "order" in Luke 1:8.
+
+The seven-day week is named in many languages by a word derived from "seven". The archaism sennight ("seven-night") preserves the old Germanic practice of reckoning time by nights, as in the more common fortnight ("fourteen-night").[1] Hebdomad and hebdomadal week both derive from the Greek hebdomás (ἑβδομάς, "a seven"). Septimana is cognate with the Romance terms derived from Latin septimana ("a seven").
+
+Slavic has a formation *tъ(žь)dьnь (Serbian тједан, Croatian tjedan, Ukrainian тиждень, Czech týden, Polish tydzień), from *tъ "this" + *dьnь "day". Chinese has 星期, as it were "planetary time unit".
+
+A week is defined as an interval of exactly seven days,[b] so that technically, except at daylight saving time transitions or leap seconds,
+
+With respect to the Gregorian calendar:
+
+In a Gregorian mean year, there are 365.2425 days, and thus exactly ​52 71⁄400 or 52.1775 weeks (unlike the Julian year of 365.25 days or ​52 5⁄28 ≈ 52.1786 weeks, which cannot be represented by a finite decimal expansion). There are exactly 20,871 weeks in 400 Gregorian years, so 27 July 1620 was a Monday just as was 27 July 2020.
+
+Relative to the path of the Moon, a week is 23.659% of an average lunation or 94.637% of an average quarter lunation.
+
+Historically, the system of dominical letters (letters A to G identifying the weekday of the first day of a given year) has been used to facilitate calculation of the day of week.
+The day of the week can be easily calculated given a date's Julian day number (JD, i.e. the integer value at noon UT):
+Adding one to the remainder after dividing the Julian day number by seven (JD modulo 7 + 1) yields that date's ISO 8601 day of the week. For example, the Julian day number of 27 July 2020 is 2459058. Calculating 2459058 mod 7 + 1 yields 1, corresponding to Monday.[2]
+
+The days of the week were originally named for the classical planets. This naming system persisted alongside an "ecclesiastical" tradition of numbering the days, in ecclesiastical Latin beginning with dominica (the Lord's Day) as the first day. The Greco-Roman gods associated with the classical planets were rendered in their interpretatio germanica at some point during the late Roman Empire, yielding the Germanic tradition of names based on indigenous deities.
+
+The ordering of the weekday names is not the classical order of the planets (by distance in the planetary spheres model, nor, equivalently, by their apparent speed of movement in the night sky). Instead, the planetary hours systems resulted in succeeding days being named for planets that are three places apart in their traditional listing. This characteristic was apparently discussed in Plutarch in a treatise written in c. AD 100, which is reported to have addressed the question of Why are the days named after the planets reckoned in a different order from the actual order? (the text of Plutarch's treatise has been lost).[3]
+
+An ecclesiastical, non-astrological, system of numbering the days of the week was adopted in Late Antiquity. This model also seems to have influenced (presumably via Gothic) the designation of Wednesday as "mid-week" in Old High German (mittawehha) and Old Church Slavonic (срѣда). Old Church Slavonic may have also modeled the name of Monday, понєдѣльникъ, after the Latin feria secunda.[5] The ecclesiastical system became prevalent in Eastern Christianity, but in the Latin West it remains extant only in modern Icelandic, Galician, and Portuguese.[6]
+
+A continuous seven-day cycle that runs throughout history without reference to the phases of the moon was first practised in Judaism, dated to the 6th century BC at the latest.[8][9]
+
+There are several hypotheses concerning the origin of the biblical seven-day cycle.
+
+Friedrich Delitzsch and others suggested that the seven-day week being approximately a quarter of a lunation is the implicit astronomical origin of the seven-day week,[10] and indeed the Babylonian calendar used intercalary days to synchronize the last week of a month with the new moon.[11] According to this theory, the Jewish week was adopted from the Babylonians while removing the moon-dependency.
+
+However, Niels-Erik Andreasen, Jeffrey H. Tigay, and others claimed that the Biblical Sabbath is mentioned as a day of rest in some of the earliest layers of the Pentateuch dated to the 9th century BC at the latest, centuries before Judea's Babylonian exile. They also find the resemblance between the Biblical Sabbath and the Babylonian system to be weak. Therefore, they suggested that the seven-day week may reflect an independent Israelite tradition.[12][13][14][15] Tigay writes:
+
+It is clear that among neighboring nations that were in position to have an influence over Israel – and in fact which did influence it in various matters – there is no precise parallel to the Israelite Sabbatical week. This leads to the conclusion that the Sabbatical week, which is as unique to Israel as the Sabbath from which it flows, is an independent Israelite creation.[14][16]
+
+The seven-day week seems to have been adopted, at different stages, by the Persian Empire, in Hellenistic astrology, and (via Greek transmission) in Gupta India and Tang China.[c][citation needed]
+The Babylonian system was received by the Greeks in the 4th century BC (notably via Eudoxus of Cnidus). However the designation of the seven days of the week to the seven planets is an innovation introduced in the time of Augustus.[18] The astrological concept of planetary hours is rather an original innovation of Hellenistic astrology, probably first conceived in the 2nd century BC.[19]
+
+The seven-day week was widely known throughout the Roman Empire by the 1st century AD,[18] along with references to the Jewish Sabbath by Roman authors such as Seneca and Ovid.[20] When the seven-day week came into use in Rome during the early imperial period, it did not immediately replace the older eight-day nundinal system.[21] The nundinal system had probably fallen out of use by the time Emperor Constantine adopted the seven-day week for official use in CE 321, making the Day of the Sun (dies Solis) a legal holiday.[22]
+
+The earliest evidence of an astrological significance of a seven-day period is connected to Gudea, priest-king of Lagash in Sumer during the Gutian dynasty, who built a seven-room temple, which he dedicated with a seven-day festival. In the flood story of the Assyro-Babylonian epic of Gilgamesh, the storm lasts for seven days, the dove is sent out after seven days, and the Noah-like character of Utnapishtim leaves the ark seven days after it reaches firm ground.[d]
+
+It is possible that the Hebrew seven-day week is based on the Babylonian tradition, although going through certain adaptations.[contradictory] George Aaron Barton speculated that the seven-day creation account of Genesis is connected to the Babylonian creation epic, Enûma Eliš, which is recorded on seven tablets.[26]
+
+Counting from the new moon, the Babylonians celebrated the 7th, 14th, 21st and 28th as "holy-days", also called "evil days" (meaning "unsuitable" for prohibited activities). On these days, officials were prohibited from various activities and common men were forbidden to "make a wish", and at least the 28th was known as a "rest-day".[27]
+On each of them, offerings were made to a different god and goddess.
+
+In a frequently-quoted suggestion going back to the early 20th century,[28] the Hebrew Sabbath is compared to the Sumerian sa-bat "mid-rest", a term for the full moon. The Sumerian term has been reconstructed as rendered Sapattum or Sabattum in Babylonian, possibly present in the lost fifth tablet of the Enûma Eliš, tentatively reconstructed[according to whom?] "[Sa]bbath shalt thou then encounter, mid[month]ly".[27]
+
+The Zoroastrian calendar follows the Babylonian in relating the 7th, 14th, 21st, and 28th of the month to Ahura Mazda.[29]
+The forerunner of all modern Zoroastrian calendars is the system used to determine dates in the Persian Empire, adopted from the Babylonian calendar by the 4th century BC.
+
+Frank C. Senn in his book Christian Liturgy: Catholic and Evangelical points to data suggesting evidence of an early continuous use of a seven-day week; referring to the Jews during the Babylonian captivity in the 6th century BC,[9] after the destruction of the Temple of Solomon.
+While the seven-day week in Judaism is tied to Creation account in the Book of Genesis in the Hebrew Bible (where God creates the heavens and the earth in six days and rests on the seventh; Genesis 1:1–2:3, in the Book of Exodus, the fourth of the Ten Commandments is to rest on the seventh day, Shabbat, which can be seen as implying a socially instituted seven-day week), it is not clear whether the Genesis narrative predates the Babylonian captivity of the Jews in the 6th century BC. At least since the Second Temple period under Persian rule, Judaism relied on the seven-day cycle of recurring Sabbaths.[30]
+
+Tablets[citation needed] from the Achaemenid period indicate that the lunation of 29 or 30 days basically contained three seven-day weeks, and a final week of eight or nine days inclusive, breaking the continuous seven-day cycle.[27]
+The Babylonians additionally celebrated the 19th as a special "evil day", the "day of anger", because it was roughly the 49th day of the (preceding) month, completing a "week of weeks", also with sacrifices and prohibitions.[27]
+
+Difficulties with Friedrich Delitzsch's origin theory connecting Hebrew Shabbat with the Babylonian lunar cycle[31] include reconciling the differences between an unbroken week and a lunar week, and explaining the absence of texts naming the lunar week as Shabbat in any language.[32]
+
+In Jewish sources by the time of the Septuagint, the term "Sabbath" (Greek Sabbaton) by synecdoche also came to refer to an entire seven-day week,[33] the interval between two weekly Sabbaths.
+Jesus's parable of the Pharisee and the Publican (Luke 18:12) describes the Pharisee as fasting "twice in the week" (Greek δὶς τοῦ σαββάτου dis tou sabbatou).
+
+The ancient Romans traditionally used the eight-day nundinum but, after the Julian calendar had come into effect in 45 BC, the seven-day week came into increasing use. For a while, the week and the nundinal cycle coexisted, but by the time the week was officially adopted by Constantine in AD 321, the nundinal cycle had fallen out of use. The association of the days of the week with the Sun, the Moon and the five planets visible to the naked eye dates to the Roman era (2nd century).[34][30]
+
+The continuous seven-day cycle of the days of the week can be traced back to the reign of Augustus; the first identifiable date cited complete with day of the week is 6 February AD 60, identified as a "Sunday" (as viii idus Februarius dies solis "eighth day before the ides of February, day of the Sun") in a Pompeiian graffito. According to the (contemporary) Julian calendar, 6 February 60 was, however, a Wednesday. This is explained by the existence of two conventions of naming days of the weeks based on the planetary hours system: 6 February was a "Sunday" based on the sunset naming convention, and a "Wednesday" based on the sunrise naming convention.[35]
+
+The earliest known reference in Chinese writings to a seven-day week is attributed to Fan Ning, who lived in the late 4th century in the Jin Dynasty, while diffusions from the Manichaeans are documented with the writings of the Chinese Buddhist monk Yi Jing and the Ceylonese or Central Asian Buddhist monk Bu Kong of the 7th century (Tang Dynasty).
+
+The Chinese variant of the planetary system was brought to Japan by the Japanese monk Kūkai (9th century). Surviving diaries of the Japanese statesman Fujiwara Michinaga show the seven-day system in use in Heian Period Japan as early as 1007. In Japan, the seven-day system was kept in use for astrological purposes until its promotion to a full-fledged Western-style calendrical basis during the Meiji Period.
+
+The seven-day week was known in India by the 6th century, referenced in the Pañcasiddhāntikā.[citation needed] Shashi (2000) mentions the Garga Samhita, which he places in the 1st century BC or AD, as a possible earlier reference to a seven-day week in India. He concludes "the above references furnish a terminus ad quem (viz. 1st century) The terminus a quo cannot be stated with certainty".[36][37]
+
+In Arabia, a similar seven-week system was adopted, that may be influenced by the Hebrew week (via Christianity).[citation needed]
+
+The seven-day weekly cycle has remained unbroken in Christendom, and hence in Western history, for almost two millennia, despite changes to the Coptic, Julian, and Gregorian calendars, demonstrated by the date of Easter Sunday having been traced back through numerous computistic tables to an Ethiopic copy of an early Alexandrian table beginning with the Easter of AD 311.[38][39]
+
+A tradition of divinations arranged for the days of the week on which certain feast days occur develops in the Early Medieval period. There are many later variants of this, including the German Bauern-Praktik and the versions of Erra Pater published in 16th to 17th century England, mocked in Samuel Butler's Hudibras. South and East Slavic versions are known as koliadniki (from koliada, a loan of Latin calendae), with Bulgarian copies dating from the 13th century, and Serbian versions from the 14th century.[40]
+
+Medieval Christian traditions associated with the lucky or unlucky nature of certain days of the week survived into the modern period. This concerns primarily Friday, associated with the crucifixion of Jesus. Sunday, sometimes personified as Saint Anastasia, was itself an object of worship in Russia, a practice denounced in a sermon extant in copies going back to the 14th century.[41]
+
+Sunday, in the ecclesiastical numbering system also counted as the feria prima or the first day of the week; yet, at the same time, figures as the "eighth day", and has occasionally been so called in Christian liturgy.
+[e]
+
+Justin Martyr wrote: "the first day after the Sabbath, remaining the first of all the days, is called, however, the eighth, according to the number of all the days of the cycle, and [yet] remains the first."[42]
+
+A period of eight days, usually (but not always, mainly because of Christmas Day) starting and ending on a Sunday, is called an octave, particularly in Roman Catholic liturgy. In German, the phrase heute in acht Tagen (literally "today in eight days") means one week from today (i.e. on the same weekday). The same is true of the Italian phrase oggi otto (literally "today eight").
+
+Weeks in a Gregorian calendar year can be numbered for each year. This style of numbering is often used in European and Asian countries. It is less common in the U.S. and elsewhere.
+
+The system for numbering weeks is the ISO week date system, which is included in ISO 8601. This system dictates that each week begins on a Monday and is associated with the year that contains that week's Thursday.
+
+In practice week 1 (W01 in ISO notation) of any year can be determined as follows:
+
+Examples:
+
+It is also possible to determine if the last week of the previous year was Week 52 or Week 53 as follows:
+
+In some countries, though, the numbering system is different from the ISO standard. At least six numberings are in use:[43][44][dubious  – discuss]
+
+The semiconductor package date code is often a 4 digit date code YYWW where the first two digits YY are the last 2 digits of the calendar year and the last two digits WW are the two-digit week number.[45][46]
+
+The tire date code mandated by the US DOT is a 4 digit date code WWYY with two digits of the week number WW followed by the last two digits of the calendar year YY.[47]
+
+The term "week" is sometimes expanded to refer to other time units comprising a few days. Such "weeks" of between four and ten days have been used historically in various places.[48] Intervals longer than 10 days are not usually termed "weeks" as they are closer in length to the fortnight or the month than to the seven-day week.
+
+Calendars unrelated to the Chaldean, Hellenistic, Christian, or Jewish traditions often have time cycles between the day and the month of varying lengths, sometimes also called "weeks".
+
+An eight-day week was used in Ancient Rome and possibly in the pre-Christian Celtic calendar. Traces of a nine-day week are found in Baltic languages and in Welsh. The ancient Chinese calendar had a ten-day week, as did the ancient Egyptian calendar (and, incidentally, the French Republican Calendar, dividing its 30-day months into thirds).
+
+A six-day week is found in the Akan Calendar. Several cultures used a five-day week, including the 10th century Icelandic calendar, the Javanese calendar, and the traditional cycle of market days in Korea.[citation needed] The Igbo have a "market week" of four days. Evidence of a "three-day week" has been derived from the names of the days of the week in Guipuscoan Basque.[49]
+
+The Aztecs and Mayas used the Mesoamerican calendars. The most important of these calendars divided a ritual cycle of 260 days (known as Tonalpohualli in Nahuatl and Tzolk'in in Yucatec Maya) into 20 weeks of 13 days (known in Spanish as trecenas). They also divided the solar year into 18 periods (winal) of 20 days and five nameless days (wayebʼ), creating a 20-day month divided into four five-day weeks. The end of each five-day week was a market day.[50][51]
+
+The Balinese Pawukon is a 210-day calendar consisting of 10 different simultaneously running weeks of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 days, of which the weeks of 4, 8, and 9 days are interrupted to fit into the 210-day cycle.
+
+A 10-day week, called décade, was used in France for nine and a half years from October 1793 to April 1802; furthermore, the Paris Commune adopted the Revolutionary Calendar for 18 days in 1871.
+
+The Bahá'í calendar features a 19-day period which some classify as a month and others classify as a week.[52]
+
+The International Fixed Calendar (also known as the "Eastman plan") fixed every date always on the same weekday. This plan kept a 7-day week while defining a year of 13 months with 28 days each. It was the official calendar of the Eastman Kodak Company for decades.
+
+In the Soviet Union between 1929 and 1940, most factory and enterprise workers, but not collective farm workers, used five- and six-day work weeks while the country as a whole continued to use the traditional seven-day week.[53][54][55] From 1929 to 1951, five national holidays were days of rest (22 January, 1–2 May, 7–8 November). From autumn 1929 to summer 1931, the remaining 360 days of the year were subdivided into 72 five-day work weeks beginning on 1 January. Workers were assigned any one of the five days as their day off, even if their spouse or friends might be assigned a different day off. Peak usage of the five-day work week occurred on 1 October 1930 at 72% of industrial workers. From summer 1931 until 26 June 1940, each Gregorian month was subdivided into five six-day work weeks, more-or-less, beginning with the first day of each month. The sixth day of each six-day work week was a uniform day of rest. On 1 July 1935 74.2% of industrial workers were on non-continuous schedules, mostly six-day work weeks, while 25.8% were still on continuous schedules, mostly five-day work weeks. The Gregorian calendar with its irregular month lengths and the traditional seven-day week were used in the Soviet Union during its entire existence, including 1929–1940; for example, in the masthead of Pravda, the official Communist newspaper, and in both Soviet calendars displayed here. The traditional names of the seven-day week continued to be used, including "Resurrection" (Воскресенье) for Sunday and "Sabbath" (Суббота) for Saturday, despite the government's official atheism.

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+
+
+Hebrew (Hebrew: עִבְרִית‎, romanized: Ivrit, IPA: [ivˈʁit] or [ʕivˈɾit] (listen)) is a Northwest Semitic language native to Israel. In 2013, Modern Hebrew was spoken by over nine million people worldwide.[8] Historically, it is regarded as the language of the Israelites and their ancestors, although the language was not referred to by the name "Hebrew" in the Tanakh itself.[note 1] The earliest examples of written Paleo-Hebrew date from the 10th century BCE.[9] Hebrew belongs to the West Semitic branch of the Afroasiatic language family. Hebrew is the only Canaanite language still spoken and the only truly successful example of a revived dead language.[10][11]
+
+Hebrew ceased to be an everyday spoken language somewhere between 200 and 400 CE, declining since the aftermath of the Bar Kokhba revolt.[2][12][note 2] Aramaic and, to a lesser extent, Greek were already in use as international languages, especially among elites and immigrants.[14] Hebrew survived into the medieval period as the language of Jewish liturgy, rabbinic literature, intra-Jewish commerce and poetry. With the rise of Zionism in the 19th century, it was revived as a spoken and literary language, becoming the main language of the Yishuv and subsequently of the State of Israel. According to Ethnologue, in 1998, Hebrew was the language of five million people worldwide.[5] After Israel, the United States has the second-largest Hebrew-speaking population, with about 220,000 fluent speakers,[15] mostly from Israel.
+
+Modern Hebrew is the official language of the State of Israel, while premodern Hebrew is used for prayer or study in Jewish communities around the world today. The Samaritan dialect is also the liturgical tongue of the Samaritans, while modern Hebrew or Arabic is their vernacular. As a foreign language, it is studied mostly by Jews and students of Judaism and Israel and by archaeologists and linguists specializing in the Middle East and its civilizations, as well as by theologians in Christian seminaries.
+
+Nearly all of the Hebrew Bible is written in Biblical Hebrew, with much of its present form in the dialect that scholars believe flourished around the 6th century BCE, around the time of the Babylonian captivity. For this reason, Hebrew has been referred to by Jews as Lashon Hakodesh (לשון הקודש), "the holy language" or "the language of holiness", since ancient times.
+
+The modern English word "Hebrew" is derived from Old French Ebrau, via Latin from the Greek Ἑβραῖος (Hebraîos) and Aramaic 'ibrāy, all ultimately derived from Biblical Hebrew Ivri (עברי), one of several names for the Israelite (Jewish and Samaritan) people (Hebrews). It is traditionally understood to be an adjective based on the name of Abraham's ancestor, Eber, mentioned in Genesis 10:21. The name is believed to be based on the Semitic root ʕ-b-r (עבר) meaning "beyond", "other side", "across";[16] interpretations of the term "Hebrew" generally render its meaning as roughly "from the other side [of the river/desert]"—i.e., an exonym for the inhabitants of the land of Israel/Judah, perhaps from the perspective of Mesopotamia, Phoenicia or the Transjordan (with the river referenced perhaps the Euphrates, Jordan or Litani; or maybe the northern Arabian Desert between Babylonia and Canaan).[17] Compare the word Habiru or cognate Assyrian ebru, of identical meaning.[18]
+
+One of the earliest references to the language's name as "Ivrit" is found in the prologue to the Book of Ben Sira,[a] from the 2nd century BCE.[19] The Hebrew Bible does not use the term "Hebrew" in reference to the language of the Hebrew people;[20] its later historiography, in the Book of Kings, refers to it as ‏יְהוּדִית‎ Yehudit 'Judahite (language)'.[21]
+
+Hebrew belongs to the Canaanite group of languages. Canaanite languages are a branch of the Northwest Semitic family of languages.[22]
+
+According to Avraham Ben-Yosef, Hebrew flourished as a spoken language in the Kingdoms of Israel and Judah during the period from about 1200 to 586 BCE.[23] Scholars debate the degree to which Hebrew was a spoken vernacular in ancient times following the Babylonian exile, when the predominant international language in the region was Old Aramaic.
+
+Hebrew was extinct as a colloquial language by Late Antiquity, but it continued to be used as a literary language and as the liturgical language of Judaism, evolving various dialects of literary Medieval Hebrew, until its revival as a spoken language in the late 19th century.[24][25]
+
+In July 2008, Israeli archaeologist Yossi Garfinkel discovered a ceramic shard at Khirbet Qeiyafa that he claimed may be the earliest Hebrew writing yet discovered, dating from around 3,000 years ago.[26] Hebrew University archaeologist Amihai Mazar said that the inscription was "proto-Canaanite" but cautioned that "The differentiation between the scripts, and between the languages themselves in that period, remains unclear," and suggested that calling the text Hebrew might be going too far.[27]
+
+The Gezer calendar also dates back to the 10th century BCE at the beginning of the Monarchic Period, the traditional time of the reign of David and Solomon. Classified as Archaic Biblical Hebrew, the calendar presents a list of seasons and related agricultural activities. The Gezer calendar (named after the city in whose proximity it was found) is written in an old Semitic script, akin to the Phoenician one that, through the Greeks and Etruscans, later became the Roman script. The Gezer calendar is written without any vowels, and it does not use consonants to imply vowels even in the places in which later Hebrew spelling requires them.
+
+Numerous older tablets have been found in the region with similar scripts written in other Semitic languages, for example, Protosinaitic. It is believed that the original shapes of the script go back to Egyptian hieroglyphs, though the phonetic values are instead inspired by the acrophonic principle. The common ancestor of Hebrew and Phoenician is called Canaanite, and was the first to use a Semitic alphabet distinct from that of Egyptian. One ancient document is the famous Moabite Stone, written in the Moabite dialect; the Siloam Inscription, found near Jerusalem, is an early example of Hebrew. Less ancient samples of Archaic Hebrew include the ostraca found near Lachish, which describe events preceding the final capture of Jerusalem by Nebuchadnezzar and the Babylonian captivity of 586 BCE.
+
+In its widest sense, Biblical Hebrew refers to the spoken language of ancient Israel flourishing between the 10th century BCE and the turn of the 4th century CE.[28] It comprises several evolving and overlapping dialects. The phases of Classical Hebrew are often named after important literary works associated with them.
+
+Sometimes the above phases of spoken Classical Hebrew are simplified into "Biblical Hebrew" (including several dialects from the 10th century BCE to 2nd century BCE and extant in certain Dead Sea Scrolls) and "Mishnaic Hebrew" (including several dialects from the 3rd century BCE to the 3rd century CE and extant in certain other Dead Sea Scrolls).[29] However, today most Hebrew linguists classify Dead Sea Scroll Hebrew as a set of dialects evolving out of Late Biblical Hebrew and into Mishnaic Hebrew, thus including elements from both but remaining distinct from either.[30]
+
+By the start of the Byzantine Period in the 4th century CE, Classical Hebrew ceased as a regularly spoken language, roughly a century after the publication of the Mishnah, apparently declining since the aftermath of the catastrophic Bar Kokhba War around 135 CE.
+
+In the early 6th century BCE, the Neo-Babylonian Empire conquered the ancient Kingdom of Judah, destroying much of Jerusalem and exiling its population far to the East in Babylon. During the Babylonian captivity, many Israelites learned Aramaic, the closely related Semitic language of their captors. Thus for a significant period, the Jewish elite became influenced by Aramaic.[31]
+
+After Cyrus the Great conquered Babylon, he allowed the Jewish people to return from captivity. As a result,[improper synthesis?] a local version of Aramaic came to be spoken in Israel alongside Hebrew. By the beginning of the Common Era, Aramaic was the primary colloquial language of Samarian, Babylonian and Galileean Jews, and western and intellectual Jews spoke Greek,[citation needed] but a form of so-called Rabbinic Hebrew continued to be used as a vernacular in Judea until it was displaced by Aramaic, probably in the 3rd century CE. Certain Sadducee, Pharisee, Scribe, Hermit, Zealot and Priest classes maintained an insistence on Hebrew, and all Jews maintained their identity with Hebrew songs and simple quotations from Hebrew texts.[13][32][33]
+
+While there is no doubt that at a certain point, Hebrew was displaced as the everyday spoken language of most Jews, and that its chief successor in the Middle East was the closely related Aramaic language, then Greek,[32][note 2] scholarly opinions on the exact dating of that shift have changed very much.[12] In the first half of the 20th century, most scholars followed Geiger and Dalman in thinking that Aramaic became a spoken language in the land of Israel as early as the beginning of Israel's Hellenistic Period in the 4th century BCE, and that as a corollary Hebrew ceased to function as a spoken language around the same time. Segal, Klausner and Ben Yehuda are notable exceptions to this view. During the latter half of the 20th century, accumulating archaeological evidence and especially linguistic analysis of the Dead Sea Scrolls has disproven that view. The Dead Sea Scrolls, uncovered in 1946–1948 near Qumran revealed ancient Jewish texts overwhelmingly in Hebrew, not Aramaic.
+
+The Qumran scrolls indicate that Hebrew texts were readily understandable to the average Israelite, and that the language had evolved since Biblical times as spoken languages do.[note 3] Recent scholarship recognizes that reports of Jews speaking in Aramaic indicate a multilingual society, not necessarily the primary language spoken. Alongside Aramaic, Hebrew co-existed within Israel as a spoken language.[35] Most scholars now date the demise of Hebrew as a spoken language to the end of the Roman Period, or about 200 CE.[36] It continued on as a literary language down through the Byzantine Period from the 4th century CE.
+
+The exact roles of Aramaic and Hebrew remain hotly debated. A trilingual scenario has been proposed for the land of Israel. Hebrew functioned as the local mother tongue with powerful ties to Israel's history, origins and golden age and as the language of Israel's religion; Aramaic functioned as the international language with the rest of the Middle East; and eventually Greek functioned as another international language with the eastern areas of the Roman Empire.[citation needed] William Schniedewind argues that after waning in the Persian Period, the religious importance of Hebrew grew in the Hellenistic and Roman periods, and cites epigraphical evidence that Hebrew survived as a vernacular language — though both its grammar and its writing system had been substantially influenced by Aramaic.[37] According to another summary, Greek was the language of government, Hebrew the language of prayer, study and religious texts, and Aramaic was the language of legal contracts and trade.[38] There was also a geographic pattern: according to Spolsky, by the beginning of the Common Era, "Judeo-Aramaic was mainly used in Galilee in the north, Greek was concentrated in the former colonies and around governmental centers, and Hebrew monolingualism continued mainly in the southern villages of Judea."[32] In other words, "in terms of dialect geography, at the time of the tannaim Palestine could be divided into the Aramaic-speaking regions of Galilee and Samaria and a smaller area, Judaea, in which Rabbinic Hebrew was used among the descendants of returning exiles."[13][33] In addition, it has been surmised that Koine Greek was the primary vehicle of communication in coastal cities and among the upper class of Jerusalem, while Aramaic was prevalent in the lower class of Jerusalem, but not in the surrounding countryside.[38] After the suppression of the Bar Kokhba revolt in the 2nd century CE, Judaeans were forced to disperse. Many relocated to Galilee, so most remaining native speakers of Hebrew at that last stage would have been found in the north.[39]
+
+The Christian New Testament contains some Semitic place names and quotes.[40] The language of such Semitic glosses (and in general the language spoken by Jews in scenes from the New Testament) is often referred to as "Hebrew" in the text,[41] although this term is often re-interpreted as referring to Aramaic instead[note 4][note 5] and is rendered accordingly in recent translations.[43] Nonetheless, these glosses can be interpreted as Hebrew as well.[44] It has been argued that Hebrew, rather than Aramaic or Koine Greek, lay behind the composition of the Gospel of Matthew.[45] (See the Hebrew Gospel hypothesis or Language of Jesus for more details on Hebrew and Aramaic in the gospels.)
+
+The term "Mishnaic Hebrew" generally refers to the Hebrew dialects found in the Talmud, excepting quotations from the Hebrew Bible. The dialects organize into Mishnaic Hebrew (also called Tannaitic Hebrew, Early Rabbinic Hebrew, or Mishnaic Hebrew I), which was a spoken language, and Amoraic Hebrew (also called Late Rabbinic Hebrew or Mishnaic Hebrew II), which was a literary language. The earlier section of the Talmud is the Mishnah that was published around 200 CE, although many of the stories take place much earlier, and was written in the earlier Mishnaic dialect. The dialect is also found in certain Dead Sea Scrolls. Mishnaic Hebrew is considered to be one of the dialects of Classical Hebrew that functioned as a living language in the land of Israel. A transitional form of the language occurs in the other works of Tannaitic literature dating from the century beginning with the completion of the Mishnah. These include the halachic Midrashim (Sifra, Sifre, Mechilta etc.) and the expanded collection of Mishnah-related material known as the Tosefta. The Talmud contains excerpts from these works, as well as further Tannaitic material not attested elsewhere; the generic term for these passages is Baraitot. The dialect of all these works is very similar to Mishnaic Hebrew.
+
+About a century after the publication of the Mishnah, Mishnaic Hebrew fell into disuse as a spoken language. The later section of the Talmud, the Gemara, generally comments on the Mishnah and Baraitot in two forms of Aramaic. Nevertheless, Hebrew survived as a liturgical and literary language in the form of later Amoraic Hebrew, which sometimes occurs in the text of the Gemara.
+
+Hebrew was always regarded as the language of Israel's religion, history and national pride, and after it faded as a spoken language, it continued to be used as a lingua franca among scholars and Jews traveling in foreign countries.[46] After the 2nd century CE when the Roman Empire exiled most of the Jewish population of Jerusalem following the Bar Kokhba revolt, they adapted to the societies in which they found themselves, yet letters, contracts, commerce, science, philosophy, medicine, poetry and laws continued to be written mostly in Hebrew, which adapted by borrowing and inventing terms.
+
+After the Talmud, various regional literary dialects of Medieval Hebrew evolved. The most important is Tiberian Hebrew or Masoretic Hebrew, a local dialect of Tiberias in Galilee that became the standard for vocalizing the Hebrew Bible and thus still influences all other regional dialects of Hebrew. This Tiberian Hebrew from the 7th to 10th century CE is sometimes called "Biblical Hebrew" because it is used to pronounce the Hebrew Bible; however, properly it should be distinguished from the historical Biblical Hebrew of the 6th century BCE, whose original pronunciation must be reconstructed. Tiberian Hebrew incorporates the remarkable scholarship of the Masoretes (from masoret meaning "tradition"), who added vowel points and grammar points to the Hebrew letters to preserve much earlier features of Hebrew, for use in chanting the Hebrew Bible. The Masoretes inherited a biblical text whose letters were considered too sacred to be altered, so their markings were in the form of pointing in and around the letters. The Syriac alphabet, precursor to the Arabic alphabet, also developed vowel pointing systems around this time. The Aleppo Codex, a Hebrew Bible with the Masoretic pointing, was written in the 10th century, likely in Tiberias, and survives to this day. It is perhaps the most important Hebrew manuscript in existence.
+
+During the Golden age of Jewish culture in Spain, important work was done by grammarians in explaining the grammar and vocabulary of Biblical Hebrew; much of this was based on the work of the grammarians of Classical Arabic. Important Hebrew grammarians were Judah ben David Hayyuj, Jonah ibn Janah, Abraham ibn Ezra[47] and later (in Provence), David Kimhi. A great deal of poetry was written, by poets such as Dunash ben Labrat, Solomon ibn Gabirol, Judah ha-Levi, Moses ibn Ezra and Abraham ibn Ezra, in a "purified" Hebrew based on the work of these grammarians, and in Arabic quantitative or strophic meters. This literary Hebrew was later used by Italian Jewish poets.[48]
+
+The need to express scientific and philosophical concepts from Classical Greek and Medieval Arabic motivated Medieval Hebrew to borrow terminology and grammar from these other languages, or to coin equivalent terms from existing Hebrew roots, giving rise to a distinct style of philosophical Hebrew. This is used in the translations made by the Ibn Tibbon family. (Original Jewish philosophical works were usually written in Arabic.[citation needed]) Another important influence was Maimonides, who developed a simple style based on Mishnaic Hebrew for use in his law code, the Mishneh Torah.  Subsequent rabbinic literature is written in a blend between this style and the Aramaized Rabbinic Hebrew of the Talmud.
+
+Hebrew persevered through the ages as the main language for written purposes by all Jewish communities around the world for a large range of uses—not only liturgy, but also poetry, philosophy, science and medicine, commerce, daily correspondence and contracts. There have been many deviations from this generalization such as Bar Kokhba's letters to his lieutenants, which were mostly in Aramaic,[49] and Maimonides' writings, which were mostly in Arabic;[50] but overall, Hebrew did not cease to be used for such purposes. For example, the first Middle East printing press, in Safed (modern Israel), produced a small number of books in Hebrew in 1577, which were then sold to the nearby Jewish world.[51] This meant not only that well-educated Jews in all parts of the world could correspond in a mutually intelligible language, and that books and legal documents published or written in any part of the world could be read by Jews in all other parts, but that an educated Jew could travel and converse with Jews in distant places, just as priests and other educated Christians could converse in Latin. For example, Rabbi Avraham Danzig wrote the Chayei Adam in Hebrew, as opposed to Yiddish, as a guide to Halacha for the "average 17-year-old" (Ibid. Introduction 1). Similarly, the Chofetz Chaim, Rabbi Yisrael Meir Kagan's purpose in writing the Mishna Berurah was to "produce a work that could be studied daily so that Jews might know the proper procedures to follow minute by minute".  The work was nevertheless written in Talmudic Hebrew and Aramaic, since, "the ordinary Jew [of Eastern Europe] of a century ago, was fluent enough in this idiom to be able to follow the Mishna Berurah without any trouble."[52]
+
+Hebrew has been revived several times as a literary language, most significantly by the Haskalah (Enlightenment) movement of early and mid-19th-century Germany. In the early 19th century, a form of spoken Hebrew had emerged in the markets of Jerusalem between Jews of different linguistic backgrounds to communicate for commercial purposes. This Hebrew dialect was to a certain extent a pidgin.[53] Near the end of that century the Jewish activist Eliezer Ben-Yehuda, owing to the ideology of the national revival (שיבת ציון, Shivat Tziyon, later Zionism), began reviving Hebrew as a modern spoken language. Eventually, as a result of the local movement he created, but more significantly as a result of the new groups of immigrants known under the name of the Second Aliyah, it replaced a score of languages spoken by Jews at that time. Those languages were Jewish dialects of local languages, including Judaeo-Spanish (also called "Judezmo" and "Ladino"), Yiddish, Judeo-Arabic and Bukhori (Tajiki), or local languages spoken in the Jewish diaspora such as Russian, Persian and Arabic.
+
+The major result of the literary work of the Hebrew intellectuals along the 19th century was a lexical modernization of Hebrew. New words and expressions were adapted as neologisms from the large corpus of Hebrew writings since the Hebrew Bible, or borrowed from Arabic (mainly by Eliezer Ben-Yehuda) and older Aramaic and Latin. Many new words were either borrowed from or coined after European languages, especially English, Russian, German, and French. Modern Hebrew became an official language in British-ruled Palestine in 1921 (along with English and Arabic), and then in 1948 became an official language of the newly declared State of Israel. Hebrew is the most widely spoken language in Israel today.
+
+In the Modern Period, from the 19th century onward, the literary Hebrew tradition revived as the spoken language of modern Israel, called variously Israeli Hebrew, Modern Israeli Hebrew, Modern Hebrew, New Hebrew, Israeli Standard Hebrew, Standard Hebrew and so on. Israeli Hebrew exhibits some features of Sephardic Hebrew from its local Jerusalemite tradition but adapts it with numerous neologisms, borrowed terms (often technical) from European languages and adopted terms (often colloquial) from Arabic.
+
+The literary and narrative use of Hebrew was revived beginning with the Haskalah movement. The first secular periodical in Hebrew, HaMe'assef (The Gatherer), was published by maskilim in Königsberg (today's Kaliningrad) from 1783 onwards.[54] In the mid-19th century, publications of several Eastern European Hebrew-language newspapers (e.g. Hamagid, founded in Ełk in 1856) multiplied. Prominent poets were Hayim Nahman Bialik and Shaul Tchernichovsky; there were also novels written in the language.
+
+The revival of the Hebrew language as a mother tongue was initiated in the late 19th century by the efforts of Eliezer Ben-Yehuda. He joined the Jewish national movement and in 1881 immigrated to Palestine, then a part of the Ottoman Empire. Motivated by the surrounding ideals of renovation and rejection of the diaspora "shtetl" lifestyle, Ben-Yehuda set out to develop tools for making the literary and liturgical language into everyday spoken language. However, his brand of Hebrew followed norms that had been replaced in Eastern Europe by different grammar and style, in the writings of people like Ahad Ha'am and others. His organizational efforts and involvement with the establishment of schools and the writing of textbooks pushed the vernacularization activity into a gradually accepted movement. It was not, however, until the 1904–1914 Second Aliyah that Hebrew had caught real momentum in Ottoman Palestine with the more highly organized enterprises set forth by the new group of immigrants. When the British Mandate of Palestine recognized Hebrew as one of the country's three official languages (English, Arabic, and Hebrew, in 1922), its new formal status contributed to its diffusion. A constructed modern language with a truly Semitic vocabulary and written appearance, although often European in phonology, was to take its place among the current languages of the nations.
+
+While many saw his work as fanciful or even blasphemous[55] (because Hebrew was the holy language of the Torah and therefore some thought that it should not be used to discuss everyday matters), many soon understood the need for a common language amongst Jews of the British Mandate who at the turn of the 20th century were arriving in large numbers from diverse countries and speaking different languages. A Committee of the Hebrew Language was established. After the establishment of Israel, it became the Academy of the Hebrew Language. The results of Ben-Yehuda's lexicographical work were published in a dictionary (The Complete Dictionary of Ancient and Modern Hebrew). The seeds of Ben-Yehuda's work fell on fertile ground, and by the beginning of the 20th century, Hebrew was well on its way to becoming the main language of the Jewish population of both Ottoman and British Palestine. At the time, members of the Old Yishuv and a very few Hasidic sects, most notably those under the auspices of Satmar, refused to speak Hebrew and spoke only Yiddish.
+
+In the Soviet Union, the use of Hebrew, along with other Jewish cultural and religious activities, was suppressed. Soviet authorities considered the use of Hebrew "reactionary" since it was associated with Zionism, and the teaching of Hebrew at primary and secondary schools was officially banned by the People's Commissariat for Education as early as 1919, as part of an overall agenda aiming to secularize education (the language itself did not cease to be studied at universities for historical and linguistic purposes[56]). The official ordinance stated that Yiddish, being the spoken language of the Russian Jews, should be treated as their only national language, while Hebrew was to be treated as a foreign language.[57] Hebrew books and periodicals ceased to be published and were seized from the libraries, although liturgical texts were still published until the 1930s. Despite numerous protests,[58] a policy of suppression of the teaching of Hebrew operated from the 1930s on. Later in the 1980s in the USSR, Hebrew studies reappeared due to people struggling for permission to go to Israel (refuseniks). Several of the teachers were imprisoned, e.g. Yosef Begun, Ephraim Kholmyansky, Yevgeny Korostyshevsky and others responsible for a Hebrew learning network connecting many cities of the USSR.
+
+Standard Hebrew, as developed by Eliezer Ben-Yehuda, was based on Mishnaic spelling and Sephardi Hebrew pronunciation. However, the earliest speakers of Modern Hebrew had Yiddish as their native language and often introduced calques from Yiddish and phono-semantic matchings of international words.
+
+Despite using Sephardic Hebrew pronunciation as its primary basis, modern Israeli Hebrew has adapted to Ashkenazi Hebrew phonology in some respects, mainly the following:
+
+The vocabulary of Israeli Hebrew is much larger than that of earlier periods. According to Ghil'ad Zuckermann:
+
+The number of attested Biblical Hebrew words is 8198, of which some 2000 are hapax legomena (the number of Biblical Hebrew roots, on which many of these words are based, is 2099). The number of attested Rabbinic Hebrew words is less than 20,000, of which (i) 7879 are Rabbinic par excellence, i.e. they did not appear in the Old Testament (the number of new Rabbinic Hebrew roots is 805); (ii) around 6000 are a subset of Biblical Hebrew; and (iii) several thousand are Aramaic words which can have a Hebrew form. Medieval Hebrew added 6421 words to (Modern) Hebrew. The approximate number of new lexical items in Israeli is 17,000 (cf. 14,762 in Even-Shoshan 1970 [...]). With the inclusion of foreign and technical terms [...], the total number of Israeli words, including words of biblical, rabbinic and medieval descent, is more than 60,000.[61]:64–65
+
+In Israel, Modern Hebrew is currently taught in institutions called Ulpanim (singular: Ulpan). There are government-owned, as well as private, Ulpanim offering online courses and face-to-face programs.
+
+Modern Hebrew is the primary official language of the State of Israel. As of 2013[update], there are about 9 million Hebrew speakers worldwide,[62] of whom 7 million speak it fluently.[63][64][65]
+
+Currently, 90% of Israeli Jews are proficient in Hebrew, and 70% are highly proficient.[66] Some 60% of Israeli Arabs are also proficient in Hebrew,[66] and 30% report having a higher proficiency in Hebrew than in Arabic.[8] In total, about 53% of the Israeli population speaks Hebrew as a native language,[67] while most of the rest speak it fluently. However, in 2013 Hebrew was the native language of only 49% of Israelis over the age of 20, with Russian, Arabic, French, English, Yiddish and Ladino being the native tongues of most of the rest. Some 26% of immigrants from the former Soviet Union and 12% of Arabs reported speaking Hebrew poorly or not at all.[66][68]
+
+Steps have been taken to keep Hebrew the primary language of use, and to prevent large-scale incorporation of English words into the Hebrew vocabulary. The Academy of the Hebrew Language of the Hebrew University of Jerusalem currently invents about 2,000 new Hebrew words each year for modern words by finding an original Hebrew word that captures the meaning, as an alternative to incorporating more English words into Hebrew vocabulary. The Haifa municipality has banned officials from using English words in official documents, and is fighting to stop businesses from using only English signs to market their services.[69] In 2012, a Knesset bill for the preservation of the Hebrew language was proposed, which includes the stipulation that all signage in Israel must first and foremost be in Hebrew, as with all speeches by Israeli officials abroad. The bill's author, MK Akram Hasson, stated that the bill was proposed as a response to Hebrew "losing its prestige" and children incorporating more English words into their vocabulary.[70]
+
+Hebrew is also an official national minority language in Poland, since 6 January 2005.[71]
+
+Biblical Hebrew had a typical Semitic consonant inventory, with pharyngeal /ʕ ħ/, a series of "emphatic" consonants (possibly ejective, but this is debated), lateral fricative /ɬ/, and in its older stages also uvular /χ ʁ/. /χ ʁ/ merged into /ħ ʕ/ in later Biblical Hebrew, and /b ɡ d k p t/ underwent allophonic spirantization to [v ɣ ð x f θ] (known as begadkefat). The earliest Biblical Hebrew vowel system contained the Proto-Semitic vowels /a aː i iː u uː/ as well as /oː/, but this system changed dramatically over time.
+
+By the time of the Dead Sea Scrolls, /ɬ/ had shifted to /s/ in the Jewish traditions, though for the Samaritans it merged with /ʃ/ instead.[30] The Tiberian reading tradition of the Middle Ages had the vowel system /a ɛ e i ɔ o u ă ɔ̆ ɛ̆/, though other Medieval reading traditions had fewer vowels.
+
+A number of reading traditions have been preserved in liturgical use. In Oriental (Sephardi and Mizrahi) Jewish reading traditions, the emphatic consonants are realized as pharyngealized, while the Ashkenazi (northern and eastern European) traditions have lost emphatics and pharyngeals (although according to Ashkenazi law, pharyngeal articulation is preferred over uvular or glottal articulation when representing the community in religious service such as prayer and Torah reading), and show the shift of /w/ to /v/. The Samaritan tradition has a complex vowel system that does not correspond closely to the Tiberian systems.
+
+Modern Hebrew pronunciation developed from a mixture of the different Jewish reading traditions, generally tending towards simplification. In line with Sephardi Hebrew pronunciation, emphatic consonants have shifted to their ordinary counterparts, /w/ to /v/, and [ɣ ð θ] are not present. Most Israelis today also merge /ʕ ħ/ with /ʔ χ/, do not have contrastive gemination, and pronounce /r/ as a uvular fricative [ʁ] or a voiced velar fricative [ɣ] rather than an alveolar trill, because of Ashkenazi Hebrew influences. The consonants /tʃ/ and /dʒ/ have become phonemic due to loan words, and /w/ has similarly been re-introduced.
+
+Notes:
+
+Hebrew grammar is partly analytic, expressing such forms as dative, ablative and accusative using prepositional particles rather than grammatical cases. However, inflection plays a decisive role in the formation of the verbs and nouns. For example, nouns have a construct state, called "smikhut", to denote the relationship of "belonging to": this is the converse of the genitive case of more inflected languages. Words in smikhut are often combined with hyphens.  In modern speech, the use of the construct is sometimes interchangeable with the preposition "shel", meaning "of".  There are many cases, however, where older declined forms are retained (especially in idiomatic expressions and the like), and "person"-enclitics are widely used to "decline" prepositions.
+
+Like all Semitic languages, the Hebrew language exhibits a pattern of stems consisting typically of "triliteral", or 3-consonant consonantal roots, from which nouns, adjectives, and verbs are formed in various ways: e.g. by inserting vowels, doubling consonants, lengthening vowels and/or adding prefixes, suffixes or infixes. 4-consonant roots also exist and became more frequent in the modern language due to a process of coining verbs from nouns that are themselves constructed from 3-consonant verbs. Some triliteral roots lose one of their consonants in most forms and are called "Nehim" (Resting).
+
+Hebrew uses a number of one-letter prefixes that are added to words for various purposes. These are called inseparable prepositions or "Letters of Use" (Hebrew: אותיות השימוש‎, romanized: Otiyot HaShimush). Such items include: the definite article ha- (/ha/) (="the"); prepositions be- (/bə/) (="in"), le- (/lə/) (="to"; a shortened version of the preposition el), mi- (/mi/) (="from"; a shortened version of the preposition min); conjunctions ve- (/və/) (="and"), she- (/ʃe/) (="that"; a shortened version of the Biblical conjunction asher), ke- (/kə/) (="as", "like"; a shortened version of the conjunction kmo).
+
+The vowel accompanying each of these letters may differ from those listed above, depending on the first letter or vowel following it. The rules governing these changes, hardly observed in colloquial speech as most speakers tend to employ the regular form, may be heard in more formal circumstances. For example, if a preposition is put before a word that begins with a moving Shva, then the preposition takes the vowel /i/ (and the initial consonant may be weakened): colloquial be-kfar (="in a village") corresponds to the more formal bi-khfar.
+
+The definite article may be inserted between a preposition or a conjunction and the word it refers to, creating composite words like mé-ha-kfar (="from the village"). The latter also demonstrates the change in the vowel of mi-. With be,  le and ke, the definite article is assimilated into the prefix, which then becomes ba, la or ka. Thus *be-ha-matos becomes ba-matos (="in the plane"). Note that this does not happen to mé (the form of "min" or "mi-" used before the letter "he"), therefore mé-ha-matos is a valid form, which means "from the airplane".
+
+Like most other languages, the vocabulary of the Hebrew language is divided into verbs, nouns, adjectives and so on, and its sentence structure can be analyzed by terms like object, subject and so on.
+
+Modern Hebrew is written from right to left using the Hebrew alphabet, which is an "impure" abjad, or consonant-only script, of 22 letters. The ancient paleo-Hebrew alphabet is similar to those used for Canaanite and Phoenician.[citation needed]  Modern scripts are based on the "square" letter form, known as Ashurit (Assyrian), which was developed from the Aramaic script. A cursive Hebrew script is used in handwriting: the letters tend to be more circular in form when written in cursive, and sometimes vary markedly from their printed equivalents.  The medieval version of the cursive script forms the basis of another style, known as Rashi script.  When necessary, vowels are indicated by diacritic marks above or below the letter representing the syllabic onset, or by use of matres lectionis, which are consonantal letters used as vowels.  Further diacritics are used to indicate variations in the pronunciation of the consonants (e.g. bet/vet, shin/sin); and, in some contexts, to indicate the punctuation, accentuation and musical rendition of Biblical texts (see Cantillation).
+
+Hebrew has always been used as the language of prayer and study, and the following pronunciation systems are found.
+
+Ashkenazi Hebrew, originating in Central and Eastern Europe, is still widely used in Ashkenazi Jewish religious services and studies in Israel and abroad, particularly in the Haredi and other Orthodox communities. It was influenced by the Yiddish language.
+
+Sephardi Hebrew is the traditional pronunciation of the Spanish and Portuguese Jews and Sephardi Jews in the countries of the former Ottoman Empire, with the exception of Yemenite Hebrew.  This pronunciation, in the form used by the Jerusalem Sephardic community, is the basis of the Hebrew phonology of Israeli native speakers. It was influenced by the Judezmo language.
+
+Mizrahi (Oriental) Hebrew is actually a collection of dialects spoken liturgically by Jews in various parts of the Arab and Islamic world. It was derived from the old Arabic language, and in some cases influenced by Sephardi Hebrew. The same claim is sometimes made for Yemenite Hebrew or Temanit, which differs from other Mizrahi dialects by having a radically different vowel system, and distinguishing between different diacritically marked consonants that are pronounced identically in other dialects (for example gimel and "ghimel".)
+
+These pronunciations are still used in synagogue ritual and religious study, in Israel and elsewhere, mostly by people who are not native speakers of Hebrew, though some traditionalist Israelis use liturgical pronunciations in prayer.
+
+Many synagogues in the diaspora, even though Ashkenazi by rite and by ethnic composition, have adopted the "Sephardic" pronunciation in deference to Israeli Hebrew. However, in many British and American schools and synagogues, this pronunciation retains several elements of its Ashkenazi substrate, especially the distinction between tsere and segol.

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+
+A pet, or companion animal, is an animal kept primarily for a person's company or entertainment rather than as a working animal, livestock or a laboratory animal. Popular pets are often considered to have attractive appearances, intelligence and relatable personalities, but some pets may be taken in on an altruistic basis (such as a stray animal) and accepted by the owner regardless of these characteristics.
+
+Two of the most popular pets are dogs and cats; the technical term for a cat lover is an ailurophile and a dog lover a cynophile. Other animals commonly kept include: rabbits; ferrets; pigs; rodents, such as gerbils, hamsters, chinchillas, rats, mice, and guinea pigs; avian pets, such as parrots, passerines and fowls; reptile pets, such as turtles, alligators, crocodiles, lizards, and snakes; aquatic pets, such as fish, freshwater and saltwater snails, amphibians like frogs and salamanders; and arthropod pets, such as tarantulas and hermit crabs. Small pets may be grouped together as pocket pets, while the equine and bovine group include the largest companion animals.
+
+Pets provide their owners (or "guardians")[1] both physical and emotional benefits. Walking a dog can provide both the human and the dog with exercise, fresh air and social interaction. Pets can give companionship to people who are living alone or elderly adults who do not have adequate social interaction with other people. There is a medically approved class of therapy animals, mostly dogs or cats, that are brought to visit confined humans, such as children in hospitals or elders in nursing homes. Pet therapy utilizes trained animals and handlers to achieve specific physical, social, cognitive or emotional goals with patients.
+
+People most commonly get pets for companionship, to protect a home or property or because of the perceived beauty or attractiveness of the animals.[2] A 1994 Canadian study found that the most common reasons for not owning a pet were lack of ability to care for the pet when traveling (34.6%), lack of time (28.6%) and lack of suitable housing (28.3%), with dislike of pets being less common (19.6%).[2] Some scholars, ethicists and animal rights organizations have raised concerns over keeping pets because of the lack of autonomy and the objectification of non-human animals.[3]
+
+In China, spending on domestic animals has grown from an estimated $3.12 billion in 2010 to $25 billion in 2018. The Chinese people own 51 million dogs and 41 million cats, with pet owners often preferring to source pet food internationally.[4] There are a total of 755 million pets, increased from 389 million in 2013.[5]
+
+According to a survey promoted by Italian family associations in 2009, it is estimated that there are approximately 45 million pets in Italy. This includes 7 million dogs, 7.5 million cats, 16 million fish, 12 million birds, and 10 million snakes.[6]
+
+A 2007 survey by the University of Bristol found that 26% of UK households owned cats and 31% owned dogs, estimating total domestic populations of approximately 10.3 million cats and 10.5 million dogs in 2006.[7] The survey also found that 47.2% of households with a cat had at least one person educated to degree level, compared with 38.4% of homes with dogs.[8]
+
+Sixty-eight percent of U.S. households, or about 85 million families, own a pet, according to the 2017-2018 National Pet Owners Survey conducted by the American Pet Products Association (APPA). This is up from 56 percent of U.S. households in 1988, the first year the survey was conducted.[9]There are approximately 86.4 million pet cats and approximately 78.2 million pet dogs in the United States,[10][11] and a United States 2007–2008 survey showed that dog-owning households outnumbered those owning cats, but that the total number of pet cats was higher than that of dogs. The same was true for 2011.[12] In 2013, pets outnumbered children four to one in the United States.[13]
+
+Keeping animals as pets may be detrimental to their health if certain requirements are not met. An important issue is inappropriate feeding, which may produce clinical effects. The consumption of chocolate or grapes by dogs, for example, may prove fatal.
+
+Certain species of houseplants can also prove toxic if consumed by pets. Examples include philodendrons and Easter lilies (which can cause severe kidney damage to cats)[16][17] and poinsettias, begonia, and aloe vera (which are mildly toxic to dogs).[18][19]
+
+Housepets, particularly dogs and cats in industrialized societies, are also highly susceptible to obesity. Overweight pets have been shown to be at a higher risk of developing diabetes, liver problems, joint pain, kidney failure, and cancer. Lack of exercise and high-caloric diets are considered to be the primary contributors to pet obesity.[20][21][22]
+
+It is widely believed among the public, and among many scientists, that pets probably bring mental and physical health benefits to their owners;[23] a 1987 NIH statement cautiously argued that existing data was "suggestive" of a significant benefit.[24] A recent dissent comes from a 2017 RAND study, which found that at least in the case of children, having a pet per se failed to improve physical or mental health by a statistically significant amount; instead, the study found children who were already prone to being healthy were more likely to get pets in the first place.[23][25][26] Unfortunately, conducting long-term randomized trials to settle the issue would be costly or infeasible.[24][26]
+
+Pets might have the ability to stimulate their caregivers, in particular the elderly, giving people someone to take care of, someone to exercise with, and someone to help them heal from a physically or psychologically troubled past.[24][27][28] Animal company can also help people to preserve acceptable levels of happiness despite the presence of mood symptoms like anxiety or depression.[29] Having a pet may also help people achieve health goals, such as lowered blood pressure, or mental goals, such as decreased stress.[30][31][32][33][34][35] There is evidence that having a pet can help a person lead a longer, healthier life. In a 1986 study of 92 people hospitalized for coronary ailments, within a year, 11 of the 29 patients without pets had died, compared to only 3 of the 52 patients who had pets.[28] Having pet(s) was shown to significantly reduce triglycerides, and thus heart disease risk, in the elderly.[36] A study by the National Institute of Health found that people who owned dogs were less likely to die as a result of a heart attack than those who did not own one.[37] There is some evidence that pets may have a therapeutic effect in dementia cases.[38] Other studies have shown that for the elderly, good health may be a requirement for having a pet, and not a result.[39] Dogs trained to be guide dogs can help people with vision impairment. Dogs trained in the field of Animal-Assisted Therapy (AAT) can also benefit people with other disabilities.[24][40]
+
+People residing in a long-term care facility, such as a hospice or nursing home, may experience health benefits from pets. Pets help them to cope with the emotional issues related to their illness. They also offer physical contact with another living creature, something that is often missing in an elder's life.[10][41] Pets for nursing homes are chosen based on the size of the pet, the amount of care that the breed needs, and the population and size of the care institution.[28] Appropriate pets go through a screening process and, if it is a dog, additional training programs to become a therapy dog.[42] There are three types of therapy dogs: facility therapy dogs, animal-assisted therapy dogs, and therapeutic visitation dogs. The most common therapy dogs are therapeutic visitation dogs. These dogs are household pets whose handlers take time to visit hospitals, nursing homes, detention facilities, and rehabilitation facilities.[27] Different pets require varying amounts of attention and care; for example, cats may have lower maintenance requirements than dogs.[43]
+
+In addition to providing health benefits for their owners, pets also impact the social lives of their owners and their connection to their community. There is some evidence that pets can facilitate social interaction.[44] Assistant Professor of Sociology at the University of Colorado at Boulder, Leslie Irvine has focused her attention on pets of the homeless population. Her studies of pet ownership among the homeless found that many modify their life activities for fear of losing their pets. Pet ownership prompts them to act responsibly, with many making a deliberate choice not to drink or use drugs, and to avoid contact with substance abusers or those involved in any criminal activity for fear of being separated from their pet. Additionally, many refuse to house in shelters if their pet is not allowed to stay with them.[45]
+
+Health risks that are associated with pets include:
+
+The European Convention for the Protection of Pet Animals is a 1987 treaty of the Council of Europe – but accession to the treaty is open to all states in the world – to promote the welfare of pet animals and ensure minimum standards for their treatment and protection. It went into effect on 1 May 1992, and as of June 2020, it has been ratified by 24 states.[47]
+
+States, cities, and towns in Western nations commonly enact local ordinances to limit the number or kind of pets a person may keep personally or for business purposes. Prohibited pets may be specific to certain breeds (such as pit bulls or Rottweilers), they may apply to general categories of animals (such as livestock, exotic animals, wild animals, and canid or felid hybrids), or they may simply be based on the animal's size. Additional or different maintenance rules and regulations may also apply. Condominium associations and owners of rental properties also commonly limit or forbid tenants' keeping of pets.[citation needed]
+
+The keeping of animals as pets can cause concerns with regard to animal rights and welfare.[48][49][50] Pets have commonly been considered private property, owned by individual persons. However, many legal protections have existed (historically and today) with the intention of safeguarding pets' (and other animals') well-being.[51][52][53][54] Since the year 2000, a small but increasing number of jurisdictions in North America have enacted laws redefining pet's owners as guardians. Intentions have been characterized as simply changing attitudes and perceptions (but not legal consequences) to working toward legal personhood for pets themselves. Some veterinarians and breeders have opposed these moves. The question of pets' legal status can arise with concern to purchase or adoption, custody, divorce, estate and inheritance, injury, damage, and veterinary malpractice.[55][56][57][58]
+
+In Belgium and the Netherlands, the government publishes white lists and black lists (called 'positive' and 'negative lists') with animal species that are designated to be appropriate to be kept as pets (positive) or not (negative). The Dutch Ministry of Economic Affairs and Climate Policy originally established its first positive list (positieflijst) per 1 February 2015 for a set of 100 mammals (including cats, dogs and production animals) deemed appropriate as pets on the recommendations of Wageningen University.[59] Parliamentary debates about such a pet list date back to the 1980s, with continuous disagreements about which species should be included and how the law should be enforced.[60] In January 2017, the white list was expanded to 123 species, while the black list that had been set up was expanded (with animals like the brown bear and two great kangaroo species) to contain 153 species unfit for petting, such as the armadillo, the sloth, the European hare and the wild boar.[61]
+
+Pets have a considerable environmental impact, especially in countries where they are common or held in high densities. For instance, the 163 million dogs and cats kept in the United States consume about 20% of the amount of dietary energy that humans do and an estimated 33% of the animal-derived energy.[62] They produce about 30% ± 13%, by mass, as much feces as Americans, and through their diet, constitute about 25–30% of the environmental impacts from animal production in terms of the use of land, water, fossil fuel, phosphate, and biocides. Dog and cat animal product consumption is responsible for the release of up to 64 ± 16 million tons CO2-equivalent methane and nitrous oxide, two powerful greenhouse gasses. Americans are the largest pet owners in the world, but pet ownership in the US has considerable environmental costs.[62]
+
+While many people have kept many different species of animals in captivity over the course of human history, only a relative few have been kept long enough to be considered domesticated. Other types of animals, notably monkeys, have never been domesticated but are still sold and kept as pets. There are also inanimate objects that have been kept as "pets", either as a form of a game or humorously (e.g. the Pet Rock or Chia Pet). Some wild animals are kept as pets, such as tigers, even though this is illegal. There is a market for illegal pets.
+
+Domesticated pets are most common. A domesticated animal is a species that has been made fit for a human environment[63] by being consistently kept in captivity and selectively bred over a long enough period of time that it exhibits marked differences in behavior and appearance from its wild relatives. Domestication contrasts with taming, which is simply when an un-domesticated, wild animal has become tolerant of human presence, and perhaps, even enjoys it.
+
+Wild animals are kept as pets. The term “wild” in this context specifically applies to any species of animal which has not undergone a fundamental change in behavior to facilitate a close co-existence with humans. Some species may have been bred in captivity for a considerable length of time, but are still not recognized as domesticated.
+
+Generally, wild animals are recognized as not suitable to keep as pets, and this practice is completely banned in many places. In other areas, certain species are allowed to be kept, and it is usually required for the owner to obtain a permit. It is considered animal cruelty by some, as most often, wild animals require precise and constant care that is very difficult to meet in captive conditions. Many large and instinctively aggressive animals are extremely dangerous, and numerous times have they killed their handlers.
+
+Archaeology suggests that human ownership of dogs as pets may date back to at least 12,000 years ago.[64]
+
+Ancient Greeks and Romans would openly grieve for the loss of a dog, evidenced by inscriptions left on tombstones commemorating their loss.[65] The surviving epitaphs dedicated to horses are more likely to reference a gratitude for the companionship that had come from war horses rather than race horses. The latter may have chiefly been commemorated as a way to further the owner's fame and glory.[66] In Ancient Egypt, dogs and baboons were kept as pets and buried with their owners. Dogs were given names, which is significant as Egyptians considered names to have magical properties. [67]
+
+Throughout the seventeenth and eighteenth-century pet keeping in the modern sense gradually became accepted throughout Britain. Initially, aristocrats kept dogs for both companionship and hunting. Thus, pet keeping was a sign of elitism within society. By the nineteenth century, the rise of the middle class stimulated the development of pet keeping and it became inscribed within the bourgeois culture.[68]
+
+As the popularity of pet-keeping in the modern sense rose during the Victorian era, animals became a fixture within urban culture as commodities and decorative objects.[69] Pet keeping generated a commercial opportunity for entrepreneurs. By the mid-nineteenth century, nearly twenty thousand street vendors in London dealt with live animals.[70] Also, the popularity of animals developed a demand for animal goods such as accessories and guides for pet keeping. Pet care developed into a big business by the end of the nineteenth century.[71]
+
+Profiteers also sought out pet stealing as a means for economic gain. Utilizing the affection that owners had for their pets, professional dog stealers would capture animals and hold them for ransom.[72] The development of dog stealing reflects the increased value of pets. Pets gradually became defined as the property of their owners. Laws were created that punished offenders for their burglary.[73]
+
+Pets and animals also had social and cultural implications throughout the nineteenth century. The categorization of dogs by their breeds reflected the hierarchical, social order of the Victorian era. The pedigree of a dog represented the high status and lineage of their owners and reinforced social stratification.[74] Middle-class owners, however, valued the ability to associate with the upper-class through ownership of their pets. The ability to care for a pet signified respectability and the capability to be self-sufficient.[75] According to Harriet Ritvo, the identification of “elite animal and elite owner was not a confirmation of the owner’s status but a way of redefining it.”[76]
+
+The popularity of dog and pet keeping generated animal fancy. Dog fanciers showed enthusiasm for owning pets, breeding dogs, and showing dogs in various shows. The first dog show took place on 28 June 1859 in Newcastle and focused mostly on sporting and hunting dogs.[77] However, pet owners produced an eagerness to demonstrate their pets as well as have an outlet to compete.[78] Thus, pet animals gradually were included within dog shows. The first large show, which would host one thousand entries, took place in Chelsea in 1863.[79] The Kennel Club was created in 1873 to ensure fairness and organization within dog shows. The development of the Stud Book by the Kennel Club defined policies, presented a national registry system of purebred dogs, and essentially institutionalized dog shows.[80]
+
+Pet ownership by animals in the wild, as an analogue to the human phenomenon, has not been observed and is likely non-existent in nature.[81][82] One group of capuchin monkeys was observed appearing to care for a marmoset, a fellow New World monkey species, however observations of chimpanzees apparently "playing" with small animals like hyraxes have ended with the chimpanzees killing the animals and tossing the corpses around.[83]
+
+A 2010 study states that human relationships with animals have an exclusive human cognitive component and that pet-keeping is a fundamental and ancient attribute of the human species. Anthropomorphism, or the projection of human feelings, thoughts and attributes on to animals, is a defining feature of human pet-keeping. The study identifies it as the same trait in evolution responsible for domestication and concern for animal welfare. It is estimated to have arisen at least 100,000 years before present (ybp) in Homo sapiens sapiens.[82]
+
+It is debated whether this redirection of human nurturing behaviour towards non-human animals, in the form of pet-keeping, was maladaptive, due to being biologically costly, or whether it was positively selected for.[84][85][82] Two studies suggest that the human ability to domesticate and keep pets came from the same fundamental evolutionary trait and that this trait provided a material benefit in the form of domestication that was sufficiently adaptive to be positively selected for.[82][85]:300 A 2011 study suggests that the practical functions that some pets provide, such as assisting hunting or removing pests, could've resulted in enough evolutionary advantage to allow for the persistence of this behaviour in humans and outweigh the economic burden held by pets kept as playthings for immediate emotional rewards.[86] Two other studies suggest that the behaviour constitutes an error, side effect or misapplication of the evolved mechanisms responsible for human empathy and theory of mind to cover non-human animals which has not sufficiently impacted its evolutionary advantage in the long run.[85]:300
+
+Animals in captivity, with the help of caretakers, have been considered to have owned "pets". Examples of this include Koko the gorilla and several pet cats, Tonda the orangutan and a pet cat and Tarra the elephant and a dog named Bella.[83]
+
+Katharine of Aragon with a monkey
+
+The Girl with the Marmot by Jean-Honoré Fragonard
+
+- Young Lady with parrot by Édouard Manet 1866
+
+Antoinette Metayer (1732–88) and her pet dog
+
+The Lady with an Ermine
+
+Sir Henry Raeburn - Boy and Rabbit
+
+Eos, A Favorite Greyhound of Prince Albert
+
+A Neapolitan Woman
+
+Signal, a Grey Arab, with a Groom in the Desert
+
+Eduardo Leon Garrido. An Elegant Lady with her Dog
+
+The Fireplace depicting a Pug, James Tissot
+
+Rosa Bonheur - Portrait of William F. Cody
+
+Hunt
+
+
+
+
+
+
+

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+Hebrew (Hebrew: עִבְרִית‎, romanized: Ivrit, IPA: [ivˈʁit] or [ʕivˈɾit] (listen)) is a Northwest Semitic language native to Israel. In 2013, Modern Hebrew was spoken by over nine million people worldwide.[8] Historically, it is regarded as the language of the Israelites and their ancestors, although the language was not referred to by the name "Hebrew" in the Tanakh itself.[note 1] The earliest examples of written Paleo-Hebrew date from the 10th century BCE.[9] Hebrew belongs to the West Semitic branch of the Afroasiatic language family. Hebrew is the only Canaanite language still spoken and the only truly successful example of a revived dead language.[10][11]
+
+Hebrew ceased to be an everyday spoken language somewhere between 200 and 400 CE, declining since the aftermath of the Bar Kokhba revolt.[2][12][note 2] Aramaic and, to a lesser extent, Greek were already in use as international languages, especially among elites and immigrants.[14] Hebrew survived into the medieval period as the language of Jewish liturgy, rabbinic literature, intra-Jewish commerce and poetry. With the rise of Zionism in the 19th century, it was revived as a spoken and literary language, becoming the main language of the Yishuv and subsequently of the State of Israel. According to Ethnologue, in 1998, Hebrew was the language of five million people worldwide.[5] After Israel, the United States has the second-largest Hebrew-speaking population, with about 220,000 fluent speakers,[15] mostly from Israel.
+
+Modern Hebrew is the official language of the State of Israel, while premodern Hebrew is used for prayer or study in Jewish communities around the world today. The Samaritan dialect is also the liturgical tongue of the Samaritans, while modern Hebrew or Arabic is their vernacular. As a foreign language, it is studied mostly by Jews and students of Judaism and Israel and by archaeologists and linguists specializing in the Middle East and its civilizations, as well as by theologians in Christian seminaries.
+
+Nearly all of the Hebrew Bible is written in Biblical Hebrew, with much of its present form in the dialect that scholars believe flourished around the 6th century BCE, around the time of the Babylonian captivity. For this reason, Hebrew has been referred to by Jews as Lashon Hakodesh (לשון הקודש), "the holy language" or "the language of holiness", since ancient times.
+
+The modern English word "Hebrew" is derived from Old French Ebrau, via Latin from the Greek Ἑβραῖος (Hebraîos) and Aramaic 'ibrāy, all ultimately derived from Biblical Hebrew Ivri (עברי), one of several names for the Israelite (Jewish and Samaritan) people (Hebrews). It is traditionally understood to be an adjective based on the name of Abraham's ancestor, Eber, mentioned in Genesis 10:21. The name is believed to be based on the Semitic root ʕ-b-r (עבר) meaning "beyond", "other side", "across";[16] interpretations of the term "Hebrew" generally render its meaning as roughly "from the other side [of the river/desert]"—i.e., an exonym for the inhabitants of the land of Israel/Judah, perhaps from the perspective of Mesopotamia, Phoenicia or the Transjordan (with the river referenced perhaps the Euphrates, Jordan or Litani; or maybe the northern Arabian Desert between Babylonia and Canaan).[17] Compare the word Habiru or cognate Assyrian ebru, of identical meaning.[18]
+
+One of the earliest references to the language's name as "Ivrit" is found in the prologue to the Book of Ben Sira,[a] from the 2nd century BCE.[19] The Hebrew Bible does not use the term "Hebrew" in reference to the language of the Hebrew people;[20] its later historiography, in the Book of Kings, refers to it as ‏יְהוּדִית‎ Yehudit 'Judahite (language)'.[21]
+
+Hebrew belongs to the Canaanite group of languages. Canaanite languages are a branch of the Northwest Semitic family of languages.[22]
+
+According to Avraham Ben-Yosef, Hebrew flourished as a spoken language in the Kingdoms of Israel and Judah during the period from about 1200 to 586 BCE.[23] Scholars debate the degree to which Hebrew was a spoken vernacular in ancient times following the Babylonian exile, when the predominant international language in the region was Old Aramaic.
+
+Hebrew was extinct as a colloquial language by Late Antiquity, but it continued to be used as a literary language and as the liturgical language of Judaism, evolving various dialects of literary Medieval Hebrew, until its revival as a spoken language in the late 19th century.[24][25]
+
+In July 2008, Israeli archaeologist Yossi Garfinkel discovered a ceramic shard at Khirbet Qeiyafa that he claimed may be the earliest Hebrew writing yet discovered, dating from around 3,000 years ago.[26] Hebrew University archaeologist Amihai Mazar said that the inscription was "proto-Canaanite" but cautioned that "The differentiation between the scripts, and between the languages themselves in that period, remains unclear," and suggested that calling the text Hebrew might be going too far.[27]
+
+The Gezer calendar also dates back to the 10th century BCE at the beginning of the Monarchic Period, the traditional time of the reign of David and Solomon. Classified as Archaic Biblical Hebrew, the calendar presents a list of seasons and related agricultural activities. The Gezer calendar (named after the city in whose proximity it was found) is written in an old Semitic script, akin to the Phoenician one that, through the Greeks and Etruscans, later became the Roman script. The Gezer calendar is written without any vowels, and it does not use consonants to imply vowels even in the places in which later Hebrew spelling requires them.
+
+Numerous older tablets have been found in the region with similar scripts written in other Semitic languages, for example, Protosinaitic. It is believed that the original shapes of the script go back to Egyptian hieroglyphs, though the phonetic values are instead inspired by the acrophonic principle. The common ancestor of Hebrew and Phoenician is called Canaanite, and was the first to use a Semitic alphabet distinct from that of Egyptian. One ancient document is the famous Moabite Stone, written in the Moabite dialect; the Siloam Inscription, found near Jerusalem, is an early example of Hebrew. Less ancient samples of Archaic Hebrew include the ostraca found near Lachish, which describe events preceding the final capture of Jerusalem by Nebuchadnezzar and the Babylonian captivity of 586 BCE.
+
+In its widest sense, Biblical Hebrew refers to the spoken language of ancient Israel flourishing between the 10th century BCE and the turn of the 4th century CE.[28] It comprises several evolving and overlapping dialects. The phases of Classical Hebrew are often named after important literary works associated with them.
+
+Sometimes the above phases of spoken Classical Hebrew are simplified into "Biblical Hebrew" (including several dialects from the 10th century BCE to 2nd century BCE and extant in certain Dead Sea Scrolls) and "Mishnaic Hebrew" (including several dialects from the 3rd century BCE to the 3rd century CE and extant in certain other Dead Sea Scrolls).[29] However, today most Hebrew linguists classify Dead Sea Scroll Hebrew as a set of dialects evolving out of Late Biblical Hebrew and into Mishnaic Hebrew, thus including elements from both but remaining distinct from either.[30]
+
+By the start of the Byzantine Period in the 4th century CE, Classical Hebrew ceased as a regularly spoken language, roughly a century after the publication of the Mishnah, apparently declining since the aftermath of the catastrophic Bar Kokhba War around 135 CE.
+
+In the early 6th century BCE, the Neo-Babylonian Empire conquered the ancient Kingdom of Judah, destroying much of Jerusalem and exiling its population far to the East in Babylon. During the Babylonian captivity, many Israelites learned Aramaic, the closely related Semitic language of their captors. Thus for a significant period, the Jewish elite became influenced by Aramaic.[31]
+
+After Cyrus the Great conquered Babylon, he allowed the Jewish people to return from captivity. As a result,[improper synthesis?] a local version of Aramaic came to be spoken in Israel alongside Hebrew. By the beginning of the Common Era, Aramaic was the primary colloquial language of Samarian, Babylonian and Galileean Jews, and western and intellectual Jews spoke Greek,[citation needed] but a form of so-called Rabbinic Hebrew continued to be used as a vernacular in Judea until it was displaced by Aramaic, probably in the 3rd century CE. Certain Sadducee, Pharisee, Scribe, Hermit, Zealot and Priest classes maintained an insistence on Hebrew, and all Jews maintained their identity with Hebrew songs and simple quotations from Hebrew texts.[13][32][33]
+
+While there is no doubt that at a certain point, Hebrew was displaced as the everyday spoken language of most Jews, and that its chief successor in the Middle East was the closely related Aramaic language, then Greek,[32][note 2] scholarly opinions on the exact dating of that shift have changed very much.[12] In the first half of the 20th century, most scholars followed Geiger and Dalman in thinking that Aramaic became a spoken language in the land of Israel as early as the beginning of Israel's Hellenistic Period in the 4th century BCE, and that as a corollary Hebrew ceased to function as a spoken language around the same time. Segal, Klausner and Ben Yehuda are notable exceptions to this view. During the latter half of the 20th century, accumulating archaeological evidence and especially linguistic analysis of the Dead Sea Scrolls has disproven that view. The Dead Sea Scrolls, uncovered in 1946–1948 near Qumran revealed ancient Jewish texts overwhelmingly in Hebrew, not Aramaic.
+
+The Qumran scrolls indicate that Hebrew texts were readily understandable to the average Israelite, and that the language had evolved since Biblical times as spoken languages do.[note 3] Recent scholarship recognizes that reports of Jews speaking in Aramaic indicate a multilingual society, not necessarily the primary language spoken. Alongside Aramaic, Hebrew co-existed within Israel as a spoken language.[35] Most scholars now date the demise of Hebrew as a spoken language to the end of the Roman Period, or about 200 CE.[36] It continued on as a literary language down through the Byzantine Period from the 4th century CE.
+
+The exact roles of Aramaic and Hebrew remain hotly debated. A trilingual scenario has been proposed for the land of Israel. Hebrew functioned as the local mother tongue with powerful ties to Israel's history, origins and golden age and as the language of Israel's religion; Aramaic functioned as the international language with the rest of the Middle East; and eventually Greek functioned as another international language with the eastern areas of the Roman Empire.[citation needed] William Schniedewind argues that after waning in the Persian Period, the religious importance of Hebrew grew in the Hellenistic and Roman periods, and cites epigraphical evidence that Hebrew survived as a vernacular language — though both its grammar and its writing system had been substantially influenced by Aramaic.[37] According to another summary, Greek was the language of government, Hebrew the language of prayer, study and religious texts, and Aramaic was the language of legal contracts and trade.[38] There was also a geographic pattern: according to Spolsky, by the beginning of the Common Era, "Judeo-Aramaic was mainly used in Galilee in the north, Greek was concentrated in the former colonies and around governmental centers, and Hebrew monolingualism continued mainly in the southern villages of Judea."[32] In other words, "in terms of dialect geography, at the time of the tannaim Palestine could be divided into the Aramaic-speaking regions of Galilee and Samaria and a smaller area, Judaea, in which Rabbinic Hebrew was used among the descendants of returning exiles."[13][33] In addition, it has been surmised that Koine Greek was the primary vehicle of communication in coastal cities and among the upper class of Jerusalem, while Aramaic was prevalent in the lower class of Jerusalem, but not in the surrounding countryside.[38] After the suppression of the Bar Kokhba revolt in the 2nd century CE, Judaeans were forced to disperse. Many relocated to Galilee, so most remaining native speakers of Hebrew at that last stage would have been found in the north.[39]
+
+The Christian New Testament contains some Semitic place names and quotes.[40] The language of such Semitic glosses (and in general the language spoken by Jews in scenes from the New Testament) is often referred to as "Hebrew" in the text,[41] although this term is often re-interpreted as referring to Aramaic instead[note 4][note 5] and is rendered accordingly in recent translations.[43] Nonetheless, these glosses can be interpreted as Hebrew as well.[44] It has been argued that Hebrew, rather than Aramaic or Koine Greek, lay behind the composition of the Gospel of Matthew.[45] (See the Hebrew Gospel hypothesis or Language of Jesus for more details on Hebrew and Aramaic in the gospels.)
+
+The term "Mishnaic Hebrew" generally refers to the Hebrew dialects found in the Talmud, excepting quotations from the Hebrew Bible. The dialects organize into Mishnaic Hebrew (also called Tannaitic Hebrew, Early Rabbinic Hebrew, or Mishnaic Hebrew I), which was a spoken language, and Amoraic Hebrew (also called Late Rabbinic Hebrew or Mishnaic Hebrew II), which was a literary language. The earlier section of the Talmud is the Mishnah that was published around 200 CE, although many of the stories take place much earlier, and was written in the earlier Mishnaic dialect. The dialect is also found in certain Dead Sea Scrolls. Mishnaic Hebrew is considered to be one of the dialects of Classical Hebrew that functioned as a living language in the land of Israel. A transitional form of the language occurs in the other works of Tannaitic literature dating from the century beginning with the completion of the Mishnah. These include the halachic Midrashim (Sifra, Sifre, Mechilta etc.) and the expanded collection of Mishnah-related material known as the Tosefta. The Talmud contains excerpts from these works, as well as further Tannaitic material not attested elsewhere; the generic term for these passages is Baraitot. The dialect of all these works is very similar to Mishnaic Hebrew.
+
+About a century after the publication of the Mishnah, Mishnaic Hebrew fell into disuse as a spoken language. The later section of the Talmud, the Gemara, generally comments on the Mishnah and Baraitot in two forms of Aramaic. Nevertheless, Hebrew survived as a liturgical and literary language in the form of later Amoraic Hebrew, which sometimes occurs in the text of the Gemara.
+
+Hebrew was always regarded as the language of Israel's religion, history and national pride, and after it faded as a spoken language, it continued to be used as a lingua franca among scholars and Jews traveling in foreign countries.[46] After the 2nd century CE when the Roman Empire exiled most of the Jewish population of Jerusalem following the Bar Kokhba revolt, they adapted to the societies in which they found themselves, yet letters, contracts, commerce, science, philosophy, medicine, poetry and laws continued to be written mostly in Hebrew, which adapted by borrowing and inventing terms.
+
+After the Talmud, various regional literary dialects of Medieval Hebrew evolved. The most important is Tiberian Hebrew or Masoretic Hebrew, a local dialect of Tiberias in Galilee that became the standard for vocalizing the Hebrew Bible and thus still influences all other regional dialects of Hebrew. This Tiberian Hebrew from the 7th to 10th century CE is sometimes called "Biblical Hebrew" because it is used to pronounce the Hebrew Bible; however, properly it should be distinguished from the historical Biblical Hebrew of the 6th century BCE, whose original pronunciation must be reconstructed. Tiberian Hebrew incorporates the remarkable scholarship of the Masoretes (from masoret meaning "tradition"), who added vowel points and grammar points to the Hebrew letters to preserve much earlier features of Hebrew, for use in chanting the Hebrew Bible. The Masoretes inherited a biblical text whose letters were considered too sacred to be altered, so their markings were in the form of pointing in and around the letters. The Syriac alphabet, precursor to the Arabic alphabet, also developed vowel pointing systems around this time. The Aleppo Codex, a Hebrew Bible with the Masoretic pointing, was written in the 10th century, likely in Tiberias, and survives to this day. It is perhaps the most important Hebrew manuscript in existence.
+
+During the Golden age of Jewish culture in Spain, important work was done by grammarians in explaining the grammar and vocabulary of Biblical Hebrew; much of this was based on the work of the grammarians of Classical Arabic. Important Hebrew grammarians were Judah ben David Hayyuj, Jonah ibn Janah, Abraham ibn Ezra[47] and later (in Provence), David Kimhi. A great deal of poetry was written, by poets such as Dunash ben Labrat, Solomon ibn Gabirol, Judah ha-Levi, Moses ibn Ezra and Abraham ibn Ezra, in a "purified" Hebrew based on the work of these grammarians, and in Arabic quantitative or strophic meters. This literary Hebrew was later used by Italian Jewish poets.[48]
+
+The need to express scientific and philosophical concepts from Classical Greek and Medieval Arabic motivated Medieval Hebrew to borrow terminology and grammar from these other languages, or to coin equivalent terms from existing Hebrew roots, giving rise to a distinct style of philosophical Hebrew. This is used in the translations made by the Ibn Tibbon family. (Original Jewish philosophical works were usually written in Arabic.[citation needed]) Another important influence was Maimonides, who developed a simple style based on Mishnaic Hebrew for use in his law code, the Mishneh Torah.  Subsequent rabbinic literature is written in a blend between this style and the Aramaized Rabbinic Hebrew of the Talmud.
+
+Hebrew persevered through the ages as the main language for written purposes by all Jewish communities around the world for a large range of uses—not only liturgy, but also poetry, philosophy, science and medicine, commerce, daily correspondence and contracts. There have been many deviations from this generalization such as Bar Kokhba's letters to his lieutenants, which were mostly in Aramaic,[49] and Maimonides' writings, which were mostly in Arabic;[50] but overall, Hebrew did not cease to be used for such purposes. For example, the first Middle East printing press, in Safed (modern Israel), produced a small number of books in Hebrew in 1577, which were then sold to the nearby Jewish world.[51] This meant not only that well-educated Jews in all parts of the world could correspond in a mutually intelligible language, and that books and legal documents published or written in any part of the world could be read by Jews in all other parts, but that an educated Jew could travel and converse with Jews in distant places, just as priests and other educated Christians could converse in Latin. For example, Rabbi Avraham Danzig wrote the Chayei Adam in Hebrew, as opposed to Yiddish, as a guide to Halacha for the "average 17-year-old" (Ibid. Introduction 1). Similarly, the Chofetz Chaim, Rabbi Yisrael Meir Kagan's purpose in writing the Mishna Berurah was to "produce a work that could be studied daily so that Jews might know the proper procedures to follow minute by minute".  The work was nevertheless written in Talmudic Hebrew and Aramaic, since, "the ordinary Jew [of Eastern Europe] of a century ago, was fluent enough in this idiom to be able to follow the Mishna Berurah without any trouble."[52]
+
+Hebrew has been revived several times as a literary language, most significantly by the Haskalah (Enlightenment) movement of early and mid-19th-century Germany. In the early 19th century, a form of spoken Hebrew had emerged in the markets of Jerusalem between Jews of different linguistic backgrounds to communicate for commercial purposes. This Hebrew dialect was to a certain extent a pidgin.[53] Near the end of that century the Jewish activist Eliezer Ben-Yehuda, owing to the ideology of the national revival (שיבת ציון, Shivat Tziyon, later Zionism), began reviving Hebrew as a modern spoken language. Eventually, as a result of the local movement he created, but more significantly as a result of the new groups of immigrants known under the name of the Second Aliyah, it replaced a score of languages spoken by Jews at that time. Those languages were Jewish dialects of local languages, including Judaeo-Spanish (also called "Judezmo" and "Ladino"), Yiddish, Judeo-Arabic and Bukhori (Tajiki), or local languages spoken in the Jewish diaspora such as Russian, Persian and Arabic.
+
+The major result of the literary work of the Hebrew intellectuals along the 19th century was a lexical modernization of Hebrew. New words and expressions were adapted as neologisms from the large corpus of Hebrew writings since the Hebrew Bible, or borrowed from Arabic (mainly by Eliezer Ben-Yehuda) and older Aramaic and Latin. Many new words were either borrowed from or coined after European languages, especially English, Russian, German, and French. Modern Hebrew became an official language in British-ruled Palestine in 1921 (along with English and Arabic), and then in 1948 became an official language of the newly declared State of Israel. Hebrew is the most widely spoken language in Israel today.
+
+In the Modern Period, from the 19th century onward, the literary Hebrew tradition revived as the spoken language of modern Israel, called variously Israeli Hebrew, Modern Israeli Hebrew, Modern Hebrew, New Hebrew, Israeli Standard Hebrew, Standard Hebrew and so on. Israeli Hebrew exhibits some features of Sephardic Hebrew from its local Jerusalemite tradition but adapts it with numerous neologisms, borrowed terms (often technical) from European languages and adopted terms (often colloquial) from Arabic.
+
+The literary and narrative use of Hebrew was revived beginning with the Haskalah movement. The first secular periodical in Hebrew, HaMe'assef (The Gatherer), was published by maskilim in Königsberg (today's Kaliningrad) from 1783 onwards.[54] In the mid-19th century, publications of several Eastern European Hebrew-language newspapers (e.g. Hamagid, founded in Ełk in 1856) multiplied. Prominent poets were Hayim Nahman Bialik and Shaul Tchernichovsky; there were also novels written in the language.
+
+The revival of the Hebrew language as a mother tongue was initiated in the late 19th century by the efforts of Eliezer Ben-Yehuda. He joined the Jewish national movement and in 1881 immigrated to Palestine, then a part of the Ottoman Empire. Motivated by the surrounding ideals of renovation and rejection of the diaspora "shtetl" lifestyle, Ben-Yehuda set out to develop tools for making the literary and liturgical language into everyday spoken language. However, his brand of Hebrew followed norms that had been replaced in Eastern Europe by different grammar and style, in the writings of people like Ahad Ha'am and others. His organizational efforts and involvement with the establishment of schools and the writing of textbooks pushed the vernacularization activity into a gradually accepted movement. It was not, however, until the 1904–1914 Second Aliyah that Hebrew had caught real momentum in Ottoman Palestine with the more highly organized enterprises set forth by the new group of immigrants. When the British Mandate of Palestine recognized Hebrew as one of the country's three official languages (English, Arabic, and Hebrew, in 1922), its new formal status contributed to its diffusion. A constructed modern language with a truly Semitic vocabulary and written appearance, although often European in phonology, was to take its place among the current languages of the nations.
+
+While many saw his work as fanciful or even blasphemous[55] (because Hebrew was the holy language of the Torah and therefore some thought that it should not be used to discuss everyday matters), many soon understood the need for a common language amongst Jews of the British Mandate who at the turn of the 20th century were arriving in large numbers from diverse countries and speaking different languages. A Committee of the Hebrew Language was established. After the establishment of Israel, it became the Academy of the Hebrew Language. The results of Ben-Yehuda's lexicographical work were published in a dictionary (The Complete Dictionary of Ancient and Modern Hebrew). The seeds of Ben-Yehuda's work fell on fertile ground, and by the beginning of the 20th century, Hebrew was well on its way to becoming the main language of the Jewish population of both Ottoman and British Palestine. At the time, members of the Old Yishuv and a very few Hasidic sects, most notably those under the auspices of Satmar, refused to speak Hebrew and spoke only Yiddish.
+
+In the Soviet Union, the use of Hebrew, along with other Jewish cultural and religious activities, was suppressed. Soviet authorities considered the use of Hebrew "reactionary" since it was associated with Zionism, and the teaching of Hebrew at primary and secondary schools was officially banned by the People's Commissariat for Education as early as 1919, as part of an overall agenda aiming to secularize education (the language itself did not cease to be studied at universities for historical and linguistic purposes[56]). The official ordinance stated that Yiddish, being the spoken language of the Russian Jews, should be treated as their only national language, while Hebrew was to be treated as a foreign language.[57] Hebrew books and periodicals ceased to be published and were seized from the libraries, although liturgical texts were still published until the 1930s. Despite numerous protests,[58] a policy of suppression of the teaching of Hebrew operated from the 1930s on. Later in the 1980s in the USSR, Hebrew studies reappeared due to people struggling for permission to go to Israel (refuseniks). Several of the teachers were imprisoned, e.g. Yosef Begun, Ephraim Kholmyansky, Yevgeny Korostyshevsky and others responsible for a Hebrew learning network connecting many cities of the USSR.
+
+Standard Hebrew, as developed by Eliezer Ben-Yehuda, was based on Mishnaic spelling and Sephardi Hebrew pronunciation. However, the earliest speakers of Modern Hebrew had Yiddish as their native language and often introduced calques from Yiddish and phono-semantic matchings of international words.
+
+Despite using Sephardic Hebrew pronunciation as its primary basis, modern Israeli Hebrew has adapted to Ashkenazi Hebrew phonology in some respects, mainly the following:
+
+The vocabulary of Israeli Hebrew is much larger than that of earlier periods. According to Ghil'ad Zuckermann:
+
+The number of attested Biblical Hebrew words is 8198, of which some 2000 are hapax legomena (the number of Biblical Hebrew roots, on which many of these words are based, is 2099). The number of attested Rabbinic Hebrew words is less than 20,000, of which (i) 7879 are Rabbinic par excellence, i.e. they did not appear in the Old Testament (the number of new Rabbinic Hebrew roots is 805); (ii) around 6000 are a subset of Biblical Hebrew; and (iii) several thousand are Aramaic words which can have a Hebrew form. Medieval Hebrew added 6421 words to (Modern) Hebrew. The approximate number of new lexical items in Israeli is 17,000 (cf. 14,762 in Even-Shoshan 1970 [...]). With the inclusion of foreign and technical terms [...], the total number of Israeli words, including words of biblical, rabbinic and medieval descent, is more than 60,000.[61]:64–65
+
+In Israel, Modern Hebrew is currently taught in institutions called Ulpanim (singular: Ulpan). There are government-owned, as well as private, Ulpanim offering online courses and face-to-face programs.
+
+Modern Hebrew is the primary official language of the State of Israel. As of 2013[update], there are about 9 million Hebrew speakers worldwide,[62] of whom 7 million speak it fluently.[63][64][65]
+
+Currently, 90% of Israeli Jews are proficient in Hebrew, and 70% are highly proficient.[66] Some 60% of Israeli Arabs are also proficient in Hebrew,[66] and 30% report having a higher proficiency in Hebrew than in Arabic.[8] In total, about 53% of the Israeli population speaks Hebrew as a native language,[67] while most of the rest speak it fluently. However, in 2013 Hebrew was the native language of only 49% of Israelis over the age of 20, with Russian, Arabic, French, English, Yiddish and Ladino being the native tongues of most of the rest. Some 26% of immigrants from the former Soviet Union and 12% of Arabs reported speaking Hebrew poorly or not at all.[66][68]
+
+Steps have been taken to keep Hebrew the primary language of use, and to prevent large-scale incorporation of English words into the Hebrew vocabulary. The Academy of the Hebrew Language of the Hebrew University of Jerusalem currently invents about 2,000 new Hebrew words each year for modern words by finding an original Hebrew word that captures the meaning, as an alternative to incorporating more English words into Hebrew vocabulary. The Haifa municipality has banned officials from using English words in official documents, and is fighting to stop businesses from using only English signs to market their services.[69] In 2012, a Knesset bill for the preservation of the Hebrew language was proposed, which includes the stipulation that all signage in Israel must first and foremost be in Hebrew, as with all speeches by Israeli officials abroad. The bill's author, MK Akram Hasson, stated that the bill was proposed as a response to Hebrew "losing its prestige" and children incorporating more English words into their vocabulary.[70]
+
+Hebrew is also an official national minority language in Poland, since 6 January 2005.[71]
+
+Biblical Hebrew had a typical Semitic consonant inventory, with pharyngeal /ʕ ħ/, a series of "emphatic" consonants (possibly ejective, but this is debated), lateral fricative /ɬ/, and in its older stages also uvular /χ ʁ/. /χ ʁ/ merged into /ħ ʕ/ in later Biblical Hebrew, and /b ɡ d k p t/ underwent allophonic spirantization to [v ɣ ð x f θ] (known as begadkefat). The earliest Biblical Hebrew vowel system contained the Proto-Semitic vowels /a aː i iː u uː/ as well as /oː/, but this system changed dramatically over time.
+
+By the time of the Dead Sea Scrolls, /ɬ/ had shifted to /s/ in the Jewish traditions, though for the Samaritans it merged with /ʃ/ instead.[30] The Tiberian reading tradition of the Middle Ages had the vowel system /a ɛ e i ɔ o u ă ɔ̆ ɛ̆/, though other Medieval reading traditions had fewer vowels.
+
+A number of reading traditions have been preserved in liturgical use. In Oriental (Sephardi and Mizrahi) Jewish reading traditions, the emphatic consonants are realized as pharyngealized, while the Ashkenazi (northern and eastern European) traditions have lost emphatics and pharyngeals (although according to Ashkenazi law, pharyngeal articulation is preferred over uvular or glottal articulation when representing the community in religious service such as prayer and Torah reading), and show the shift of /w/ to /v/. The Samaritan tradition has a complex vowel system that does not correspond closely to the Tiberian systems.
+
+Modern Hebrew pronunciation developed from a mixture of the different Jewish reading traditions, generally tending towards simplification. In line with Sephardi Hebrew pronunciation, emphatic consonants have shifted to their ordinary counterparts, /w/ to /v/, and [ɣ ð θ] are not present. Most Israelis today also merge /ʕ ħ/ with /ʔ χ/, do not have contrastive gemination, and pronounce /r/ as a uvular fricative [ʁ] or a voiced velar fricative [ɣ] rather than an alveolar trill, because of Ashkenazi Hebrew influences. The consonants /tʃ/ and /dʒ/ have become phonemic due to loan words, and /w/ has similarly been re-introduced.
+
+Notes:
+
+Hebrew grammar is partly analytic, expressing such forms as dative, ablative and accusative using prepositional particles rather than grammatical cases. However, inflection plays a decisive role in the formation of the verbs and nouns. For example, nouns have a construct state, called "smikhut", to denote the relationship of "belonging to": this is the converse of the genitive case of more inflected languages. Words in smikhut are often combined with hyphens.  In modern speech, the use of the construct is sometimes interchangeable with the preposition "shel", meaning "of".  There are many cases, however, where older declined forms are retained (especially in idiomatic expressions and the like), and "person"-enclitics are widely used to "decline" prepositions.
+
+Like all Semitic languages, the Hebrew language exhibits a pattern of stems consisting typically of "triliteral", or 3-consonant consonantal roots, from which nouns, adjectives, and verbs are formed in various ways: e.g. by inserting vowels, doubling consonants, lengthening vowels and/or adding prefixes, suffixes or infixes. 4-consonant roots also exist and became more frequent in the modern language due to a process of coining verbs from nouns that are themselves constructed from 3-consonant verbs. Some triliteral roots lose one of their consonants in most forms and are called "Nehim" (Resting).
+
+Hebrew uses a number of one-letter prefixes that are added to words for various purposes. These are called inseparable prepositions or "Letters of Use" (Hebrew: אותיות השימוש‎, romanized: Otiyot HaShimush). Such items include: the definite article ha- (/ha/) (="the"); prepositions be- (/bə/) (="in"), le- (/lə/) (="to"; a shortened version of the preposition el), mi- (/mi/) (="from"; a shortened version of the preposition min); conjunctions ve- (/və/) (="and"), she- (/ʃe/) (="that"; a shortened version of the Biblical conjunction asher), ke- (/kə/) (="as", "like"; a shortened version of the conjunction kmo).
+
+The vowel accompanying each of these letters may differ from those listed above, depending on the first letter or vowel following it. The rules governing these changes, hardly observed in colloquial speech as most speakers tend to employ the regular form, may be heard in more formal circumstances. For example, if a preposition is put before a word that begins with a moving Shva, then the preposition takes the vowel /i/ (and the initial consonant may be weakened): colloquial be-kfar (="in a village") corresponds to the more formal bi-khfar.
+
+The definite article may be inserted between a preposition or a conjunction and the word it refers to, creating composite words like mé-ha-kfar (="from the village"). The latter also demonstrates the change in the vowel of mi-. With be,  le and ke, the definite article is assimilated into the prefix, which then becomes ba, la or ka. Thus *be-ha-matos becomes ba-matos (="in the plane"). Note that this does not happen to mé (the form of "min" or "mi-" used before the letter "he"), therefore mé-ha-matos is a valid form, which means "from the airplane".
+
+Like most other languages, the vocabulary of the Hebrew language is divided into verbs, nouns, adjectives and so on, and its sentence structure can be analyzed by terms like object, subject and so on.
+
+Modern Hebrew is written from right to left using the Hebrew alphabet, which is an "impure" abjad, or consonant-only script, of 22 letters. The ancient paleo-Hebrew alphabet is similar to those used for Canaanite and Phoenician.[citation needed]  Modern scripts are based on the "square" letter form, known as Ashurit (Assyrian), which was developed from the Aramaic script. A cursive Hebrew script is used in handwriting: the letters tend to be more circular in form when written in cursive, and sometimes vary markedly from their printed equivalents.  The medieval version of the cursive script forms the basis of another style, known as Rashi script.  When necessary, vowels are indicated by diacritic marks above or below the letter representing the syllabic onset, or by use of matres lectionis, which are consonantal letters used as vowels.  Further diacritics are used to indicate variations in the pronunciation of the consonants (e.g. bet/vet, shin/sin); and, in some contexts, to indicate the punctuation, accentuation and musical rendition of Biblical texts (see Cantillation).
+
+Hebrew has always been used as the language of prayer and study, and the following pronunciation systems are found.
+
+Ashkenazi Hebrew, originating in Central and Eastern Europe, is still widely used in Ashkenazi Jewish religious services and studies in Israel and abroad, particularly in the Haredi and other Orthodox communities. It was influenced by the Yiddish language.
+
+Sephardi Hebrew is the traditional pronunciation of the Spanish and Portuguese Jews and Sephardi Jews in the countries of the former Ottoman Empire, with the exception of Yemenite Hebrew.  This pronunciation, in the form used by the Jerusalem Sephardic community, is the basis of the Hebrew phonology of Israeli native speakers. It was influenced by the Judezmo language.
+
+Mizrahi (Oriental) Hebrew is actually a collection of dialects spoken liturgically by Jews in various parts of the Arab and Islamic world. It was derived from the old Arabic language, and in some cases influenced by Sephardi Hebrew. The same claim is sometimes made for Yemenite Hebrew or Temanit, which differs from other Mizrahi dialects by having a radically different vowel system, and distinguishing between different diacritically marked consonants that are pronounced identically in other dialects (for example gimel and "ghimel".)
+
+These pronunciations are still used in synagogue ritual and religious study, in Israel and elsewhere, mostly by people who are not native speakers of Hebrew, though some traditionalist Israelis use liturgical pronunciations in prayer.
+
+Many synagogues in the diaspora, even though Ashkenazi by rite and by ethnic composition, have adopted the "Sephardic" pronunciation in deference to Israeli Hebrew. However, in many British and American schools and synagogues, this pronunciation retains several elements of its Ashkenazi substrate, especially the distinction between tsere and segol.

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+
+
+
+Mass is both a property of a physical body and a measure of its resistance to acceleration (a change in its state of motion) when a net force is applied.[1] An object's mass also determines the strength of its gravitational attraction to other bodies.
+
+The basic SI unit of mass is the kilogram (kg). In physics, mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
+
+There are several distinct phenomena which can be used to measure mass. Although some theorists have speculated that some of these phenomena could be independent of each other,[2] current experiments have found no difference in results regardless of how it is measured:
+
+The mass of an object determines its acceleration in the presence of an applied force. The inertia and the inertial mass describe the same properties of physical bodies at the qualitative and quantitative level respectively, by other words, the mass quantitatively describes the inertia.  According to Newton's second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A body's mass also determines the degree to which it generates or is affected by a gravitational field. If a first body of mass mA is placed at a distance r (center of mass to center of mass) from a second body of mass mB, each body is subject to an attractive force Fg = GmAmB/r2, where G = 6.67×10−11 N kg−2 m2 is the "universal gravitational constant". This is sometimes referred to as gravitational mass.[note 1] Repeated experiments since the 17th century have demonstrated that inertial and gravitational mass are identical; since 1915, this observation has been entailed a priori in the equivalence principle of general relativity.
+
+The standard International System of Units (SI) unit of mass is the kilogram (kg). The kilogram is 1000 grams (g), first defined in 1795 as one cubic decimeter of water at the melting point of ice. However, because precise measurement of a cubic decimeter of water at the proper temperature and pressure was difficult, in 1889 the kilogram was redefined as the mass of the international prototype of the kilogram of cast iron, and thus became independent of the meter and the properties of water.
+
+However, the mass of the international prototype and its supposedly identical national copies have been found to be drifting over time. The re-definition of the kilogram and several other units occurred on May 20, 2019, following a final vote by the CGPM in November 2018.[3] The new definition uses only invariant quantities of nature: the speed of light, the caesium hyperfine frequency, and the Planck constant.[4]
+
+Other units are accepted for use in SI:
+
+Outside the SI system, other units of mass include:
+
+In physical science, one may distinguish conceptually between at least seven different aspects of mass, or seven physical notions that involve the concept of mass.[5] Every experiment to date has shown these seven values to be proportional, and in some cases equal, and this proportionality gives rise to the abstract concept of mass.  There are a number of ways mass can be measured or operationally defined:
+
+In everyday usage, mass and "weight" are often used interchangeably. For instance, a person's weight may be stated as 75 kg. In a constant gravitational field, the weight of an object is proportional to its mass, and it is unproblematic to use the same unit for both concepts. But because of slight differences in the strength of the Earth's gravitational field at different places, the distinction becomes important for measurements with a precision better than a few percent, and for places far from the surface of the Earth, such as in space or on other planets. Conceptually, "mass" (measured in kilograms) refers to an intrinsic property of an object, whereas "weight" (measured in newtons) measures an object's resistance to deviating from its natural course of free fall, which can be influenced by the nearby gravitational field. No matter how strong the gravitational field, objects in free fall are weightless, though they still have mass.[6]
+
+The force known as "weight" is proportional to mass and acceleration in all situations where the mass is accelerated away from free fall. For example, when a body is at rest in a gravitational field (rather than in free fall), it must be accelerated by a force from a scale or the surface of a planetary body such as the Earth or the Moon. This force keeps the object from going into free fall. Weight is the opposing force in such circumstances, and is thus determined by the acceleration of free fall. On the surface of the Earth, for example, an object with a mass of 50 kilograms weighs 491 newtons, which means that 491 newtons is being applied to keep the object from going into free fall. By contrast, on the surface of the Moon, the same object still has a mass of 50 kilograms but weighs only 81.5 newtons, because only 81.5 newtons is required to keep this object from going into a free fall on the moon. Restated in mathematical terms, on the surface of the Earth, the weight W of an object is related to its mass m by W = mg, where g = 9.80665 m/s2 is the acceleration due to Earth's gravitational field, (expressed as the acceleration experienced by a free-falling object).
+
+For other situations, such as when objects are subjected to mechanical accelerations from forces other than the resistance of a planetary surface, the weight force is proportional to the mass of an object multiplied by the total acceleration away from free fall, which is called the proper acceleration. Through such mechanisms, objects in elevators, vehicles, centrifuges, and the like, may experience weight forces many times those caused by resistance to the effects of gravity on objects, resulting from planetary surfaces. In such cases, the generalized equation for weight W of an object is related to its mass m by the equation W = –ma, where a is the proper acceleration of the object caused by all influences other than gravity. (Again, if gravity is the only influence, such as occurs when an object falls freely, its weight will be zero).
+
+Although inertial mass, passive gravitational mass and active gravitational mass are conceptually distinct, no experiment has ever unambiguously demonstrated any difference between them. In classical mechanics, Newton's third law implies that active and passive gravitational mass must always be identical (or at least proportional), but the classical theory offers no compelling reason why the gravitational mass has to equal the inertial mass. That it does is merely an empirical fact.
+
+Albert Einstein developed his general theory of relativity starting with the assumption of the intentionality of correspondence between inertial and passive gravitational mass, and that no experiment will ever detect a difference between them, in essence the equivalence principle.
+
+This particular equivalence often referred to as the "Galilean equivalence principle" or the "weak equivalence principle" has the most important consequence for freely falling objects. Suppose an object has inertial and gravitational masses m and M, respectively. If the only force acting on the object comes from a gravitational field g, the force on the object is:
+
+Given this force, the acceleration of the object can be determined by Newton's second law:
+
+Putting these together, the gravitational acceleration is given by:
+
+This says that the ratio of gravitational to inertial mass of any object is equal to some constant K if and only if all objects fall at the same rate in a given gravitational field. This phenomenon is referred to as the "universality of free-fall". In addition, the constant K can be taken as 1 by defining our units appropriately.
+
+The first experiments demonstrating the universality of free-fall were—according to scientific ‘folklore’—conducted by Galileo obtained by dropping objects from the Leaning Tower of Pisa. This is most likely apocryphal: he is more likely to have performed his experiments with balls rolling down nearly frictionless inclined planes to slow the motion and increase the timing accuracy. Increasingly precise experiments have been performed, such as those performed by Loránd Eötvös,[7] using the torsion balance pendulum, in 1889. As of 2008[update], no deviation from universality, and thus from Galilean equivalence, has ever been found, at least to the precision 10−12. More precise experimental efforts are still being carried out.[citation needed]
+
+The universality of free-fall only applies to systems in which gravity is the only acting force. All other forces, especially friction and air resistance, must be absent or at least negligible. For example, if a hammer and a feather are dropped from the same height through the air on Earth, the feather will take much longer to reach the ground; the feather is not really in free-fall because the force of air resistance upwards against the feather is comparable to the downward force of gravity. On the other hand, if the experiment is performed in a vacuum, in which there is no air resistance, the hammer and the feather should hit the ground at exactly the same time (assuming the acceleration of both objects towards each other, and of the ground towards both objects, for its own part, is negligible). This can easily be done in a high school laboratory by dropping the objects in transparent tubes that have the air removed with a vacuum pump. It is even more dramatic when done in an environment that naturally has a vacuum, as David Scott did on the surface of the Moon during Apollo 15.
+
+A stronger version of the equivalence principle, known as the Einstein equivalence principle or the strong equivalence principle, lies at the heart of the general theory of relativity. Einstein's equivalence principle states that within sufficiently small regions of space-time, it is impossible to distinguish between a uniform acceleration and a uniform gravitational field. Thus, the theory postulates that the force acting on a massive object caused by a gravitational field is a result of the object's tendency to move in a straight line (in other words its inertia) and should therefore be a function of its inertial mass and the strength of the gravitational field.
+
+In theoretical physics, a mass generation mechanism is a theory which attempts to explain the origin of mass from the most fundamental laws of physics. To date, a number of different models have been proposed which advocate different views of the origin of mass. The problem is complicated by the fact that the notion of mass is strongly related to the gravitational interaction but a theory of the latter has not been yet reconciled with the currently popular model of particle physics, known as the Standard Model.
+
+The concept of amount is very old and predates recorded history.  Humans, at some early era, realized that the weight of a collection of similar objects was directly proportional to the number of objects in the collection:
+
+where W is the weight of the collection of similar objects and n is the number of objects in the collection. Proportionality, by definition, implies that two values have a constant ratio:
+
+An early use of this relationship is a balance scale, which balances the force of one object's weight against the force of another object's weight. The two sides of a balance scale are close enough that the objects experience similar gravitational fields.  Hence, if they have similar masses then their weights will also be similar.  This allows the scale, by comparing weights, to also compare masses.
+
+Consequently, historical weight standards were often defined in terms of amounts.  The Romans, for example, used the carob seed (carat or siliqua) as a measurement standard.  If an object's weight was equivalent to 1728 carob seeds, then the object was said to weigh one Roman pound.  If, on the other hand, the object's weight was equivalent to 144 carob seeds then the object was said to weigh one Roman ounce (uncia).  The Roman pound and ounce were both defined in terms of different sized collections of the same common mass standard, the carob seed.  The ratio of a Roman ounce (144 carob seeds) to a Roman pound (1728 carob seeds) was:
+
+In 1600 AD, Johannes Kepler sought employment with Tycho Brahe, who had some of the most precise astronomical data available.  Using Brahe's precise observations of the planet Mars, Kepler spent the next five years developing his own method for characterizing planetary motion. In 1609, Johannes Kepler published his three laws of planetary motion, explaining how the planets orbit the Sun. In Kepler's final planetary model, he described planetary orbits as following elliptical paths with the Sun at a focal point of the ellipse. Kepler discovered that the square of the orbital period of each planet is directly proportional to the cube of the semi-major axis of its orbit, or equivalently, that the ratio of these two values is constant for all planets in the Solar System.[note 4]
+
+On 25 August 1609, Galileo Galilei demonstrated his first telescope to a group of Venetian merchants, and in early January 1610, Galileo observed four dim objects near Jupiter, which he mistook for stars.  However, after a few days of observation, Galileo realized that these "stars" were in fact orbiting Jupiter.  These four objects (later named the Galilean moons in honor of their discoverer) were the first celestial bodies observed to orbit something other than the Earth or Sun.  Galileo continued to observe these moons over the next eighteen months, and by the middle of 1611 he had obtained remarkably accurate estimates for their periods.
+
+Sometime prior to 1638, Galileo turned his attention to the phenomenon of objects in free fall, attempting to characterize these motions. Galileo was not the first to investigate Earth's gravitational field, nor was he the first to accurately describe its fundamental characteristics.  However, Galileo's reliance on scientific experimentation to establish physical principles would have a profound effect on future generations of scientists. It is unclear if these were just hypothetical experiments used to illustrate a concept, or if they were real experiments performed by Galileo,[8] but the results obtained from these experiments were both realistic and compelling.  A biography by Galileo's pupil Vincenzo Viviani stated that Galileo had dropped balls of the same material, but different masses, from the Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass.[note 5] In support of this conclusion, Galileo had advanced the following theoretical argument: He asked if two bodies of different masses and different rates of fall are tied by a string, does the combined system fall faster because it is now more massive, or does the lighter body in its slower fall hold back the heavier body?  The only convincing resolution to this question is that all bodies must fall at the same rate.[9]
+
+A later experiment was described in Galileo's Two New Sciences published in 1638.  One of Galileo's fictional characters, Salviati, describes an experiment using a bronze ball and a wooden ramp.  The wooden ramp was "12 cubits long, half a cubit wide and three finger-breadths thick" with a straight, smooth, polished groove.  The groove was lined with "parchment, also smooth and polished as possible".  And into this groove was placed "a hard, smooth and very round bronze ball".  The ramp was inclined at various angles to slow the acceleration enough so that the elapsed time could be measured.  The ball was allowed to roll a known distance down the ramp, and the time taken for the ball to move the known distance was measured.  The time was measured using a water clock described as follows:
+
+Galileo found that for an object in free fall, the distance that the object has fallen is always proportional to the square of the elapsed time:
+
+Galileo had shown that objects in free fall under the influence of the Earth's gravitational field have a constant acceleration, and Galileo's contemporary, Johannes Kepler, had shown that the planets follow elliptical paths under the influence of the Sun's gravitational mass.  However, Galileo's free fall motions and Kepler's planetary motions remained distinct during Galileo's lifetime.
+
+Robert Hooke had published his concept of gravitational forces in 1674, stating that all celestial bodies have an attraction or gravitating power towards their own centers, and also attract all the other celestial bodies that are within the sphere of their activity. He further stated that gravitational attraction increases by how much nearer the body wrought upon is to their own center.[11] In correspondence with Isaac Newton from 1679 and 1680, Hooke conjectured that gravitational forces might decrease according to the double of the distance between the two bodies.[12] Hooke urged Newton, who was a pioneer in the development of calculus, to work through the mathematical details of Keplerian orbits to determine if Hooke's hypothesis was correct.  Newton's own investigations verified that Hooke was correct, but due to personal differences between the two men, Newton chose not to reveal this to Hooke.  Isaac Newton kept quiet about his discoveries until 1684, at which time he told a friend, Edmond Halley, that he had solved the problem of gravitational orbits, but had misplaced the solution in his office.[13] After being encouraged by Halley, Newton decided to develop his ideas about gravity and publish all of his findings.  In November 1684, Isaac Newton sent a document to Edmund Halley, now lost but presumed to have been titled De motu corporum in gyrum (Latin for "On the motion of bodies in an orbit").[14] Halley presented Newton's findings to the Royal Society of London, with a promise that a fuller presentation would follow.  Newton later recorded his ideas in a three book set, entitled Philosophiæ Naturalis Principia Mathematica (Latin: Mathematical Principles of Natural Philosophy).  The first was received by the Royal Society on 28 April 1685–86; the second on 2 March 1686–87; and the third on 6 April 1686–87.  The Royal Society published Newton's entire collection at their own expense in May 1686–87.[15]:31
+
+Isaac Newton had bridged the gap between Kepler's gravitational mass and Galileo's gravitational acceleration, resulting in the discovery of the following relationship which governed both of these:
+
+where g is the apparent acceleration of a body as it passes through a region of space where gravitational fields exist, μ is the gravitational mass (standard gravitational parameter) of the body causing gravitational fields, and R is the radial coordinate (the distance between the centers of the two bodies).
+
+By finding the exact relationship between a body's gravitational mass and its gravitational field, Newton provided a second method for measuring gravitational mass.  The mass of the Earth can be determined using Kepler's method (from the orbit of Earth's Moon), or it can be determined by measuring the gravitational acceleration on the Earth's surface, and multiplying that by the square of the Earth's radius.  The mass of the Earth is approximately three millionths of the mass of the Sun.  To date, no other accurate method for measuring gravitational mass has been discovered.[16]
+
+Newton's cannonball was a thought experiment used to bridge the gap between Galileo's gravitational acceleration and Kepler's elliptical orbits.  It appeared in Newton's 1728 book A Treatise of the System of the World.  According to Galileo's concept of gravitation, a dropped stone falls with constant acceleration down towards the Earth.  However, Newton explains that when a stone is thrown horizontally (meaning sideways or perpendicular to Earth's gravity) it follows a curved path.  "For a stone projected is by the pressure of its own weight forced out of the rectilinear path, which by the projection alone it should have pursued, and made to describe a curve line in the air; and through that crooked way is at last brought down to the ground.  And the greater the velocity is with which it is projected, the farther it goes before it falls to the Earth."[15]:513 Newton further reasons that if an object were "projected in an horizontal direction from the top of a high mountain" with sufficient velocity, "it would reach at last quite beyond the circumference of the Earth, and return to the mountain from which it was projected."[citation needed]
+
+In contrast to earlier theories (e.g. celestial spheres) which stated that the heavens were made of entirely different material, Newton's theory of mass was groundbreaking partly because it introduced universal gravitational mass: every object has gravitational mass, and therefore, every object generates a gravitational field.  Newton further assumed that the strength of each object's gravitational field would decrease according to the square of the distance to that object. If a large collection of small objects were formed into a giant spherical body such as the Earth or Sun, Newton calculated the collection would create a gravitational field proportional to the total mass of the body,[15]:397 and inversely proportional to the square of the distance to the body's center.[15]:221[note 6]
+
+For example, according to Newton's theory of universal gravitation, each carob seed produces a gravitational field.  Therefore, if one were to gather an immense number of carob seeds and form them into an enormous sphere, then the gravitational field of the sphere would be proportional to the number of carob seeds in the sphere.  Hence, it should be theoretically possible to determine the exact number of carob seeds that would be required to produce a gravitational field similar to that of the Earth or Sun.  In fact, by unit conversion it is a simple matter of abstraction to realize that any traditional mass unit can theoretically be used to measure gravitational mass.
+
+Measuring gravitational mass in terms of traditional mass units is simple in principle, but extremely difficult in practice.  According to Newton's theory all objects produce gravitational fields and it is theoretically possible to collect an immense number of small objects and form them into an enormous gravitating sphere.  However, from a practical standpoint, the gravitational fields of small objects are extremely weak and difficult to measure. Newton's books on universal gravitation were published in the 1680s, but the first successful measurement of the Earth's mass in terms of traditional mass units, the Cavendish experiment, did not occur until 1797, over a hundred years later. Cavendish found that the Earth's density was 5.448 ± 0.033 times that of water.  As of 2009, the Earth's mass in kilograms is only known to around five digits of accuracy, whereas its gravitational mass is known to over nine significant figures.[clarification needed]
+
+Given two objects A and B, of masses MA and MB, separated by a displacement RAB, Newton's law of gravitation states that each object exerts a gravitational force on the other, of magnitude
+
+where G is the universal gravitational constant. The above statement may be reformulated in the following way: if g is the magnitude at a given location in a gravitational field, then the gravitational force on an object with gravitational mass M is
+
+This is the basis by which masses are determined by weighing. In simple spring scales, for example, the force F is proportional to the displacement of the spring beneath the weighing pan, as per Hooke's law, and the scales are calibrated to take g into account, allowing the mass M to be read off. Assuming the gravitational field is equivalent on both sides of the balance, a balance measures relative weight, giving the relative gravitation mass of each object.
+
+Inertial mass is the mass of an object measured by its resistance to acceleration. This definition has been championed by Ernst Mach[17][18] and has since been developed into the notion of operationalism by Percy W. Bridgman.[19][20] The simple classical mechanics definition of mass is slightly different than the definition in the theory of special relativity, but the essential meaning is the same.
+
+In classical mechanics, according to Newton's second law, we say that a body has a mass m if, at any instant of time, it obeys the equation of motion
+
+where F is the resultant force acting on the body and a is the acceleration of the body's centre of mass.[note 7] For the moment, we will put aside the question of what "force acting on the body" actually means.
+
+This equation illustrates how mass relates to the inertia of a body. Consider two objects with different masses. If we apply an identical force to each, the object with a bigger mass will experience a smaller acceleration, and the object with a smaller mass will experience a bigger acceleration. We might say that the larger mass exerts a greater "resistance" to changing its state of motion in response to the force.
+
+However, this notion of applying "identical" forces to different objects brings us back to the fact that we have not really defined what a force is. We can sidestep this difficulty with the help of Newton's third law, which states that if one object exerts a force on a second object, it will experience an equal and opposite force. To be precise, suppose we have two objects of constant inertial masses m1 and m2. We isolate the two objects from all other physical influences, so that the only forces present are the force exerted on m1 by m2, which we denote F12, and the force exerted on m2 by m1, which we denote F21. Newton's second law states that
+
+where a1 and a2 are the accelerations of m1 and m2, respectively. Suppose that these accelerations are non-zero, so that the forces between the two objects are non-zero. This occurs, for example, if the two objects are in the process of colliding with one another. Newton's third law then states that
+
+and thus
+
+If |a1| is non-zero, the fraction is well-defined, which allows us to measure the inertial mass of m1. In this case, m2 is our "reference" object, and we can define its mass m as (say) 1 kilogram. Then we can measure the mass of any other object in the universe by colliding it with the reference object and measuring the accelerations.
+
+Additionally, mass relates a body's momentum p to its linear velocity v:
+
+and the body's kinetic energy K to its velocity:
+
+The primary difficulty with Mach's definition of mass is that it fails to take into account the potential energy (or binding energy) needed to bring two masses sufficiently close to one another to perform the measurement of mass.[18] This is most vividly demonstrated by comparing the mass of the proton in the nucleus of deuterium, to the mass of the proton in free space (which is greater by about 0.239%—this is due to the binding energy of deuterium.). Thus, for example, if the reference weight m2 is taken to be the mass of the neutron in free space, and the relative accelerations for the proton and neutron in deuterium are computed, then the above formula over-estimates the mass m1 (by 0.239%) for the proton in deuterium.  At best, Mach's formula can only be used to obtain ratios of masses, that is, as m1 /m2 = |a2| / |a1|.  An additional difficulty was pointed out by Henri Poincaré, which is that the measurement of instantaneous acceleration is impossible: unlike the measurement of time or distance, there is no way to measure acceleration with a single measurement; one must make multiple measurements (of position, time, etc.) and perform a computation to obtain the acceleration.  Poincaré termed this to be an "insurmountable flaw" in the Mach definition of mass.[21]
+
+Typically, the mass of objects is measured in relation to that of the kilogram, which is defined as the mass of the international prototype of the kilogram (IPK), a platinum alloy cylinder stored in an environmentally-monitored safe secured in a vault at the International Bureau of Weights and Measures in France. However, the IPK is not convenient for measuring the masses of atoms and particles of similar scale, as it contains trillions of trillions of atoms, and has most certainly lost or gained a little mass over time despite the best efforts to prevent this. It is much easier to precisely compare an atom's mass to that of another atom, thus scientists developed the atomic mass unit (or Dalton). By definition, 1 u is exactly one twelfth of the mass of a carbon-12 atom, and by extension a carbon-12 atom has a mass of exactly 12 u.  This definition, however, might be changed by the proposed redefinition of SI base units, which will leave the Dalton very close to one, but no longer exactly equal to it.[22][23]
+
+In some frameworks of special relativity, physicists have used different definitions of the term. In these frameworks, two kinds of mass are defined: rest mass (invariant mass),[note 8] and relativistic mass (which increases with velocity). Rest mass is the Newtonian mass as measured by an observer moving along with the object.  Relativistic mass is the total quantity of energy in a body or system divided by c2. The two are related by the following equation:
+
+where 
+
+
+
+γ
+
+
+{\displaystyle \gamma }
+
+ is the Lorentz factor:
+
+The invariant mass of systems is the same for observers in all inertial frames, while the relativistic mass depends on the observer's frame of reference. In order to formulate the equations of physics such that mass values do not change between observers, it is convenient to use rest mass. The rest mass of a body is also related to its energy E and the magnitude of its momentum p by the relativistic energy-momentum equation:
+
+So long as the system is closed with respect to mass and energy, both kinds of mass are conserved in any given frame of reference. The conservation of mass holds even as some types of particles are converted to others. Matter particles (such as atoms) may be converted to non-matter particles (such as photons of light), but this does not affect the total amount of mass or energy. Although things like heat may not be matter, all types of energy still continue to exhibit mass.[note 9][24] Thus, mass and energy do not change into one another in relativity; rather, both are names for the same thing, and neither mass nor energy appear without the other.
+
+Both rest and relativistic mass can be expressed as an energy by applying the well-known relationship E = mc2, yielding rest energy and "relativistic energy" (total system energy) respectively:
+
+The "relativistic" mass and energy concepts are related to their "rest" counterparts, but they do not have the same value as their rest counterparts in systems where there is a net momentum. Because the relativistic mass is proportional to the energy, it has gradually fallen into disuse among physicists.[25] There is disagreement over whether the concept remains useful pedagogically.[26][27][28]
+
+In bound systems, the binding energy must often be subtracted from the mass of the unbound system, because binding energy commonly leaves the system at the time it is bound. The mass of the system changes in this process merely because the system was not closed during the binding process, so the energy escaped. For example, the binding energy of atomic nuclei is often lost in the form of gamma rays when the nuclei are formed, leaving nuclides which have less mass than the free particles (nucleons) of which they are composed.
+
+Mass–energy equivalence also holds in macroscopic systems.[29]  For example, if one takes exactly one kilogram of ice, and applies heat, the mass of the resulting melt-water will be more than a kilogram: it will include the mass from the thermal energy (latent heat) used to melt the ice; this follows from the conservation of energy.[30] This number is small but not negligible: about 3.7 nanograms. It is given by the latent heat of melting ice (334 kJ/kg) divided by the speed of light squared (c2 = 9×1016 m2/s2).
+
+In general relativity, the equivalence principle is the equivalence of gravitational and inertial mass. At the core of this assertion is Albert Einstein's idea that the gravitational force as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (i.e. accelerated) frame of reference.
+
+However, it turns out that it is impossible to find an objective general definition for the concept of invariant mass in general relativity. At the core of the problem is the non-linearity of the Einstein field equations, making it impossible to write the gravitational field energy as part of the stress–energy tensor in a way that is invariant for all observers. For a given observer, this can be achieved by the stress–energy–momentum pseudotensor.[31]
+
+In classical mechanics, the inert mass of a particle appears in the Euler–Lagrange equation as a parameter m:
+
+After quantization, replacing the position vector x with a wave function, the parameter m appears in the kinetic energy operator:
+
+In the ostensibly covariant (relativistically invariant) Dirac equation, and in natural units, this becomes:
+
+where the "mass" parameter m is now simply a constant associated with the quantum described by the wave function ψ.
+
+In the Standard Model of particle physics as developed in the 1960s, this term arises from the coupling of the field  ψ to an additional field Φ, the Higgs field. In the case of fermions, the Higgs mechanism results in the replacement of the term mψ in the Lagrangian with 
+
+
+
+
+G
+
+ψ
+
+
+
+
+ψ
+¯
+
+
+ϕ
+ψ
+
+
+{\displaystyle G_{\psi }{\overline {\psi }}\phi \psi }
+
+. This shifts the explanandum of the value for the mass of each elementary particle to the value of the unknown couplings Gψ.
+
+A tachyonic field, or simply tachyon, is a quantum field with an imaginary mass.[32] Although tachyons (particles that move faster than light) are a purely hypothetical concept not generally believed to exist,[32][33] fields with imaginary mass have come to play an important role in modern physics[34][34][35][36] and are discussed in popular books on physics.[32][37] Under no circumstances do any excitations ever propagate faster than light in such theories—the presence or absence of a tachyonic mass has no effect whatsoever on the maximum velocity of signals (there is no violation of causality).[38] While the field may have imaginary mass, any physical particles do not; the "imaginary mass" shows that the system becomes unstable, and sheds the instability by undergoing a type of phase transition called tachyon condensation (closely related to second order phase transitions) that results in symmetry breaking in current models of particle physics.
+
+The term "tachyon" was coined by Gerald Feinberg in a 1967 paper,[39] but it was soon realized that Feinberg's model in fact did not allow for superluminal speeds.[38] Instead, the imaginary mass creates an instability in the configuration:- any configuration in which one or more field excitations are tachyonic will spontaneously decay, and the resulting configuration contains no physical tachyons.  This process is known as tachyon condensation.  Well known examples include the condensation of the Higgs boson in particle physics, and ferromagnetism in condensed matter physics.
+
+Although the notion of a tachyonic imaginary mass might seem troubling because there is no classical interpretation of an imaginary mass, the mass is not quantized.  Rather, the scalar field is; even for tachyonic quantum fields, the field operators at spacelike separated points still commute (or anticommute), thus preserving causality. Therefore, information still does not propagate faster than light,[39]  and solutions grow exponentially, but not superluminally (there is no violation of causality). Tachyon condensation drives a physical system that has reached a local limit and might naively be expected to produce physical tachyons, to an alternate stable state where no physical tachyons exist. Once the tachyonic field reaches the minimum of the potential, its quanta are not tachyons any more but rather are ordinary particles with a positive mass-squared.[40]
+
+This is a special case of the general rule, where unstable massive particles are formally described as having a complex mass, with the real part being their mass in the usual sense, and the imaginary part being the decay rate in natural units.[40] However, in quantum field theory, a particle (a "one-particle state") is roughly defined as a state which is constant over time; i.e., an eigenvalue of the Hamiltonian. An unstable particle is a state which is only approximately constant over time; If it exists long enough to be measured, it can be formally described as having a complex mass, with the real part of the mass greater than its imaginary part. If both parts are of the same magnitude, this is interpreted as a resonance appearing in a scattering process rather than a particle, as it is considered not to exist long enough to be measured independently of the scattering process. In the case of a tachyon the real part of the mass is zero, and hence no concept of a particle can be attributed to it.
+
+In a Lorentz invariant theory, the same formulas that apply to ordinary slower-than-light particles (sometimes called "bradyons" in discussions of tachyons) must also apply to tachyons. In particular the energy–momentum relation:
+
+(where p is the relativistic momentum of the bradyon and m is its rest mass) should still apply, along with the formula for the total energy of a particle:
+
+This equation shows that the total energy of a particle (bradyon or tachyon) contains a contribution from its rest mass (the "rest mass–energy") and a contribution from its motion, the kinetic energy.
+When v is larger than c, the denominator in the equation for the energy is "imaginary", as the value under the radical is negative. Because the total energy must be real, the numerator must also be imaginary:  i.e. the rest mass m must be imaginary, as a pure imaginary number divided by another pure imaginary number is a real number.
+
+The negative mass exists in the model to describe dark energy (phantom energy) and radiation in negative-index metamaterial in a unified way.[41] In this way, the negative mass is associated with negative momentum, negative pressure, negative kinetic energy and FTL (faster-than-light).

en/2512.html.txt

@@ -0,0 +1,63 @@
+
+
+The litre (British and Commonwealth spelling) or liter (American spelling) (SI symbols L and l,[1] other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre. A cubic decimetre (or litre) occupies a volume of 10 cm × 10 cm × 10 cm (see figure) and is thus equal to one-thousandth of a cubic metre.
+
+The original French metric system used the litre as a base unit. The word litre is derived from an older French unit, the litron, whose name came from Greek—where it was a unit of weight, not volume[2]—via Latin, and which equalled approximately 0.831 litres. The litre was also used in several subsequent versions of the metric system and is accepted for use with the SI,[3] although not an SI unit—the SI unit of volume is the cubic metre (m3). The spelling used by the International Bureau of Weights and Measures is "litre",[3] a spelling which is shared by almost all English-speaking countries. The spelling "liter" is predominantly used in American English.[a]
+
+One litre of liquid water has a mass of almost exactly one kilogram, because the kilogram was originally defined in 1795 as the mass of one cubic decimetre of water at the temperature of melting ice (0 °C). Subsequent redefinitions of the metre and kilogram mean that this relationship is no longer exact.[4]
+
+A litre is a cubic decimetre, which is the volume of a cube 10 centimetres × 10 centimetres × 10 centimetres (1 L ≡ 1 dm3 ≡ 1000 cm3). Hence 1 L ≡ 0.001 m3 ≡ 1000 cm3, and 1 m3 (i.e. a cubic metre, which is the SI unit for volume) is exactly 1000 L.
+
+From 1901 to 1964, the litre was defined as the volume of one kilogram of pure water at maximum density (+4 °C) and standard pressure. The kilogram was in turn specified as the mass of the International Prototype of the Kilogram (a specific platinum/iridium cylinder) and was intended to be of the same mass as the 1 litre of water referred to above. It was subsequently discovered that the cylinder was around 28 parts per million too large and thus, during this time, a litre was about 1.000028 dm3. Additionally, the mass–volume relationship of water (as with any fluid) depends on temperature, pressure, purity and isotopic uniformity. In 1964, the definition relating the litre to mass was superseded by the current one. Although the litre is not an SI unit, it is accepted by the CGPM (the standards body that defines the SI) for use with the SI. CGPM defines the litre and its acceptable symbols.
+
+A litre is equal in volume to the millistere, an obsolete non-SI metric unit customarily used for dry measure.
+
+Litres are most commonly used for items (such as fluids and solids that can be poured) which are measured by the capacity or size of their container, whereas cubic metres (and derived units) are most commonly used for items measured either by their dimensions or their displacements. The litre is often also used in some calculated measurements, such as density (kg/L), allowing an easy comparison with the density of water.
+
+One litre of water has a mass of almost exactly one kilogram when measured at its maximal density, which occurs at about 4 °C. It follows, therefore, that 1000th of a litre, known as one millilitre (1 mL), of water has a mass of about 1 g; 1000 litres of water has a mass of about 1000 kg (1 tonne). This relationship holds because the gram was originally defined as the mass of 1 mL of water; however, this definition was abandoned in 1799 because the density of water changes with temperature and, very slightly, with pressure.
+
+It is now known that the density of water also depends on the isotopic ratios of the oxygen and hydrogen atoms in a particular sample. Modern measurements of Vienna Standard Mean Ocean Water, which is pure distilled water with an isotopic composition representative of the average of the world's oceans, show that it has a density of 0.999975±0.000001 kg/L at its point of maximum density (3.984 °C) under one standard atmosphere (760 Torr = 101.325 kPa) of pressure.[5]
+
+The litre, though not an official SI unit, may be used with SI prefixes. The most commonly used derived unit is the millilitre, defined as one-thousandth of a litre, and also often referred to by the SI derived unit name "cubic centimetre". It is a commonly used measure, especially in medicine, cooking and automotive engineering. Other units may be found in the table below, where the more often used terms are in bold. However, some authorities advise against some of them; for example, in the United States, NIST advocates using the millilitre or litre instead of the centilitre.[6] There are two international standard symbols for the litre: L and l. In the United States the former is preferred because of the risk that (in some fonts) the letter l and the digit 1 may be confused.[7]
+
+One litre is slightly larger than a US liquid quart and slightly less than an imperial quart or one US dry quart. A mnemonic for its volume relative to an imperial pint is "a litre of water's a pint and three quarters"; this is very close, as a litre is actually 1.75975399 pints.
+
+A litre is the volume of a cube with sides of 10 cm, which is slightly less than a cube of sides 4 inches (one-third of a foot). One cubic foot would contain exactly 27 such cubes (four inches on each side), making one cubic foot approximately equal to 27 litres. One cubic foot has an exact volume of 28.316846592 litres, which is 4.88% higher than the 27-litre approximation.
+
+A litre of liquid water has a mass almost exactly equal to one kilogram. An early definition of the kilogram was set as the mass of one litre of water. Because volume changes with temperature and pressure, and pressure uses units of mass, the definition of a kilogram was changed. At standard pressure, one litre of water has a mass of 0.999975 kg at 4 °C, and 0.997 kg at 25 °C.[8]
+
+Originally, the only symbol for the litre was l (lowercase letter L), following the SI convention that only those unit symbols that abbreviate the name of a person start with a capital letter. In many English-speaking countries, however, the most common shape of a handwritten Arabic digit 1 is just a vertical stroke; that is, it lacks the upstroke added in many other cultures. Therefore, the digit "1" may easily be confused with the letter "l". In some computer typefaces, the two characters are barely distinguishable. This caused some concern, especially in the medical community.[citation needed]
+
+As a result, L (uppercase letter L) was adopted as an alternative symbol for litre in 1979.[citation needed] The United States National Institute of Standards and Technology now recommends the use of the uppercase letter L,[9] a practice that is also widely followed in Canada and Australia. In these countries, the symbol L is also used with prefixes, as in mL and μL, instead of the traditional ml and μl used in Europe. In the UK and Ireland, as well as the rest of Europe, lowercase l is used with prefixes, though whole litres are often written in full (so, "750 ml" on a wine bottle, but often "1 litre" on a juice carton). In 1990, the International Committee for Weights and Measures stated that it was too early to choose a single symbol for the litre.[10]
+
+Prior to 1979, the symbol ℓ came into common use in some countries;[citation needed] for example, it was recommended by South African Bureau of Standards publication M33 and Canada in the 1970s. This symbol can still be encountered occasionally in some English-speaking and European countries like Germany, and its use is ubiquitous in Japan and South Korea.
+
+Fonts covering the CJK characters usually include not only the script small ℓ but also four precomposed characters: ㎕, ㎖, ㎗ and ㎘ for the microlitre, millilitre, decilitre and kilolitre.
+
+The first name of the litre was "cadil"; standards are shown at the Musée des Arts et Métiers in Paris.[12]
+
+The litre was introduced in France in 1795 as one of the new "republican units of measurement" and defined as one cubic decimetre.[13]
+One litre of liquid water has a mass of almost exactly one kilogram, due to the gram being defined in 1795 as one cubic centimetre of water at the temperature of melting ice.[4]
+The original decimetre length was 44.344 lignes, which was revised in 1798 to 44.3296 lignes. This made the original litre 1.000974 of today's cubic decimetre. It was against this litre that the kilogram was constructed.
+
+In 1879, the CIPM adopted the definition of the litre, with the symbol l (lowercase letter L).
+
+In 1901, at the 3rd CGPM conference, the litre was redefined as the space occupied by 1 kg of pure water at the temperature of its maximum density (3.98 °C) under a pressure of 1 atm. This made the litre equal to about 1.000028 dm3 (earlier reference works usually put it at 1.000027 dm3).
+
+In 1964, at the 12th CGPM conference, the original definition was reverted to, and thus the litre was once again defined in exact relation to the metre, as another name for the cubic decimetre, that is, exactly 1 dm3.[14]
+
+In 1979, at the 16th CGPM conference, the alternative symbol L (uppercase letter L) was adopted. It also expressed a preference that in the future only one of these two symbols should be retained, but in 1990 said it was still too early to do so.[10]
+
+In spoken English, the symbol "mL" (for millilitre) can be pronounced as "mil". This can potentially cause confusion with some other measurement words such as:
+
+However the context is usually sufficient hint — "mL" is a unit of volume; whereas the others are units of linear or angular measurement.
+
+The abbreviation "cc" (for cubic centimetre, equal to a millilitre or mL) is a unit of the cgs system, which preceded the MKS system, which later evolved into the SI system. The abbreviation "cc" is still commonly used in many fields, including medical dosage and sizing for combustion engine displacement.
+
+The microlitre (μL) has been known in the past as the lambda (λ), but this usage is now discouraged.[15] In the medical field the microlitre is sometimes abbreviated as mcL on test results.[16]
+
+In the SI system, apart from prefixes for powers of 1000, use of the "centi" (10−2), "deci" (10−1), "deca" (10+1) and "hecto" (10+2) prefixes with litres is common. For example, in many European countries, the hectolitre is the typical unit for production and export volumes of beverages (milk, beer, soft drinks, wine, etc.) and for measuring the size of the catch and quotas for fishing boats; decilitres are common in Croatia, Switzerland and Scandinavia and often found in cookbooks, and restaurant and café menus; centilitres indicate the capacity of drinking glasses and of small bottles. In colloquial Dutch in Belgium, a "vijfentwintiger" and a "drieëndertiger" (literally "twenty-fiver" and "thirty-threer") are the common beer glasses, the corresponding bottles mention 25 cL and 33 cL. Bottles may also be 75 cL or half size at 37.5 cL for "artisanal" brews or 70 cL for wines or spirits. Cans come in 25 cL, 33 cL and 50 cL.[citation needed] Similarly, alcohol shots are often marked in cL in restaurant menus, typically 3 cL (1.06 imp fl oz; 1.01 US fl oz).
+
+In countries where the metric system was adopted as the official measuring system after the SI standard was established, common usage eschews prefixes that are not powers of 1000. For example, in Canada, Australia, and New Zealand, consumer beverages are labelled almost exclusively using litres and millilitres. Hectolitres sometimes appear in industry, but centilitres and decilitres are rarely, if ever, used.[citation needed] An exception is in pathology, where for instance blood lead level may be measured in micrograms per decilitre.[citation needed] Larger volumes are usually given in cubic metres (equivalent to 1 kL), or thousands or millions of cubic metres.[citation needed]
+
+Although kilolitres, megalitres, and gigalitres are commonly used for measuring water consumption, reservoir capacities and river flows, for larger volumes of fluids, such as annual consumption of tap water, lorry (truck) tanks, or swimming pools, the cubic metre is the general unit. It is also generally for all volumes of a non-liquid nature.[citation needed]

en/2513.html.txt

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+Helen may refer to:

en/2514.html.txt

@@ -0,0 +1 @@
+Helen may refer to:

en/2515.html.txt

@@ -0,0 +1 @@
+Helen may refer to:

en/2516.html.txt

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+
+
+
+
+
+
+A helicopter, sometimes referred to in slang as a "chopper" or "helo" (United States military usage, pronounced with a long "e"), is a type of rotorcraft in which lift and thrust are supplied by horizontally-spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward, and laterally. These attributes allow helicopters to be used in congested or isolated areas where fixed-wing aircraft and many forms of VTOL (Vertical TakeOff and Landing) aircraft cannot perform.
+
+The English word helicopter is adapted from the French word hélicoptère, coined by Gustave Ponton d'Amécourt in 1861, which originates from the Greek helix (ἕλιξ) "helix, spiral, whirl, convolution"[1] and pteron (πτερόν) "wing".[2][3][4][5] English language nicknames for helicopter include "chopper", "copter", "helo", "heli", and "whirlybird".
+
+Helicopters were developed and built during the first half-century of flight, with the Focke-Wulf Fw 61 being the first operational helicopter in 1936. Some helicopters reached limited production, but it was not until 1942 that a helicopter designed by Igor Sikorsky reached full-scale production,[6] with 131 aircraft built.[7] Though most earlier designs used more than one main rotor, it is the single main rotor with anti-torque tail rotor configuration that has become the most common helicopter configuration.  Tandem rotor helicopters are also in widespread use due to their greater payload capacity.  Coaxial helicopters, tiltrotor aircraft, and compound helicopters are all flying today.  Quadcopter helicopters were pioneered as early as 1907 in France, and other types of multicopter have been developed for specialized applications such as unmanned drones.
+
+A helicopter, sometimes referred to in slang as a "chopper", is a type of rotorcraft in which lift and thrust are supplied by one or more horizontally-spinning rotors. By contrast the autogyro (or gyroplane) and gyrodyne have a free-spinning rotor for all or part of the flight envelope, relying on a separate thrust system to propel the craft forwards, so that the airflow sets the rotor spinning to provide lift. The compound helicopter also has a separate thrust system, but continues to supply power to the rotor throughout normal flight.
+
+The rotor system, or more simply rotor, is the rotating part of a helicopter that generates lift. A rotor system may be mounted horizontally, as main rotors are, providing lift vertically, or it may be mounted vertically, such as a tail rotor, to provide horizontal thrust to counteract torque from the main rotors. The rotor consists of a mast, hub and rotor blades.
+
+The mast is a cylindrical metal shaft that extends upwards from the transmission. At the top of the mast is the attachment point for the rotor blades called the hub. Main rotor systems are classified according to how the rotor blades are attached and move relative to the hub. There are three basic types: hingeless, fully articulated, and teetering; although some modern rotor systems use a combination of these.
+
+Most helicopters have a single main rotor, but torque created by its aerodynamic drag must be countered by an opposed torque. The design that Igor Sikorsky settled on for his VS-300 was a smaller tail rotor. The tail rotor pushes or pulls against the tail to counter the torque effect, and this has become the most common configuration for helicopter design, usually at the end of a tail boom.
+
+Some helicopters use other anti-torque controls instead of the tail rotor, such as the ducted fan (called Fenestron or FANTAIL) and NOTAR. NOTAR provides anti-torque similar to the way a wing develops lift through the use of the Coandă effect on the tail boom.[8]
+
+The use of two or more horizontal rotors turning in opposite directions is another configuration used to counteract the effects of torque on the aircraft without relying on an anti-torque tail rotor. This allows the power normally required to drive the tail rotor to be applied to the main rotors, increasing the aircraft's lifting capacity. There are several common configurations that use the counter-rotating effect to benefit the rotorcraft:
+
+Tip jet designs let the rotor push itself through the air and avoid generating torque.[9]
+
+The number, size and type of engine(s) used on a helicopter determines the size, function and capability of that helicopter design. The earliest helicopter engines were simple mechanical devices, such as rubber bands or spindles, which relegated the size of helicopters to toys and small models. For a half century before the first airplane flight, steam engines were used to forward the development of the understanding of helicopter aerodynamics, but the limited power did not allow for manned flight. The introduction of the internal combustion engine at the end of the 19th century became the watershed for helicopter development as engines began to be developed and produced that were powerful enough to allow for helicopters able to lift humans.[citation needed]
+
+Early helicopter designs utilized custom-built engines or rotary engines designed for airplanes, but these were soon replaced by more powerful automobile engines and radial engines. The single, most-limiting factor of helicopter development during the first half of the 20th century was that the amount of power produced by an engine was not able to overcome the engine's weight in vertical flight. This was overcome in early successful helicopters by using the smallest engines available. When the compact, flat engine was developed, the helicopter industry found a lighter-weight powerplant easily adapted to small helicopters, although radial engines continued to be used for larger helicopters.[citation needed]
+
+Turbine engines revolutionized the aviation industry; and the turboshaft engine for helicopter use, pioneered in December 1951 by the aforementioned Kaman K-225, finally gave helicopters an engine with a large amount of power and a low weight penalty. Turboshafts are also more reliable than piston engines, especially when producing the sustained high levels of power required by a helicopter. The turboshaft engine was able to be scaled to the size of the helicopter being designed, so that all but the lightest of helicopter models are powered by turbine engines today.[citation needed]
+
+Special jet engines developed to drive the rotor from the rotor tips are referred to as tip jets. Tip jets powered by a remote compressor are referred to as cold tip jets, while those powered by combustion exhaust are referred to as hot tip jets. An example of a cold jet helicopter is the Sud-Ouest Djinn, and an example of the hot tip jet helicopter is the YH-32 Hornet.[citation needed]
+
+Some radio-controlled helicopters and smaller, helicopter-type unmanned aerial vehicles, use electric motors. Radio-controlled helicopters may also have piston engines that use fuels other than gasoline, such as nitromethane. Some turbine engines commonly used in helicopters can also use biodiesel instead of jet fuel.[10][11]
+
+There are also human-powered helicopters.
+
+A helicopter has four flight control inputs. These are the cyclic, the collective, the anti-torque pedals, and the throttle. The cyclic control is usually located between the pilot's legs and is commonly called the cyclic stick or just cyclic. On most helicopters, the cyclic is similar to a joystick. However, the Robinson R22 and Robinson R44 have a unique teetering bar cyclic control system and a few helicopters have a cyclic control that descends into the cockpit from overhead.
+
+The control is called the cyclic because it changes cyclic pitch of the main blades. The result is to tilt the rotor disk in a particular direction, resulting in the helicopter moving in that direction. If the pilot pushes the cyclic forward, the rotor disk tilts forward, and the rotor produces a thrust in the forward direction. If the pilot pushes the cyclic to the side, the rotor disk tilts to that side and produces thrust in that direction, causing the helicopter to hover sideways.
+
+The collective pitch control or collective is located on the left side of the pilot's seat with a settable friction control to prevent inadvertent movement. The collective changes the pitch angle of all the main rotor blades collectively (i.e. all at the same time) and independently of their position. Therefore, if a collective input is made, all the blades change equally, and the result is the helicopter increasing or decreasing in altitude.
+
+A swashplate controls the collective and cyclic pitch of the main blades. The swashplate moves up and down, along the main shaft, to change the pitch of both blades. This causes the helicopter to push air downward or upward, depending on the angle of attack. The swashplate can also change its angle to move the blades angle forwards or backwards, or left and right, to make the helicopter move in those directions.
+
+The anti-torque pedals are located in the same position as the rudder pedals in a fixed-wing aircraft, and serve a similar purpose, namely to control the direction in which the nose of the aircraft is pointed. Application of the pedal in a given direction changes the pitch of the tail rotor blades, increasing or reducing the thrust produced by the tail rotor and causing the nose to yaw in the direction of the applied pedal. The pedals mechanically change the pitch of the tail rotor altering the amount of thrust produced.
+
+Helicopter rotors are designed to operate in a narrow range of RPM.[12][13][14][15][16] The throttle controls the power produced by the engine, which is connected to the rotor by a fixed ratio transmission. The purpose of the throttle is to maintain enough engine power to keep the rotor RPM within allowable limits so that the rotor produces enough lift for flight. In single-engine helicopters, the throttle control is a motorcycle-style twist grip mounted on the collective control, while dual-engine helicopters have a power lever for each engine.
+
+A compound helicopter has an additional system for thrust and, typically, small stub fixed wings. This offloads the rotor in cruise, which allows its rotation to be slowed down, thus increasing the maximum speed of the aircraft. The Lockheed AH-56A Cheyenne diverted up to 90% of its engine power to a pusher propeller during forward flight.[17]
+
+There are three basic flight conditions for a helicopter: hover, forward flight and the transition between the two.
+
+Hovering is the most challenging part of flying a helicopter. This is because a helicopter generates its own gusty air while in a hover, which acts against the fuselage and flight control surfaces. The end result is constant control inputs and corrections by the pilot to keep the helicopter where it is required to be.[18] Despite the complexity of the task, the control inputs in a hover are simple. The cyclic is used to eliminate drift in the horizontal plane, that is to control forward and back, right and left. The collective is used to maintain altitude. The pedals are used to control nose direction or heading. It is the interaction of these controls that makes hovering so difficult, since an adjustment in any one control requires an adjustment of the other two, creating a cycle of constant correction.
+
+As a helicopter moves from hover to forward flight it enters a state called translational lift which provides extra lift without increasing power. This state, most typically, occurs when the airspeed reaches approximately 16–24 knots (30–44 km/h; 18–28 mph), and may be necessary for a helicopter to obtain flight.
+
+In forward flight a helicopter's flight controls behave more like those of a fixed-wing aircraft. Displacing the cyclic forward will cause the nose to pitch down, with a resultant increase in airspeed and loss of altitude. Aft cyclic will cause the nose to pitch up, slowing the helicopter and causing it to climb. Increasing collective (power) while maintaining a constant airspeed will induce a climb while decreasing collective will cause a descent. Coordinating these two inputs, down collective plus aft cyclic or up collective plus forward cyclic, will result in airspeed changes while maintaining a constant altitude. The pedals serve the same function in both a helicopter and a fixed-wing aircraft, to maintain balanced flight. This is done by applying a pedal input in whichever direction is necessary to center the ball in the turn and bank indicator.
+
+Due to the operating characteristics of the helicopter—its ability to take off and land vertically, and to hover for extended periods of time, as well as the aircraft's handling properties under low airspeed conditions—it has been chosen to conduct tasks that were previously not possible with other aircraft, or were time- or work-intensive to accomplish on the ground. Today, helicopter uses include transportation of people and cargo, military uses, construction, firefighting, search and rescue, tourism, medical transport, law enforcement, agriculture, news and media, and aerial observation, among others.[19]
+They can be used for Reflection seismology or recreation.
+
+A helicopter used to carry loads connected to long cables or slings is called an aerial crane. Aerial cranes are used to place heavy equipment, like radio transmission towers and large air conditioning units, on the tops of tall buildings, or when an item must be raised up in a remote area, such as a radio tower raised on the top of a hill or mountain. Helicopters are used as aerial cranes in the logging industry to lift trees out of terrain where vehicles cannot travel and where environmental concerns prohibit the building of roads.[20] These operations are referred to as longline because of the long, single sling line used to carry the load.[21]
+
+The largest single non-combat helicopter operation in history was the disaster management operation following the 1986 Chernobyl nuclear disaster. Hundreds of pilots were involved in airdrop and observation missions, making dozens of sorties a day for several months.
+
+"Helitack" is the use of helicopters to combat wildland fires.[22] The helicopters are used for aerial firefighting (water bombing) and may be fitted with tanks or carry helibuckets. Helibuckets, such as the Bambi bucket, are usually filled by submerging the bucket into lakes, rivers, reservoirs, or portable tanks. Tanks fitted onto helicopters are filled from a hose while the helicopter is on the ground or water is siphoned from lakes or reservoirs through a hanging snorkel as the helicopter hovers over the water source. Helitack helicopters are also used to deliver firefighters, who rappel down to inaccessible areas, and to resupply firefighters. Common firefighting helicopters include variants of the Bell 205 and the Erickson S-64 Aircrane helitanker.
+
+Helicopters are used as air ambulances for emergency medical assistance in situations when an ambulance cannot easily or quickly reach the scene, or cannot transport the patient to a medical facility in time. Helicopters are also used when patients need to be transported between medical facilities and air transportation is the most practical method. An air ambulance helicopter is equipped to stabilize and provide limited medical treatment to a patient while in flight. The use of helicopters as air ambulances is often referred to as "MEDEVAC", and patients are referred to as being "airlifted", or "medevaced". This use was pioneered in the Korean War, when time to reach a medical facility was reduced to three hours from the eight hours needed in World War II, and further reduced to two hours by the Vietnam War.[23]
+
+Police departments and other law enforcement agencies use helicopters to pursue suspects. Since helicopters can achieve a unique aerial view, they are often used in conjunction with police on the ground to report on suspects' locations and movements. They are often mounted with lighting and heat-sensing equipment for night pursuits.
+
+Military forces use attack helicopters to conduct aerial attacks on ground targets. Such helicopters are mounted with missile launchers and miniguns. Transport helicopters are used to ferry troops and supplies where the lack of an airstrip would make transport via fixed-wing aircraft impossible. The use of transport helicopters to deliver troops as an attack force on an objective is referred to as "air assault". Unmanned aerial systems (UAS) helicopter systems of varying sizes are developed by companies for military reconnaissance and surveillance duties. Naval forces also use helicopters equipped with dipping sonar for anti-submarine warfare, since they can operate from small ships.
+
+Oil companies charter helicopters to move workers and parts quickly to remote drilling sites located at sea or in remote locations. The speed advantage over boats makes the high operating cost of helicopters cost-effective in ensuring that oil platforms continue to operate. Various companies specialize in this type of operation.
+
+NASA is developing the Mars Helicopter, a 1.8 kg (4.0 lb) helicopter to be launched to survey Mars (along with a rover) in 2020. Given that the Martian atmosphere is 100 times thinner than that of Earth's, its two blades will spin at close to 3,000 revolutions a minute, approximately 10 times faster than that of a terrestrial helicopter.[24]
+
+In 2017, 926 civil helicopters were shipped for $3.68 Billion, led by Airbus Helicopters with $1.87 Billion for 369 rotorcraft, Leonardo Helicopters with $806 Million for 102 (first three-quarters only), Bell Helicopter with $696 Million for 132, then Robinson Helicopter with $161 Million for 305.[25]
+
+By October 2018, the in-service and stored helicopter fleet of 38,570 with civil or government operators was led Robinson Helicopter with 24.7% followed by Airbus Helicopters with 24.4%, then Bell with 20.5 and Leonardo with 8.4%, Russian Helicopters with 7.7%, Sikorsky Aircraft with 7.2%, MD Helicopters with 3.4% and other with 2.2%.
+The most widespread model is the piston Robinson R44 with 5,600, then the H125/AS350 with 3,600 units, followed by the Bell 206 with 3,400.
+Most were in North America with 34.3% then in Europe with 28.0% followed by Asia-Pacific with 18.6%, Latin America with 11.6%, Africa with 5.3% and Middle East with 1.7%.[26]
+
+The earliest references for vertical flight came from China. Since around 400 BC,[27] Chinese children have played with bamboo flying toys (or Chinese top).[28][29][30] This bamboo-copter is spun by rolling a stick attached to a rotor. The spinning creates lift, and the toy flies when released.[27] The 4th-century AD Daoist book Baopuzi by Ge Hong (抱朴子 "Master who Embraces Simplicity") reportedly describes some of the ideas inherent to rotary wing aircraft.[31]
+
+Designs similar to the Chinese helicopter toy appeared in some Renaissance paintings and other works.[32] In the 18th and early 19th centuries Western scientists developed flying machines based on the Chinese toy.[33]
+
+It was not until the early 1480s, when Italian polymath Leonardo da Vinci created a design for a machine that could be described as an "aerial screw", that any recorded advancement was made towards vertical flight. His notes suggested that he built small flying models, but there were no indications for any provision to stop the rotor from making the craft rotate.[34][35] As scientific knowledge increased and became more accepted, people continued to pursue the idea of vertical flight.
+
+In July 1754, Russian Mikhail Lomonosov had developed a small coaxial modeled after the Chinese top but powered by a wound-up spring device[33] and demonstrated it to the Russian Academy of Sciences. It was powered by a spring, and was suggested as a method to lift meteorological instruments. In 1783, Christian de Launoy, and his mechanic, Bienvenu, used a coaxial version of the Chinese top in a model consisting of contrarotating turkey flight feathers[33] as rotor blades, and in 1784, demonstrated it to the French Academy of Sciences. Sir George Cayley, influenced by a childhood fascination with the Chinese flying top, developed a model of feathers, similar to that of Launoy and Bienvenu, but powered by rubber bands. By the end of the century, he had progressed to using sheets of tin for rotor blades and springs for power. His writings on his experiments and models would become influential on future aviation pioneers.[34] Alphonse Pénaud would later develop coaxial rotor model helicopter toys in 1870, also powered by rubber bands. One of these toys, given as a gift by their father, would inspire the Wright brothers to pursue the dream of flight.[36]
+
+In 1861, the word "helicopter" was coined by Gustave de Ponton d'Amécourt, a French inventor who demonstrated a small steam-powered model. While celebrated as an innovative use of a new metal, aluminum, the model never lifted off the ground. D'Amecourt's linguistic contribution would survive to eventually describe the vertical flight he had envisioned. Steam power was popular with other inventors as well. In 1878 the Italian Enrico Forlanini's unmanned vehicle, also powered by a steam engine, rose to a height of 12 meters (39 feet), where it hovered for some 20 seconds after a vertical take-off. Emmanuel Dieuaide's steam-powered design featured counter-rotating rotors powered through a hose from a boiler on the ground.[34] In 1887 Parisian inventor, Gustave Trouvé, built and flew a tethered electric model helicopter.[citation needed]
+
+In July 1901, the maiden flight of Hermann Ganswindt's helicopter took place in Berlin-Schöneberg; this was probably the first heavier-than-air motor-driven flight carrying humans. A movie covering the event was taken by Max Skladanowsky, but it remains lost.[37]
+
+In 1885, Thomas Edison was given US$1,000 (equivalent to $28,000 today) by James Gordon Bennett, Jr., to conduct experiments towards developing flight. Edison built a helicopter and used the paper for a stock ticker to create guncotton, with which he attempted to power an internal combustion engine. The helicopter was damaged by explosions and one of his workers was badly burned. Edison reported that it would take a motor with a ratio of three to four pounds per horsepower produced to be successful, based on his experiments.[38] Ján Bahýľ, a Slovak inventor, adapted the internal combustion engine to power his helicopter model that reached a height of 0.5 meters (1.6 feet) in 1901. On 5 May 1905, his helicopter reached 4 meters (13 feet) in altitude and flew for over 1,500 meters (4,900 feet).[39] In 1908, Edison patented his own design for a helicopter powered by a gasoline engine with box kites attached to a mast by cables for a rotor,[40] but it never flew.[41]
+
+In 1906, two French brothers, Jacques and Louis Breguet, began experimenting with airfoils for helicopters. In 1907, those experiments resulted in the Gyroplane No.1, possibly as the earliest known example of a quadcopter. Although there is some uncertainty about the date, sometime between 14 August and 29 September 1907, the Gyroplane No. 1 lifted its pilot into the air about 0.6 metres (2 ft) for a minute.[6] The Gyroplane No. 1 proved to be extremely unsteady and required a man at each corner of the airframe to hold it steady. For this reason, the flights of the Gyroplane No. 1 are considered to be the first manned flight of a helicopter, but not a free or untethered flight.
+
+That same year, fellow French inventor Paul Cornu designed and built the Cornu helicopter which used two 6.1-metre (20 ft) counter-rotating rotors driven by a 24 hp (18 kW) Antoinette engine. On 13 November 1907, it lifted its inventor to 0.3 metres (1 ft) and remained aloft for 20 seconds. Even though this flight did not surpass the flight of the Gyroplane No. 1, it was reported to be the first truly free flight with a pilot.[n 1] Cornu's helicopter completed a few more flights and achieved a height of nearly 2.0 metres (6.5 ft), but it proved to be unstable and was abandoned.[6]
+
+In 1911, Slovenian philosopher and economist Ivan Slokar patented a helicopter configuration.[42][43][44]
+
+The Danish inventor Jacob Ellehammer built the Ellehammer helicopter in 1912. It consisted of a frame equipped with two counter-rotating discs, each of which was fitted with six vanes around its circumference. After indoor tests, the aircraft was demonstrated outdoors and made several free take-offs. Experiments with the helicopter continued until September 1916, when it tipped over during take-off, destroying its rotors.[45]
+
+During World War I, Austria-Hungary developed the PKZ, an experimental helicopter prototype, with two aircraft built.
+
+In the early 1920s, Argentine Raúl Pateras-Pescara de Castelluccio, while working in Europe, demonstrated one of the first successful applications of cyclic pitch.[6] Coaxial, contra-rotating, biplane rotors could be warped to cyclically increase and decrease the lift they produced. The rotor hub could also be tilted forward a few degrees, allowing the aircraft to move forward without a separate propeller to push or pull it. Pateras-Pescara was also able to demonstrate the principle of autorotation. By January 1924, Pescara's helicopter No. 1 was tested but was found to be underpowered and could not lift its own weight. His 2F fared better and set a record.[46] The British government funded further research by Pescara which resulted in helicopter No. 3, powered by a 250-horsepower (190 kW) radial engine which could fly for up to ten minutes.[47][48]
+
+On 14 April 1924, Frenchman Étienne Oehmichen set the first helicopter world record recognized by the Fédération Aéronautique Internationale (FAI), flying his quadrotor helicopter 360 meters (1,180 ft).[49] On 18 April 1924, Pescara beat Oemichen's record, flying for a distance of 736 meters (2,415 ft)[46] (nearly 0.80 kilometers or .5 miles) in 4 minutes and 11 seconds (about 13 km/h or 8 mph), maintaining a height of 1.8 meters (6 feet).[50] On 4 May, Oehmichen completed the first one-kilometer (0.62 mi) closed-circuit helicopter flight in 7 minutes 40 seconds with his No. 2 machine.[6][51]
+
+In the US, George de Bothezat built the quadrotor helicopter de Bothezat helicopter for the United States Army Air Service but the Army cancelled the program in 1924, and the aircraft was scrapped.[citation needed]
+
+Albert Gillis von Baumhauer, a Dutch aeronautical engineer, began studying rotorcraft design in 1923. His first prototype "flew" ("hopped" and hovered in reality) on 24 September 1925,[52] with Dutch Army-Air arm Captain Floris Albert van Heijst at the controls. The controls that van Heijst used were von Baumhauer's inventions, the cyclic and collective.[53][54] Patents were granted to von Baumhauer for his cyclic and collective controls by the British ministry of aviation on 31 January 1927, under patent number 265,272.[citation needed]
+
+In 1927,[55] Engelbert Zaschka from Germany built a helicopter, equipped with two rotors, in which a gyroscope was used to increase stability and serves as an energy accumulator for a gliding flight to make a landing. Zaschka's plane, the first helicopter, which ever worked so successfully in miniature, not only rises and descends vertically, but is able to remain stationary at any height.[56][57]
+
+In 1928, Hungarian aviation engineer Oszkár Asbóth constructed a helicopter prototype that took off and landed at least 182 times, with a maximum single flight duration of 53 minutes.[58][59]
+
+In 1930, the Italian engineer Corradino D'Ascanio built his D'AT3, a coaxial helicopter. His relatively large machine had two, two-bladed, counter-rotating rotors. Control was achieved by using auxiliary wings or servo-tabs on the trailing edges of the blades,[60] a concept that was later adopted by other helicopter designers, including Bleeker and Kaman. Three small propellers mounted to the airframe were used for additional pitch, roll, and yaw control. The D'AT3 held modest FAI speed and altitude records for the time, including altitude (18 m or 59 ft), duration (8 minutes 45 seconds) and distance flown (1,078 m or 3,540 ft).[60][61]
+
+Spanish aeronautical engineer and pilot Juan de la Cierva invented the autogyro in the early 1920s, becoming the first practical rotorcraft.[62] In 1928, de la Cierva successfully flew an autogyro across the English Channel, from London to Paris.[63] In 1934, an autogyro became the first rotorcraft to successfully take off and land on the deck of a ship.[64] That same year, the autogyro was employed by the Spanish military during the Asturias revolt, becoming the first military deployment of a rotocraft. Autogyros were also employed in New Jersey and Pennsylvania for delivering mail and newspapers prior to the invention of the helicopter.[65] Though lacking true vertical flight capability, work on the autogyro forms the basis for helicopter analysis.[66]
+
+In the Soviet Union, Boris N. Yuriev and Alexei M. Cheremukhin, two aeronautical engineers working at the Tsentralniy Aerogidrodinamicheskiy Institut (TsAGI or the Central Aerohydrodynamic Institute), constructed and flew the TsAGI 1-EA single lift-rotor helicopter, which used an open tubing framework, a four-blade main lift rotor, and twin sets of 1.8-meter (5.9-foot) diameter, two-bladed anti-torque rotors: one set of two at the nose and one set of two at the tail. Powered by two M-2 powerplants, up-rated copies of the Gnome Monosoupape 9 Type B-2 100 CV output rotary engine of World War I, the TsAGI 1-EA made several low altitude flights.[67] By 14 August 1932, Cheremukhin managed to get the 1-EA up to an unofficial altitude of 605 meters (1,985 feet), shattering d'Ascanio's earlier achievement. As the Soviet Union was not yet a member of the FAI, however, Cheremukhin's record remained unrecognized.[68]
+
+Nicolas Florine, a Russian engineer, built the first twin tandem rotor machine to perform a free flight. It flew in Sint-Genesius-Rode, at the Laboratoire Aérotechnique de Belgique (now von Karman Institute) in April 1933, and attained an altitude of six meters (20 feet) and an endurance of eight minutes. Florine chose a co-rotating configuration because the gyroscopic stability of the rotors would not cancel. Therefore, the rotors had to be tilted slightly in opposite directions to counter torque. Using hingeless rotors and co-rotation also minimised the stress on the hull. At the time, it was one of the most stable helicopters in existence.[69]
+
+The Bréguet-Dorand Gyroplane Laboratoire was built in 1933. It was a coaxial helicopter, contra-rotating. After many ground tests and an accident, it first took flight on 26 June 1935. Within a short time, the aircraft was setting records with pilot Maurice Claisse at the controls. On 14 December 1935, he set a record for closed-circuit flight with a 500-meter (1,600-foot) diameter.[70] The next year, on 26 September 1936, Claisse set a height record of 158 meters (518 feet).[71] And, finally, on 24 November 1936, he set a flight duration record of one hour, two minutes and 50 seconds[72] over a 44 kilometers (27 miles) closed circuit at 44.7 kilometers per hour (27.8 mph). The aircraft was destroyed in 1943 by an Allied airstrike at Villacoublay airport.[73]
+
+American inventor Arthur M. Young started work on model helicopters in 1928 using converted electric hover motors to drive the rotor head. Young invented the stabilizer bar and patented it shortly after. A mutual friend introduced Young to Lawrence Dale, who once seeing his work asked him to join the Bell Aircraft company. When Young arrived at Bell in 1941, he signed his patent over and began work on the helicopter. His budget was US$250,000 (equivalent to $4.3 million today) to build two working helicopters. In just six months they completed the first Bell Model 1, which spawned the Bell Model 30, later succeeded by the Bell 47.[74]
+
+Heinrich Focke at Focke-Wulf was licensed to produce the Cierva C.30 autogyro in 1933. Focke designed the world's first practical transverse twin-rotor helicopter, the Focke-Wulf Fw 61, which first flew on 26 June 1936. The Fw 61 broke all of the helicopter world records in 1937, demonstrating a flight envelope that had only previously been achieved by the autogyro.
+
+During World War II, Nazi Germany used helicopters in small numbers for observation, transport, and medical evacuation. The Flettner Fl 282 Kolibri synchropter—using the same basic configuration as Anton Flettner's own pioneering Fl 265—was used in the Mediterranean, while the Focke Achgelis Fa 223 Drache twin-rotor helicopter was used in Europe.[citation needed] Extensive bombing by the Allied forces prevented Germany from producing any helicopters in large quantities during the war.
+
+In the United States, Russian-born engineer Igor Sikorsky and Wynn Laurence LePage competed to produce the U.S. military's first helicopter. LePage received the patent rights to develop helicopters patterned after the Fw 61, and built the XR-1.[75] Meanwhile, Sikorsky settled on a simpler, single rotor design, the VS-300, which turned out to be the first practical single lifting-rotor helicopter design. After experimenting with configurations to counteract the torque produced by the single main rotor, Sikorsky settled on a single, smaller rotor mounted on the tail boom.
+
+Developed from the VS-300, Sikorsky's R-4 was the first large-scale mass-produced helicopter, with a production order for 100 aircraft. The R-4 was the only Allied helicopter to serve in World War II, when it was used primarily for search and rescue (by the USAAF 1st Air Commando Group) in Burma;[76] in Alaska; and in other areas with harsh terrain. Total production reached 131 helicopters before the R-4 was replaced by other Sikorsky helicopters such as the R-5 and the R-6. In all, Sikorsky produced over 400 helicopters before the end of World War II.[77]
+
+While LePage and Sikorsky built their helicopters for the military, Bell Aircraft hired Arthur Young to help build a helicopter using Young's two-blade teetering rotor design, which used a weighted stabilizer bar placed at a 90° angle to the rotor blades. The subsequent Model 30 helicopter showed the design's simplicity and ease of use. The Model 30 was developed into the Bell 47, which became the first helicopter certified for civilian use in the United States. Produced in several countries, the Bell 47 was the most popular helicopter model for nearly 30 years.
+
+In 1951, at the urging of his contacts at the Department of the Navy, Charles Kaman modified his K-225 synchropter — a design for a twin-rotor helicopter concept first pioneered by Anton Flettner in 1939, with the aforementioned Fl 265 piston-engined design in Germany – with a new kind of engine, the turboshaft engine. This adaptation of the turbine engine provided a large amount of power to Kaman's helicopter with a lower weight penalty than piston engines, with their heavy engine blocks and auxiliary components. On 11 December 1951, the Kaman K-225 became the first turbine-powered helicopter in the world. Two years later, on 26 March 1954, a modified Navy HTK-1, another Kaman helicopter, became the first twin-turbine helicopter to fly.[78] However, it was the Sud Aviation Alouette II that would become the first helicopter to be produced with a turbine-engine.[79]
+
+Reliable helicopters capable of stable hover flight were developed decades after fixed-wing aircraft. This is largely due to higher engine power density requirements than fixed-wing aircraft. Improvements in fuels and engines during the first half of the 20th century were a critical factor in helicopter development. The availability of lightweight turboshaft engines in the second half of the 20th century led to the development of larger, faster, and higher-performance helicopters. While smaller and less expensive helicopters still use piston engines, turboshaft engines are the preferred powerplant for helicopters today.
+
+There are several reasons a helicopter cannot fly as fast as a fixed-wing aircraft. When the helicopter is hovering, the outer tips of the rotor travel at a speed determined by the length of the blade and the rotational speed. In a moving helicopter, however, the speed of the blades relative to the air depends on the speed of the helicopter as well as on their rotational speed. The airspeed of the advancing rotor blade is much higher than that of the helicopter itself. It is possible for this blade to exceed the speed of sound, and thus produce vastly increased drag and vibration.
+
+At the same time, the advancing blade creates more lift traveling forward, the retreating blade produces less lift. If the aircraft were to accelerate to the air speed that the blade tips are spinning, the retreating blade passes through air moving at the same speed of the blade and produces no lift at all, resulting in very high torque stresses on the central shaft that can tip down the retreating-blade side of the vehicle, and cause a loss of control. Dual counter-rotating blades prevent this situation due to having two advancing and two retreating blades with balanced forces.
+
+Because the advancing blade has higher airspeed than the retreating blade and generates a dissymmetry of lift, rotor blades are designed to "flap" – lift and twist in such a way that the advancing blade flaps up and develops a smaller angle of attack. Conversely, the retreating blade flaps down, develops a higher angle of attack, and generates more lift. At high speeds, the force on the rotors is such that they "flap" excessively, and the retreating blade can reach too high an angle and stall. For this reason, the maximum safe forward airspeed of a helicopter is given a design rating called VNE, velocity, never exceed.[80] In addition, it is possible for the helicopter to fly at an airspeed where an excessive amount of the retreating blade stalls, which results in high vibration, pitch-up, and roll into the retreating blade.
+
+During the closing years of the 20th century designers began working on helicopter noise reduction. Urban communities have often expressed great dislike of noisy aviation or noisy aircraft, and police and passenger helicopters can be unpopular because of the sound. The redesigns followed the closure of some city heliports and government action to constrain flight paths in national parks and other places of natural beauty.
+
+Helicopters also vibrate; an unadjusted helicopter can easily vibrate so much that it will shake itself apart. To reduce vibration, all helicopters have rotor adjustments for height and weight. Blade height is adjusted by changing the pitch of the blade. Weight is adjusted by adding or removing weights on the rotor head and/or at the blade end caps. Most also have vibration dampers for height and pitch. Some also use mechanical feedback systems to sense and counter vibration. Usually the feedback system uses a mass as a "stable reference" and a linkage from the mass operates a flap to adjust the rotor's angle of attack to counter the vibration. Adjustment is difficult in part because measurement of the vibration is hard, usually requiring sophisticated accelerometers mounted throughout the airframe and gearboxes. The most common blade vibration adjustment measurement system is to use a stroboscopic flash lamp, and observe painted markings or coloured reflectors on the underside of the rotor blades. The traditional low-tech system is to mount coloured chalk on the rotor tips, and see how they mark a linen sheet. Gearbox vibration most often requires a gearbox overhaul or replacement. Gearbox or drive train vibrations can be extremely harmful to a pilot. The most severe being pain, numbness, loss of tactile discrimination and dexterity.
+
+For a standard helicopter with a single main rotor, the tips of the main rotor blades produce a vortex ring in the air, which is a spiraling and circularly rotating airflow. As the craft moves forward, these vortices trail off behind the craft.
+
+When hovering with a forward diagonal crosswind, or moving in a forward diagonal direction, the spinning vortices trailing off the main rotor blades will align with the rotation of the tail rotor and cause an instability in flight control.[81]
+
+When the trailing vortices colliding with the tail rotor are rotating in the same direction, this causes a loss of thrust from the tail rotor. When the trailing vortices rotate in the opposite direction of the tail rotor, thrust is increased. Use of the foot pedals is required to adjust the tail rotor's angle of attack, to compensate for these instabilities.
+
+These issues are due to the exposed tail rotor cutting through open air around rear of the vehicle. This issue disappears when the tail is instead ducted, using an internal impeller enclosed in the tail and a jet of high pressure air sideways out of the tail, as the main rotor vortices can not impact the operation of an internal impeller.
+
+For a standard helicopter with a single main rotor, maintaining steady flight with a crosswind presents an additional flight control problem, where strong crosswinds from certain angles will increase or decrease lift from the main rotors. This effect is also triggered in a no-wind condition when moving the craft diagonally in various directions, depending on the direction of main rotor rotation.[82]
+
+This can lead to a loss of control and a crash or hard landing when operating at low altitudes, due to the sudden unexpected loss of lift, and insufficient time and distance available to recover.
+
+Conventional rotary-wing aircraft use a set of complex mechanical gearboxes to convert the high rotation speed of gas turbines into the low speed required to drive main and tail rotors. Unlike powerplants, mechanical gearboxes cannot be duplicated (for redundancy) and have always been a major weak point in helicopter reliability. In-flight catastrophic gear failures often result in gearbox jamming and subsequent fatalities, whereas loss of lubrication can trigger onboard fire.[citation needed] Another weakness of mechanical gearboxes is their transient power limitation, due to structural fatigue limits. Recent EASA studies point to engines and transmissions as prime cause of crashes just after pilot errors.[83]
+
+By contrast, electromagnetic transmissions do not use any parts in contact; hence lubrication can be drastically simplified, or eliminated. Their inherent redundancy offers good resilience to single point of failure. The absence of gears enables high power transient without impact on service life. The concept of electric propulsion applied to helicopter and electromagnetic drive was brought to reality by Pascal Chretien who designed, built and flew world's first man-carrying, free-flying electric helicopter. The concept was taken from the conceptual computer-aided design model on 10 September 2010 to the first testing at 30% power on 1 March 2011 – less than six months. The aircraft first flew on 12 August 2011. All development was conducted in Venelles, France.[84][85]
+
+As with any moving vehicle, unsafe operation could result in loss of control, structural damage, or loss of life. The following is a list of some of the potential hazards for helicopters:

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+A helicopter, sometimes referred to in slang as a "chopper" or "helo" (United States military usage, pronounced with a long "e"), is a type of rotorcraft in which lift and thrust are supplied by horizontally-spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward, and laterally. These attributes allow helicopters to be used in congested or isolated areas where fixed-wing aircraft and many forms of VTOL (Vertical TakeOff and Landing) aircraft cannot perform.
+
+The English word helicopter is adapted from the French word hélicoptère, coined by Gustave Ponton d'Amécourt in 1861, which originates from the Greek helix (ἕλιξ) "helix, spiral, whirl, convolution"[1] and pteron (πτερόν) "wing".[2][3][4][5] English language nicknames for helicopter include "chopper", "copter", "helo", "heli", and "whirlybird".
+
+Helicopters were developed and built during the first half-century of flight, with the Focke-Wulf Fw 61 being the first operational helicopter in 1936. Some helicopters reached limited production, but it was not until 1942 that a helicopter designed by Igor Sikorsky reached full-scale production,[6] with 131 aircraft built.[7] Though most earlier designs used more than one main rotor, it is the single main rotor with anti-torque tail rotor configuration that has become the most common helicopter configuration.  Tandem rotor helicopters are also in widespread use due to their greater payload capacity.  Coaxial helicopters, tiltrotor aircraft, and compound helicopters are all flying today.  Quadcopter helicopters were pioneered as early as 1907 in France, and other types of multicopter have been developed for specialized applications such as unmanned drones.
+
+A helicopter, sometimes referred to in slang as a "chopper", is a type of rotorcraft in which lift and thrust are supplied by one or more horizontally-spinning rotors. By contrast the autogyro (or gyroplane) and gyrodyne have a free-spinning rotor for all or part of the flight envelope, relying on a separate thrust system to propel the craft forwards, so that the airflow sets the rotor spinning to provide lift. The compound helicopter also has a separate thrust system, but continues to supply power to the rotor throughout normal flight.
+
+The rotor system, or more simply rotor, is the rotating part of a helicopter that generates lift. A rotor system may be mounted horizontally, as main rotors are, providing lift vertically, or it may be mounted vertically, such as a tail rotor, to provide horizontal thrust to counteract torque from the main rotors. The rotor consists of a mast, hub and rotor blades.
+
+The mast is a cylindrical metal shaft that extends upwards from the transmission. At the top of the mast is the attachment point for the rotor blades called the hub. Main rotor systems are classified according to how the rotor blades are attached and move relative to the hub. There are three basic types: hingeless, fully articulated, and teetering; although some modern rotor systems use a combination of these.
+
+Most helicopters have a single main rotor, but torque created by its aerodynamic drag must be countered by an opposed torque. The design that Igor Sikorsky settled on for his VS-300 was a smaller tail rotor. The tail rotor pushes or pulls against the tail to counter the torque effect, and this has become the most common configuration for helicopter design, usually at the end of a tail boom.
+
+Some helicopters use other anti-torque controls instead of the tail rotor, such as the ducted fan (called Fenestron or FANTAIL) and NOTAR. NOTAR provides anti-torque similar to the way a wing develops lift through the use of the Coandă effect on the tail boom.[8]
+
+The use of two or more horizontal rotors turning in opposite directions is another configuration used to counteract the effects of torque on the aircraft without relying on an anti-torque tail rotor. This allows the power normally required to drive the tail rotor to be applied to the main rotors, increasing the aircraft's lifting capacity. There are several common configurations that use the counter-rotating effect to benefit the rotorcraft:
+
+Tip jet designs let the rotor push itself through the air and avoid generating torque.[9]
+
+The number, size and type of engine(s) used on a helicopter determines the size, function and capability of that helicopter design. The earliest helicopter engines were simple mechanical devices, such as rubber bands or spindles, which relegated the size of helicopters to toys and small models. For a half century before the first airplane flight, steam engines were used to forward the development of the understanding of helicopter aerodynamics, but the limited power did not allow for manned flight. The introduction of the internal combustion engine at the end of the 19th century became the watershed for helicopter development as engines began to be developed and produced that were powerful enough to allow for helicopters able to lift humans.[citation needed]
+
+Early helicopter designs utilized custom-built engines or rotary engines designed for airplanes, but these were soon replaced by more powerful automobile engines and radial engines. The single, most-limiting factor of helicopter development during the first half of the 20th century was that the amount of power produced by an engine was not able to overcome the engine's weight in vertical flight. This was overcome in early successful helicopters by using the smallest engines available. When the compact, flat engine was developed, the helicopter industry found a lighter-weight powerplant easily adapted to small helicopters, although radial engines continued to be used for larger helicopters.[citation needed]
+
+Turbine engines revolutionized the aviation industry; and the turboshaft engine for helicopter use, pioneered in December 1951 by the aforementioned Kaman K-225, finally gave helicopters an engine with a large amount of power and a low weight penalty. Turboshafts are also more reliable than piston engines, especially when producing the sustained high levels of power required by a helicopter. The turboshaft engine was able to be scaled to the size of the helicopter being designed, so that all but the lightest of helicopter models are powered by turbine engines today.[citation needed]
+
+Special jet engines developed to drive the rotor from the rotor tips are referred to as tip jets. Tip jets powered by a remote compressor are referred to as cold tip jets, while those powered by combustion exhaust are referred to as hot tip jets. An example of a cold jet helicopter is the Sud-Ouest Djinn, and an example of the hot tip jet helicopter is the YH-32 Hornet.[citation needed]
+
+Some radio-controlled helicopters and smaller, helicopter-type unmanned aerial vehicles, use electric motors. Radio-controlled helicopters may also have piston engines that use fuels other than gasoline, such as nitromethane. Some turbine engines commonly used in helicopters can also use biodiesel instead of jet fuel.[10][11]
+
+There are also human-powered helicopters.
+
+A helicopter has four flight control inputs. These are the cyclic, the collective, the anti-torque pedals, and the throttle. The cyclic control is usually located between the pilot's legs and is commonly called the cyclic stick or just cyclic. On most helicopters, the cyclic is similar to a joystick. However, the Robinson R22 and Robinson R44 have a unique teetering bar cyclic control system and a few helicopters have a cyclic control that descends into the cockpit from overhead.
+
+The control is called the cyclic because it changes cyclic pitch of the main blades. The result is to tilt the rotor disk in a particular direction, resulting in the helicopter moving in that direction. If the pilot pushes the cyclic forward, the rotor disk tilts forward, and the rotor produces a thrust in the forward direction. If the pilot pushes the cyclic to the side, the rotor disk tilts to that side and produces thrust in that direction, causing the helicopter to hover sideways.
+
+The collective pitch control or collective is located on the left side of the pilot's seat with a settable friction control to prevent inadvertent movement. The collective changes the pitch angle of all the main rotor blades collectively (i.e. all at the same time) and independently of their position. Therefore, if a collective input is made, all the blades change equally, and the result is the helicopter increasing or decreasing in altitude.
+
+A swashplate controls the collective and cyclic pitch of the main blades. The swashplate moves up and down, along the main shaft, to change the pitch of both blades. This causes the helicopter to push air downward or upward, depending on the angle of attack. The swashplate can also change its angle to move the blades angle forwards or backwards, or left and right, to make the helicopter move in those directions.
+
+The anti-torque pedals are located in the same position as the rudder pedals in a fixed-wing aircraft, and serve a similar purpose, namely to control the direction in which the nose of the aircraft is pointed. Application of the pedal in a given direction changes the pitch of the tail rotor blades, increasing or reducing the thrust produced by the tail rotor and causing the nose to yaw in the direction of the applied pedal. The pedals mechanically change the pitch of the tail rotor altering the amount of thrust produced.
+
+Helicopter rotors are designed to operate in a narrow range of RPM.[12][13][14][15][16] The throttle controls the power produced by the engine, which is connected to the rotor by a fixed ratio transmission. The purpose of the throttle is to maintain enough engine power to keep the rotor RPM within allowable limits so that the rotor produces enough lift for flight. In single-engine helicopters, the throttle control is a motorcycle-style twist grip mounted on the collective control, while dual-engine helicopters have a power lever for each engine.
+
+A compound helicopter has an additional system for thrust and, typically, small stub fixed wings. This offloads the rotor in cruise, which allows its rotation to be slowed down, thus increasing the maximum speed of the aircraft. The Lockheed AH-56A Cheyenne diverted up to 90% of its engine power to a pusher propeller during forward flight.[17]
+
+There are three basic flight conditions for a helicopter: hover, forward flight and the transition between the two.
+
+Hovering is the most challenging part of flying a helicopter. This is because a helicopter generates its own gusty air while in a hover, which acts against the fuselage and flight control surfaces. The end result is constant control inputs and corrections by the pilot to keep the helicopter where it is required to be.[18] Despite the complexity of the task, the control inputs in a hover are simple. The cyclic is used to eliminate drift in the horizontal plane, that is to control forward and back, right and left. The collective is used to maintain altitude. The pedals are used to control nose direction or heading. It is the interaction of these controls that makes hovering so difficult, since an adjustment in any one control requires an adjustment of the other two, creating a cycle of constant correction.
+
+As a helicopter moves from hover to forward flight it enters a state called translational lift which provides extra lift without increasing power. This state, most typically, occurs when the airspeed reaches approximately 16–24 knots (30–44 km/h; 18–28 mph), and may be necessary for a helicopter to obtain flight.
+
+In forward flight a helicopter's flight controls behave more like those of a fixed-wing aircraft. Displacing the cyclic forward will cause the nose to pitch down, with a resultant increase in airspeed and loss of altitude. Aft cyclic will cause the nose to pitch up, slowing the helicopter and causing it to climb. Increasing collective (power) while maintaining a constant airspeed will induce a climb while decreasing collective will cause a descent. Coordinating these two inputs, down collective plus aft cyclic or up collective plus forward cyclic, will result in airspeed changes while maintaining a constant altitude. The pedals serve the same function in both a helicopter and a fixed-wing aircraft, to maintain balanced flight. This is done by applying a pedal input in whichever direction is necessary to center the ball in the turn and bank indicator.
+
+Due to the operating characteristics of the helicopter—its ability to take off and land vertically, and to hover for extended periods of time, as well as the aircraft's handling properties under low airspeed conditions—it has been chosen to conduct tasks that were previously not possible with other aircraft, or were time- or work-intensive to accomplish on the ground. Today, helicopter uses include transportation of people and cargo, military uses, construction, firefighting, search and rescue, tourism, medical transport, law enforcement, agriculture, news and media, and aerial observation, among others.[19]
+They can be used for Reflection seismology or recreation.
+
+A helicopter used to carry loads connected to long cables or slings is called an aerial crane. Aerial cranes are used to place heavy equipment, like radio transmission towers and large air conditioning units, on the tops of tall buildings, or when an item must be raised up in a remote area, such as a radio tower raised on the top of a hill or mountain. Helicopters are used as aerial cranes in the logging industry to lift trees out of terrain where vehicles cannot travel and where environmental concerns prohibit the building of roads.[20] These operations are referred to as longline because of the long, single sling line used to carry the load.[21]
+
+The largest single non-combat helicopter operation in history was the disaster management operation following the 1986 Chernobyl nuclear disaster. Hundreds of pilots were involved in airdrop and observation missions, making dozens of sorties a day for several months.
+
+"Helitack" is the use of helicopters to combat wildland fires.[22] The helicopters are used for aerial firefighting (water bombing) and may be fitted with tanks or carry helibuckets. Helibuckets, such as the Bambi bucket, are usually filled by submerging the bucket into lakes, rivers, reservoirs, or portable tanks. Tanks fitted onto helicopters are filled from a hose while the helicopter is on the ground or water is siphoned from lakes or reservoirs through a hanging snorkel as the helicopter hovers over the water source. Helitack helicopters are also used to deliver firefighters, who rappel down to inaccessible areas, and to resupply firefighters. Common firefighting helicopters include variants of the Bell 205 and the Erickson S-64 Aircrane helitanker.
+
+Helicopters are used as air ambulances for emergency medical assistance in situations when an ambulance cannot easily or quickly reach the scene, or cannot transport the patient to a medical facility in time. Helicopters are also used when patients need to be transported between medical facilities and air transportation is the most practical method. An air ambulance helicopter is equipped to stabilize and provide limited medical treatment to a patient while in flight. The use of helicopters as air ambulances is often referred to as "MEDEVAC", and patients are referred to as being "airlifted", or "medevaced". This use was pioneered in the Korean War, when time to reach a medical facility was reduced to three hours from the eight hours needed in World War II, and further reduced to two hours by the Vietnam War.[23]
+
+Police departments and other law enforcement agencies use helicopters to pursue suspects. Since helicopters can achieve a unique aerial view, they are often used in conjunction with police on the ground to report on suspects' locations and movements. They are often mounted with lighting and heat-sensing equipment for night pursuits.
+
+Military forces use attack helicopters to conduct aerial attacks on ground targets. Such helicopters are mounted with missile launchers and miniguns. Transport helicopters are used to ferry troops and supplies where the lack of an airstrip would make transport via fixed-wing aircraft impossible. The use of transport helicopters to deliver troops as an attack force on an objective is referred to as "air assault". Unmanned aerial systems (UAS) helicopter systems of varying sizes are developed by companies for military reconnaissance and surveillance duties. Naval forces also use helicopters equipped with dipping sonar for anti-submarine warfare, since they can operate from small ships.
+
+Oil companies charter helicopters to move workers and parts quickly to remote drilling sites located at sea or in remote locations. The speed advantage over boats makes the high operating cost of helicopters cost-effective in ensuring that oil platforms continue to operate. Various companies specialize in this type of operation.
+
+NASA is developing the Mars Helicopter, a 1.8 kg (4.0 lb) helicopter to be launched to survey Mars (along with a rover) in 2020. Given that the Martian atmosphere is 100 times thinner than that of Earth's, its two blades will spin at close to 3,000 revolutions a minute, approximately 10 times faster than that of a terrestrial helicopter.[24]
+
+In 2017, 926 civil helicopters were shipped for $3.68 Billion, led by Airbus Helicopters with $1.87 Billion for 369 rotorcraft, Leonardo Helicopters with $806 Million for 102 (first three-quarters only), Bell Helicopter with $696 Million for 132, then Robinson Helicopter with $161 Million for 305.[25]
+
+By October 2018, the in-service and stored helicopter fleet of 38,570 with civil or government operators was led Robinson Helicopter with 24.7% followed by Airbus Helicopters with 24.4%, then Bell with 20.5 and Leonardo with 8.4%, Russian Helicopters with 7.7%, Sikorsky Aircraft with 7.2%, MD Helicopters with 3.4% and other with 2.2%.
+The most widespread model is the piston Robinson R44 with 5,600, then the H125/AS350 with 3,600 units, followed by the Bell 206 with 3,400.
+Most were in North America with 34.3% then in Europe with 28.0% followed by Asia-Pacific with 18.6%, Latin America with 11.6%, Africa with 5.3% and Middle East with 1.7%.[26]
+
+The earliest references for vertical flight came from China. Since around 400 BC,[27] Chinese children have played with bamboo flying toys (or Chinese top).[28][29][30] This bamboo-copter is spun by rolling a stick attached to a rotor. The spinning creates lift, and the toy flies when released.[27] The 4th-century AD Daoist book Baopuzi by Ge Hong (抱朴子 "Master who Embraces Simplicity") reportedly describes some of the ideas inherent to rotary wing aircraft.[31]
+
+Designs similar to the Chinese helicopter toy appeared in some Renaissance paintings and other works.[32] In the 18th and early 19th centuries Western scientists developed flying machines based on the Chinese toy.[33]
+
+It was not until the early 1480s, when Italian polymath Leonardo da Vinci created a design for a machine that could be described as an "aerial screw", that any recorded advancement was made towards vertical flight. His notes suggested that he built small flying models, but there were no indications for any provision to stop the rotor from making the craft rotate.[34][35] As scientific knowledge increased and became more accepted, people continued to pursue the idea of vertical flight.
+
+In July 1754, Russian Mikhail Lomonosov had developed a small coaxial modeled after the Chinese top but powered by a wound-up spring device[33] and demonstrated it to the Russian Academy of Sciences. It was powered by a spring, and was suggested as a method to lift meteorological instruments. In 1783, Christian de Launoy, and his mechanic, Bienvenu, used a coaxial version of the Chinese top in a model consisting of contrarotating turkey flight feathers[33] as rotor blades, and in 1784, demonstrated it to the French Academy of Sciences. Sir George Cayley, influenced by a childhood fascination with the Chinese flying top, developed a model of feathers, similar to that of Launoy and Bienvenu, but powered by rubber bands. By the end of the century, he had progressed to using sheets of tin for rotor blades and springs for power. His writings on his experiments and models would become influential on future aviation pioneers.[34] Alphonse Pénaud would later develop coaxial rotor model helicopter toys in 1870, also powered by rubber bands. One of these toys, given as a gift by their father, would inspire the Wright brothers to pursue the dream of flight.[36]
+
+In 1861, the word "helicopter" was coined by Gustave de Ponton d'Amécourt, a French inventor who demonstrated a small steam-powered model. While celebrated as an innovative use of a new metal, aluminum, the model never lifted off the ground. D'Amecourt's linguistic contribution would survive to eventually describe the vertical flight he had envisioned. Steam power was popular with other inventors as well. In 1878 the Italian Enrico Forlanini's unmanned vehicle, also powered by a steam engine, rose to a height of 12 meters (39 feet), where it hovered for some 20 seconds after a vertical take-off. Emmanuel Dieuaide's steam-powered design featured counter-rotating rotors powered through a hose from a boiler on the ground.[34] In 1887 Parisian inventor, Gustave Trouvé, built and flew a tethered electric model helicopter.[citation needed]
+
+In July 1901, the maiden flight of Hermann Ganswindt's helicopter took place in Berlin-Schöneberg; this was probably the first heavier-than-air motor-driven flight carrying humans. A movie covering the event was taken by Max Skladanowsky, but it remains lost.[37]
+
+In 1885, Thomas Edison was given US$1,000 (equivalent to $28,000 today) by James Gordon Bennett, Jr., to conduct experiments towards developing flight. Edison built a helicopter and used the paper for a stock ticker to create guncotton, with which he attempted to power an internal combustion engine. The helicopter was damaged by explosions and one of his workers was badly burned. Edison reported that it would take a motor with a ratio of three to four pounds per horsepower produced to be successful, based on his experiments.[38] Ján Bahýľ, a Slovak inventor, adapted the internal combustion engine to power his helicopter model that reached a height of 0.5 meters (1.6 feet) in 1901. On 5 May 1905, his helicopter reached 4 meters (13 feet) in altitude and flew for over 1,500 meters (4,900 feet).[39] In 1908, Edison patented his own design for a helicopter powered by a gasoline engine with box kites attached to a mast by cables for a rotor,[40] but it never flew.[41]
+
+In 1906, two French brothers, Jacques and Louis Breguet, began experimenting with airfoils for helicopters. In 1907, those experiments resulted in the Gyroplane No.1, possibly as the earliest known example of a quadcopter. Although there is some uncertainty about the date, sometime between 14 August and 29 September 1907, the Gyroplane No. 1 lifted its pilot into the air about 0.6 metres (2 ft) for a minute.[6] The Gyroplane No. 1 proved to be extremely unsteady and required a man at each corner of the airframe to hold it steady. For this reason, the flights of the Gyroplane No. 1 are considered to be the first manned flight of a helicopter, but not a free or untethered flight.
+
+That same year, fellow French inventor Paul Cornu designed and built the Cornu helicopter which used two 6.1-metre (20 ft) counter-rotating rotors driven by a 24 hp (18 kW) Antoinette engine. On 13 November 1907, it lifted its inventor to 0.3 metres (1 ft) and remained aloft for 20 seconds. Even though this flight did not surpass the flight of the Gyroplane No. 1, it was reported to be the first truly free flight with a pilot.[n 1] Cornu's helicopter completed a few more flights and achieved a height of nearly 2.0 metres (6.5 ft), but it proved to be unstable and was abandoned.[6]
+
+In 1911, Slovenian philosopher and economist Ivan Slokar patented a helicopter configuration.[42][43][44]
+
+The Danish inventor Jacob Ellehammer built the Ellehammer helicopter in 1912. It consisted of a frame equipped with two counter-rotating discs, each of which was fitted with six vanes around its circumference. After indoor tests, the aircraft was demonstrated outdoors and made several free take-offs. Experiments with the helicopter continued until September 1916, when it tipped over during take-off, destroying its rotors.[45]
+
+During World War I, Austria-Hungary developed the PKZ, an experimental helicopter prototype, with two aircraft built.
+
+In the early 1920s, Argentine Raúl Pateras-Pescara de Castelluccio, while working in Europe, demonstrated one of the first successful applications of cyclic pitch.[6] Coaxial, contra-rotating, biplane rotors could be warped to cyclically increase and decrease the lift they produced. The rotor hub could also be tilted forward a few degrees, allowing the aircraft to move forward without a separate propeller to push or pull it. Pateras-Pescara was also able to demonstrate the principle of autorotation. By January 1924, Pescara's helicopter No. 1 was tested but was found to be underpowered and could not lift its own weight. His 2F fared better and set a record.[46] The British government funded further research by Pescara which resulted in helicopter No. 3, powered by a 250-horsepower (190 kW) radial engine which could fly for up to ten minutes.[47][48]
+
+On 14 April 1924, Frenchman Étienne Oehmichen set the first helicopter world record recognized by the Fédération Aéronautique Internationale (FAI), flying his quadrotor helicopter 360 meters (1,180 ft).[49] On 18 April 1924, Pescara beat Oemichen's record, flying for a distance of 736 meters (2,415 ft)[46] (nearly 0.80 kilometers or .5 miles) in 4 minutes and 11 seconds (about 13 km/h or 8 mph), maintaining a height of 1.8 meters (6 feet).[50] On 4 May, Oehmichen completed the first one-kilometer (0.62 mi) closed-circuit helicopter flight in 7 minutes 40 seconds with his No. 2 machine.[6][51]
+
+In the US, George de Bothezat built the quadrotor helicopter de Bothezat helicopter for the United States Army Air Service but the Army cancelled the program in 1924, and the aircraft was scrapped.[citation needed]
+
+Albert Gillis von Baumhauer, a Dutch aeronautical engineer, began studying rotorcraft design in 1923. His first prototype "flew" ("hopped" and hovered in reality) on 24 September 1925,[52] with Dutch Army-Air arm Captain Floris Albert van Heijst at the controls. The controls that van Heijst used were von Baumhauer's inventions, the cyclic and collective.[53][54] Patents were granted to von Baumhauer for his cyclic and collective controls by the British ministry of aviation on 31 January 1927, under patent number 265,272.[citation needed]
+
+In 1927,[55] Engelbert Zaschka from Germany built a helicopter, equipped with two rotors, in which a gyroscope was used to increase stability and serves as an energy accumulator for a gliding flight to make a landing. Zaschka's plane, the first helicopter, which ever worked so successfully in miniature, not only rises and descends vertically, but is able to remain stationary at any height.[56][57]
+
+In 1928, Hungarian aviation engineer Oszkár Asbóth constructed a helicopter prototype that took off and landed at least 182 times, with a maximum single flight duration of 53 minutes.[58][59]
+
+In 1930, the Italian engineer Corradino D'Ascanio built his D'AT3, a coaxial helicopter. His relatively large machine had two, two-bladed, counter-rotating rotors. Control was achieved by using auxiliary wings or servo-tabs on the trailing edges of the blades,[60] a concept that was later adopted by other helicopter designers, including Bleeker and Kaman. Three small propellers mounted to the airframe were used for additional pitch, roll, and yaw control. The D'AT3 held modest FAI speed and altitude records for the time, including altitude (18 m or 59 ft), duration (8 minutes 45 seconds) and distance flown (1,078 m or 3,540 ft).[60][61]
+
+Spanish aeronautical engineer and pilot Juan de la Cierva invented the autogyro in the early 1920s, becoming the first practical rotorcraft.[62] In 1928, de la Cierva successfully flew an autogyro across the English Channel, from London to Paris.[63] In 1934, an autogyro became the first rotorcraft to successfully take off and land on the deck of a ship.[64] That same year, the autogyro was employed by the Spanish military during the Asturias revolt, becoming the first military deployment of a rotocraft. Autogyros were also employed in New Jersey and Pennsylvania for delivering mail and newspapers prior to the invention of the helicopter.[65] Though lacking true vertical flight capability, work on the autogyro forms the basis for helicopter analysis.[66]
+
+In the Soviet Union, Boris N. Yuriev and Alexei M. Cheremukhin, two aeronautical engineers working at the Tsentralniy Aerogidrodinamicheskiy Institut (TsAGI or the Central Aerohydrodynamic Institute), constructed and flew the TsAGI 1-EA single lift-rotor helicopter, which used an open tubing framework, a four-blade main lift rotor, and twin sets of 1.8-meter (5.9-foot) diameter, two-bladed anti-torque rotors: one set of two at the nose and one set of two at the tail. Powered by two M-2 powerplants, up-rated copies of the Gnome Monosoupape 9 Type B-2 100 CV output rotary engine of World War I, the TsAGI 1-EA made several low altitude flights.[67] By 14 August 1932, Cheremukhin managed to get the 1-EA up to an unofficial altitude of 605 meters (1,985 feet), shattering d'Ascanio's earlier achievement. As the Soviet Union was not yet a member of the FAI, however, Cheremukhin's record remained unrecognized.[68]
+
+Nicolas Florine, a Russian engineer, built the first twin tandem rotor machine to perform a free flight. It flew in Sint-Genesius-Rode, at the Laboratoire Aérotechnique de Belgique (now von Karman Institute) in April 1933, and attained an altitude of six meters (20 feet) and an endurance of eight minutes. Florine chose a co-rotating configuration because the gyroscopic stability of the rotors would not cancel. Therefore, the rotors had to be tilted slightly in opposite directions to counter torque. Using hingeless rotors and co-rotation also minimised the stress on the hull. At the time, it was one of the most stable helicopters in existence.[69]
+
+The Bréguet-Dorand Gyroplane Laboratoire was built in 1933. It was a coaxial helicopter, contra-rotating. After many ground tests and an accident, it first took flight on 26 June 1935. Within a short time, the aircraft was setting records with pilot Maurice Claisse at the controls. On 14 December 1935, he set a record for closed-circuit flight with a 500-meter (1,600-foot) diameter.[70] The next year, on 26 September 1936, Claisse set a height record of 158 meters (518 feet).[71] And, finally, on 24 November 1936, he set a flight duration record of one hour, two minutes and 50 seconds[72] over a 44 kilometers (27 miles) closed circuit at 44.7 kilometers per hour (27.8 mph). The aircraft was destroyed in 1943 by an Allied airstrike at Villacoublay airport.[73]
+
+American inventor Arthur M. Young started work on model helicopters in 1928 using converted electric hover motors to drive the rotor head. Young invented the stabilizer bar and patented it shortly after. A mutual friend introduced Young to Lawrence Dale, who once seeing his work asked him to join the Bell Aircraft company. When Young arrived at Bell in 1941, he signed his patent over and began work on the helicopter. His budget was US$250,000 (equivalent to $4.3 million today) to build two working helicopters. In just six months they completed the first Bell Model 1, which spawned the Bell Model 30, later succeeded by the Bell 47.[74]
+
+Heinrich Focke at Focke-Wulf was licensed to produce the Cierva C.30 autogyro in 1933. Focke designed the world's first practical transverse twin-rotor helicopter, the Focke-Wulf Fw 61, which first flew on 26 June 1936. The Fw 61 broke all of the helicopter world records in 1937, demonstrating a flight envelope that had only previously been achieved by the autogyro.
+
+During World War II, Nazi Germany used helicopters in small numbers for observation, transport, and medical evacuation. The Flettner Fl 282 Kolibri synchropter—using the same basic configuration as Anton Flettner's own pioneering Fl 265—was used in the Mediterranean, while the Focke Achgelis Fa 223 Drache twin-rotor helicopter was used in Europe.[citation needed] Extensive bombing by the Allied forces prevented Germany from producing any helicopters in large quantities during the war.
+
+In the United States, Russian-born engineer Igor Sikorsky and Wynn Laurence LePage competed to produce the U.S. military's first helicopter. LePage received the patent rights to develop helicopters patterned after the Fw 61, and built the XR-1.[75] Meanwhile, Sikorsky settled on a simpler, single rotor design, the VS-300, which turned out to be the first practical single lifting-rotor helicopter design. After experimenting with configurations to counteract the torque produced by the single main rotor, Sikorsky settled on a single, smaller rotor mounted on the tail boom.
+
+Developed from the VS-300, Sikorsky's R-4 was the first large-scale mass-produced helicopter, with a production order for 100 aircraft. The R-4 was the only Allied helicopter to serve in World War II, when it was used primarily for search and rescue (by the USAAF 1st Air Commando Group) in Burma;[76] in Alaska; and in other areas with harsh terrain. Total production reached 131 helicopters before the R-4 was replaced by other Sikorsky helicopters such as the R-5 and the R-6. In all, Sikorsky produced over 400 helicopters before the end of World War II.[77]
+
+While LePage and Sikorsky built their helicopters for the military, Bell Aircraft hired Arthur Young to help build a helicopter using Young's two-blade teetering rotor design, which used a weighted stabilizer bar placed at a 90° angle to the rotor blades. The subsequent Model 30 helicopter showed the design's simplicity and ease of use. The Model 30 was developed into the Bell 47, which became the first helicopter certified for civilian use in the United States. Produced in several countries, the Bell 47 was the most popular helicopter model for nearly 30 years.
+
+In 1951, at the urging of his contacts at the Department of the Navy, Charles Kaman modified his K-225 synchropter — a design for a twin-rotor helicopter concept first pioneered by Anton Flettner in 1939, with the aforementioned Fl 265 piston-engined design in Germany – with a new kind of engine, the turboshaft engine. This adaptation of the turbine engine provided a large amount of power to Kaman's helicopter with a lower weight penalty than piston engines, with their heavy engine blocks and auxiliary components. On 11 December 1951, the Kaman K-225 became the first turbine-powered helicopter in the world. Two years later, on 26 March 1954, a modified Navy HTK-1, another Kaman helicopter, became the first twin-turbine helicopter to fly.[78] However, it was the Sud Aviation Alouette II that would become the first helicopter to be produced with a turbine-engine.[79]
+
+Reliable helicopters capable of stable hover flight were developed decades after fixed-wing aircraft. This is largely due to higher engine power density requirements than fixed-wing aircraft. Improvements in fuels and engines during the first half of the 20th century were a critical factor in helicopter development. The availability of lightweight turboshaft engines in the second half of the 20th century led to the development of larger, faster, and higher-performance helicopters. While smaller and less expensive helicopters still use piston engines, turboshaft engines are the preferred powerplant for helicopters today.
+
+There are several reasons a helicopter cannot fly as fast as a fixed-wing aircraft. When the helicopter is hovering, the outer tips of the rotor travel at a speed determined by the length of the blade and the rotational speed. In a moving helicopter, however, the speed of the blades relative to the air depends on the speed of the helicopter as well as on their rotational speed. The airspeed of the advancing rotor blade is much higher than that of the helicopter itself. It is possible for this blade to exceed the speed of sound, and thus produce vastly increased drag and vibration.
+
+At the same time, the advancing blade creates more lift traveling forward, the retreating blade produces less lift. If the aircraft were to accelerate to the air speed that the blade tips are spinning, the retreating blade passes through air moving at the same speed of the blade and produces no lift at all, resulting in very high torque stresses on the central shaft that can tip down the retreating-blade side of the vehicle, and cause a loss of control. Dual counter-rotating blades prevent this situation due to having two advancing and two retreating blades with balanced forces.
+
+Because the advancing blade has higher airspeed than the retreating blade and generates a dissymmetry of lift, rotor blades are designed to "flap" – lift and twist in such a way that the advancing blade flaps up and develops a smaller angle of attack. Conversely, the retreating blade flaps down, develops a higher angle of attack, and generates more lift. At high speeds, the force on the rotors is such that they "flap" excessively, and the retreating blade can reach too high an angle and stall. For this reason, the maximum safe forward airspeed of a helicopter is given a design rating called VNE, velocity, never exceed.[80] In addition, it is possible for the helicopter to fly at an airspeed where an excessive amount of the retreating blade stalls, which results in high vibration, pitch-up, and roll into the retreating blade.
+
+During the closing years of the 20th century designers began working on helicopter noise reduction. Urban communities have often expressed great dislike of noisy aviation or noisy aircraft, and police and passenger helicopters can be unpopular because of the sound. The redesigns followed the closure of some city heliports and government action to constrain flight paths in national parks and other places of natural beauty.
+
+Helicopters also vibrate; an unadjusted helicopter can easily vibrate so much that it will shake itself apart. To reduce vibration, all helicopters have rotor adjustments for height and weight. Blade height is adjusted by changing the pitch of the blade. Weight is adjusted by adding or removing weights on the rotor head and/or at the blade end caps. Most also have vibration dampers for height and pitch. Some also use mechanical feedback systems to sense and counter vibration. Usually the feedback system uses a mass as a "stable reference" and a linkage from the mass operates a flap to adjust the rotor's angle of attack to counter the vibration. Adjustment is difficult in part because measurement of the vibration is hard, usually requiring sophisticated accelerometers mounted throughout the airframe and gearboxes. The most common blade vibration adjustment measurement system is to use a stroboscopic flash lamp, and observe painted markings or coloured reflectors on the underside of the rotor blades. The traditional low-tech system is to mount coloured chalk on the rotor tips, and see how they mark a linen sheet. Gearbox vibration most often requires a gearbox overhaul or replacement. Gearbox or drive train vibrations can be extremely harmful to a pilot. The most severe being pain, numbness, loss of tactile discrimination and dexterity.
+
+For a standard helicopter with a single main rotor, the tips of the main rotor blades produce a vortex ring in the air, which is a spiraling and circularly rotating airflow. As the craft moves forward, these vortices trail off behind the craft.
+
+When hovering with a forward diagonal crosswind, or moving in a forward diagonal direction, the spinning vortices trailing off the main rotor blades will align with the rotation of the tail rotor and cause an instability in flight control.[81]
+
+When the trailing vortices colliding with the tail rotor are rotating in the same direction, this causes a loss of thrust from the tail rotor. When the trailing vortices rotate in the opposite direction of the tail rotor, thrust is increased. Use of the foot pedals is required to adjust the tail rotor's angle of attack, to compensate for these instabilities.
+
+These issues are due to the exposed tail rotor cutting through open air around rear of the vehicle. This issue disappears when the tail is instead ducted, using an internal impeller enclosed in the tail and a jet of high pressure air sideways out of the tail, as the main rotor vortices can not impact the operation of an internal impeller.
+
+For a standard helicopter with a single main rotor, maintaining steady flight with a crosswind presents an additional flight control problem, where strong crosswinds from certain angles will increase or decrease lift from the main rotors. This effect is also triggered in a no-wind condition when moving the craft diagonally in various directions, depending on the direction of main rotor rotation.[82]
+
+This can lead to a loss of control and a crash or hard landing when operating at low altitudes, due to the sudden unexpected loss of lift, and insufficient time and distance available to recover.
+
+Conventional rotary-wing aircraft use a set of complex mechanical gearboxes to convert the high rotation speed of gas turbines into the low speed required to drive main and tail rotors. Unlike powerplants, mechanical gearboxes cannot be duplicated (for redundancy) and have always been a major weak point in helicopter reliability. In-flight catastrophic gear failures often result in gearbox jamming and subsequent fatalities, whereas loss of lubrication can trigger onboard fire.[citation needed] Another weakness of mechanical gearboxes is their transient power limitation, due to structural fatigue limits. Recent EASA studies point to engines and transmissions as prime cause of crashes just after pilot errors.[83]
+
+By contrast, electromagnetic transmissions do not use any parts in contact; hence lubrication can be drastically simplified, or eliminated. Their inherent redundancy offers good resilience to single point of failure. The absence of gears enables high power transient without impact on service life. The concept of electric propulsion applied to helicopter and electromagnetic drive was brought to reality by Pascal Chretien who designed, built and flew world's first man-carrying, free-flying electric helicopter. The concept was taken from the conceptual computer-aided design model on 10 September 2010 to the first testing at 30% power on 1 March 2011 – less than six months. The aircraft first flew on 12 August 2011. All development was conducted in Venelles, France.[84][85]
+
+As with any moving vehicle, unsafe operation could result in loss of control, structural damage, or loss of life. The following is a list of some of the potential hazards for helicopters:

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+Heliocentrism[a] is the astronomical model in which the Earth and planets revolve around the Sun at the center of the Solar System. Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. The notion that the Earth revolves around the Sun had been proposed as early as the 3rd century BC by Aristarchus of Samos,[1] but at least in the medieval world, Aristarchus' heliocentrism attracted little attention—possibly because of the loss of scientific works of the Hellenistic period.[b]
+
+It was not until the 16th century that a mathematical model of a heliocentric system was presented, by the Renaissance mathematician, astronomer, and Catholic cleric Nicolaus Copernicus, leading to the Copernican Revolution. In the following century, Johannes Kepler introduced elliptical orbits, and Galileo Galilei presented supporting observations made using a telescope.
+
+With the observations of William Herschel, Friedrich Bessel, and other astronomers, it was realized that the Sun, while near the barycenter of the Solar System, was not at any center of the universe.
+
+While the sphericity of the Earth was widely recognized in Greco-Roman astronomy from at least the 4th century BC,[3] the Earth's daily rotation and yearly orbit around the Sun was never universally accepted until the Copernican Revolution.
+
+While a moving Earth was proposed at least from the 4th century BC in Pythagoreanism, and a fully developed heliocentric model was developed by Aristarchus of Samos in the 3rd century BC, these ideas were not successful in replacing the view of a static spherical Earth, and from the 2nd century AD the predominant model, which would be inherited by medieval astronomy, was the geocentric model described in Ptolemy's Almagest.
+
+The Ptolemaic system was a sophisticated astronomical system that managed to calculate the positions for the planets to a fair degree of accuracy.[4] 
+Ptolemy himself, in his Almagest, points out that any model for describing the motions of the planets is merely a mathematical device, and since there is no actual way to know which is true, the simplest model that gets the right numbers should be used.[5]
+However, he rejected the idea of a spinning Earth as absurd as he believed it would create huge winds. His planetary hypotheses were sufficiently real that the distances of the Moon, Sun, planets and stars could be determined by treating orbits' celestial spheres as contiguous realities. This made the stars' distance less than 20 Astronomical Units,[6] a regression, since Aristarchus of Samos's heliocentric scheme had centuries earlier necessarily placed the stars at least two orders of magnitude more distant.
+
+Problems with Ptolemy's system were well recognized in medieval astronomy, and an increasing effort to criticize and improve it in the late medieval period eventually led to the Copernican heliocentrism developed in Renaissance astronomy.
+
+The non-geocentric model of the Universe was proposed by the Pythagorean philosopher Philolaus (d. 390 BC), who taught that at the center of the Universe was a "central fire", around which the Earth, Sun, Moon and planets revolved in uniform circular motion.  This system postulated the existence of a counter-earth collinear with the Earth and central fire, with the same period of revolution around the central fire as the Earth.  The Sun revolved around the central fire once a year, and the stars were stationary.  The Earth maintained the same hidden face towards the central fire, rendering both it and the "counter-earth" invisible from Earth.  The Pythagorean concept of uniform circular motion remained unchallenged for approximately the next 2000 years, and it was to the Pythagoreans that Copernicus referred to show that the notion of a moving Earth was neither new nor revolutionary.[7] Kepler gave an alternative explanation of the Pythagoreans' "central fire" as the Sun, "as most sects purposely hid[e] their teachings".[8]
+
+Heraclides of Pontus (4th century BC) said that the rotation of the Earth explained the apparent daily motion of the celestial sphere. It used to be thought that he believed Mercury and Venus to revolve around the Sun, which in turn (along with the other planets) revolves around the Earth.[9] Macrobius Ambrosius Theodosius (AD 395–423) later described this as the "Egyptian System," stating that "it did not escape the skill of the Egyptians," though there is no other evidence it was known in ancient Egypt.[10][11]
+
+The first person known to have proposed a heliocentric system was Aristarchus of Samos (c. 270 BC). Like his contemporary Eratosthenes, Aristarchus calculated the size of the Earth and measured the sizes and distances of the Sun and Moon. From his estimates, he concluded that the Sun was six to seven times wider than the Earth, and thought that the larger object would have the most attractive force.
+
+His writings on the heliocentric system are lost, but some information about them is known from a brief description by his contemporary, Archimedes, and from scattered references by later writers. Archimedes' description of Aristarchus' theory is given in the former's book, The Sand Reckoner. The entire description comprises just three sentences, which Thomas Heath translates as follows:[12]
+
+You [King Gelon] are aware that "universe" is the name given by most astronomers to the sphere, the centre of which is the centre of the earth, while its radius is equal to the straight line between the centre of the sun and the centre of the earth.  This is the common account (τά γραφόμενα), as you have heard from astronomers.  But Aristarchus brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the "universe" just mentioned. His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun on the circumference of a circle, the sun lying in the middle of the orbit, and that the sphere of the fixed stars, situated about the same centre as the sun, is so great that the circle in which he supposes the earth to revolve bears such a proportion to the distance of the fixed stars as the centre of the sphere bears to its surface.
+
+Aristarchus presumably took the stars to be very far away because he was aware that their parallax[13] would otherwise be observed over the course of a year. The stars are in fact so far away that stellar parallax only became detectable when sufficiently powerful telescopes had been developed.
+
+No references to Aristarchus' heliocentrism are known in any other writings from before the common era.  The earliest of the handful of other ancient references occur in two passages from the writings of Plutarch.  These mention one detail not stated explicitly in Archimedes' account[14]—namely, that Aristarchus' theory had the Earth rotating on an axis. The first of these reference occurs in On the Face in the Orb of the Moon:[15]
+
+Only do not, my good fellow, enter an action against me for impiety in the style of Cleanthes, who thought it was the duty of Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the Universe, this being the effect of his attempt to save the phenomena by supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis.
+
+Only scattered fragments of Cleanthes' writings have survived in quotations by other writers, but in Lives and Opinions of Eminent Philosophers, Diogenes Laërtius lists A reply to Aristarchus (Πρὸς Ἀρίσταρχον) as one of Cleanthes' works,[16] and some scholars[17] have suggested that this might have been where Cleanthes had accused Aristarchus of impiety.
+
+The second of the references by Plutarch is in his Platonic Questions:[18]
+
+Did Plato put the earth in motion, as he did the sun, the moon, and the five planets, which he called the instruments of time on account of their turnings, and was it necessary to conceive that the earth "which is globed about the axis stretched from pole to pole through the whole universe" was not represented as being held together and at rest, but as turning and revolving (στρεφομένην καὶ ἀνειλουμένην), as Aristarchus and Seleucus afterwards maintained that it did, the former stating this as only a hypothesis (ὑποτιθέμενος μόνον), the latter as a definite opinion (καὶ ἀποφαινόμενος)?
+
+The remaining references to Aristarchus' heliocentrism are extremely brief, and provide no more information beyond what can be gleaned from those already cited. Ones which mention Aristarchus explicitly by name occur in Aëtius' Opinions of the Philosophers, Sextus Empiricus' Against the Mathematicians,[18] and an anonymous scholiast to Aristotle.[19] Another passage in Aëtius' Opinions of the Philosophers reports that Seleucus the astronomer had affirmed the Earth's motion, but does not mention Aristarchus.[18]
+
+Since Plutarch mentions the "followers of Aristarchus" in passing, it is likely that there were other astronomers in the Classical period who also espoused heliocentrism, but whose work was lost. The only other astronomer from antiquity known by name who is known to have supported Aristarchus' heliocentric model was Seleucus of Seleucia (b. 190 BC), a Hellenistic astronomer who flourished a century after Aristarchus in the Seleucid empire.[20] Seleucus was a proponent of the heliocentric system of Aristarchus.[21] Seleucus may have proved the heliocentric theory by determining the constants of a geometric model for the heliocentric theory and developing methods to compute planetary positions using this model. He may have used early trigonometric methods that were available in his time, as he was a contemporary of Hipparchus.[22]  A fragment of a work by Seleucus has survived in Arabic translation, which was referred to by Rhazes (b. 865).[23]
+
+Alternatively, his explanation may have involved the phenomenon of tides,[24] which he supposedly theorized to be caused by the attraction to the Moon and by the revolution of the Earth around the Earth and Moon's center of mass.
+
+There were occasional speculations about heliocentrism in Europe before Copernicus. In Roman Carthage, the pagan Martianus Capella (5th century A.D.) expressed the opinion that the planets Venus and Mercury did not go about the Earth but instead circled the Sun.[25]  Capella's model was discussed in the Early Middle Ages by various anonymous 9th-century commentators[26] and Copernicus mentions him as an influence on his own work.[27]
+
+The Ptolemaic system was also received in Indian astronomy. Aryabhata (476–550), in his magnum opus Aryabhatiya (499), propounded a planetary model in which the Earth was taken to be spinning on its axis and the periods of the planets were given with respect to the Sun.[28] His immediate commentators, such as Lalla, and other later authors, rejected his innovative view about the turning Earth.[29]  He also made many astronomical calculations, such as the times of the solar and lunar eclipses, and the instantaneous motion of the Moon.[30] Early followers of Aryabhata's model included Varahamihira, Brahmagupta, and Bhaskara II.
+
+For a time, Muslim astronomers accepted the Ptolemaic system and the geocentric model, which were used by al-Battani to show that the distance between the Sun and the Earth varies.[31][32] In the 10th century, al-Sijzi accepted that the Earth rotates around its axis.[33][34] According to later astronomer al-Biruni, al-Sijzi invented an astrolabe called al-zūraqī based on a belief held by some of his contemporaries that the apparent motion of the stars was due to the Earth's movement, and not that of the firmament.[34][35] Islamic astronomers began to criticize the Ptolemaic model, including Ibn al-Haytham in his Al-Shukūk 'alā Baṭalamiyūs ("Doubts Concerning Ptolemy", c. 1028),[36][37] who branded it an impossibility.[38]
+
+Al-Biruni discussed the possibility of whether the Earth rotated about its own axis and orbited the Sun, but in his Masudic Canon (1031),[39] he expressed his faith in a geocentric and stationary Earth.[40] He was aware that if the Earth rotated on its axis, it would be consistent with his astronomical observations,[41] but considered it a problem of natural philosophy rather than one of mathematics.[34][42]
+
+In the 12th century, non-heliocentric alternatives to the Ptolemaic system were developed by some Islamic astronomers, such as Nur ad-Din al-Bitruji, who considered the Ptolemaic model mathematical, and not physical.[43][44] His system spread throughout most of Europe in the 13th century, with debates and refutations of his ideas continued to the 16th century.[44]
+
+The Maragha school of astronomy in Ilkhanid-era Persia further developed "non-Ptolemaic" planetary models involving Earth's rotation. Notable astronomers of this school are Al-Urdi (d. 1266) Al-Katibi (d. 1277),[45] and Al-Tusi (d. 1274).
+
+The arguments and evidence used resemble those used by Copernicus to support the Earth's motion.[46][47] 
+The criticism of Ptolemy as developed by Averroes and by the Maragha school explicitly address the Earth's rotation but it did not arrive at explicit heliocentrism.[48] 
+The observations of the Maragha school were further improved at the Timurid-era Samarkand observatory under Qushji (1403–1474).
+
+European scholarship in the later medieval period actively received astronomical models developed in the Islamic world and by the 13th century was well aware of the problems of the Ptolemaic model. In the 14th century, bishop Nicole Oresme discussed the possibility that the Earth rotated on its axis, while Cardinal Nicholas of Cusa in his Learned Ignorance asked whether there was any reason to assert that the Sun (or any other point) was the center of the universe.  In parallel to a mystical definition of God, Cusa wrote that "Thus the fabric of the world (machina mundi) will quasi have its center everywhere and circumference nowhere,"[49] recalling Hermes Trismegistus.[50]
+
+In India, Nilakantha Somayaji (1444–1544), in his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, developed a computational system for a geo-heliocentric planetary model, in which the planets orbit the Sun, which in turn orbits the Earth, similar to the system later proposed by Tycho Brahe. In the Tantrasamgraha (1501), Somayaji further revised his planetary system, which was mathematically more accurate at predicting the heliocentric orbits of the interior planets than both the Tychonic and Copernican models,[51][52] but did not propose any specific models of the universe.[53] Nilakantha's planetary system also incorporated the Earth's rotation on its axis.[54] Most astronomers of the Kerala school of astronomy and mathematics seem to have accepted his planetary model.[55][56]
+
+Some historians maintain that the thought of the Maragheh observatory, in particular the mathematical devices known as the Urdi lemma and the Tusi couple,  influenced Renaissance-era European astronomy, and thus was indirectly received by Renaissance-era European astronomy and thus by Copernicus.[42][57][58][59][60] 
+Copernicus used such devices in the same planetary models as found in Arabic sources.[61] 
+Furthermore, the exact replacement of the equant by two epicycles used by Copernicus in the Commentariolus was found in an earlier work by Ibn al-Shatir (d. c. 1375) of Damascus.[62] Copernicus' lunar and Mercury models are also identical to Ibn al-Shatir's.[63]
+
+Leonardo da Vinci (1452–1519) wrote "Il sole non si move." ("The Sun does not move.") [64]
+
+The state of knowledge on planetary theory received by Copernicus is summarized in Georg von Peuerbach's Theoricae Novae Planetarum (printed in 1472 by Regiomontanus). By 1470, the accuracy of observations by the Vienna school of astronomy, of which Peuerbach and Regiomontanus were members, was high enough to make the eventual development of heliocentrism inevitable, and indeed it is possible that Regiomontanus did arrive at an explicit theory of heliocentrism before his death in 1476, some 30 years before Copernicus.[65]
+While the influence of the criticism of Ptolemy by Averroes on Renaissance thought is clear and explicit, the claim of direct influence of the 
+Maragha school, postulated by Otto E. Neugebauer in 1957, remains an open question.[48][66][67] Since the Tusi couple was used by Copernicus in his reformulation of mathematical astronomy, there is a growing consensus that he became aware of this idea in some way.  It has been suggested[68][69] that the idea of the Tusi couple may have arrived in Europe leaving few manuscript traces, since it could have occurred without the translation of any Arabic text into Latin. One possible route of transmission may have been through Byzantine science, which translated some of al-Tusi's works from Arabic into Byzantine Greek. Several Byzantine Greek manuscripts containing the Tusi-couple are still extant in Italy.[70] Other scholars have argued that Copernicus could well have developed these ideas independently of the late Islamic tradition.[71] Copernicus explicitly references several astronomers of the "Islamic Golden Age" (10th to 12th centuries) in De Revolutionibus: Albategnius (Al-Battani), Averroes (Ibn Rushd), Thebit (Thabit Ibn Qurra), Arzachel (Al-Zarqali), and Alpetragius (Al-Bitruji), but he does not show awareness of the existence of any of the later astronomers of the Maragha school.[72]
+
+It has been argued that Copernicus could have independently discovered the Tusi couple or took the idea from Proclus's Commentary on the First Book of Euclid,[73] which Copernicus cited.[74] 
+Another possible source for Copernicus's knowledge of this mathematical device is the Questiones de Spera of Nicole Oresme, who described how a reciprocating linear motion of a celestial body could be produced by a combination of circular motions similar to those proposed by al-Tusi.[75]
+
+Nicolaus Copernicus in his De revolutionibus orbium coelestium ("On the revolution of heavenly spheres", first printed in 1543 in Nuremberg), presented a discussion of a heliocentric model of the universe in much the same way as Ptolemy in the 2nd century had presented his geocentric model in his Almagest.
+Copernicus discussed the philosophical implications of his proposed system, elaborated it in geometrical detail, used selected astronomical observations to derive the parameters of his model, and wrote astronomical tables which enabled one to compute the past and future positions of the stars and planets.  In doing so, Copernicus moved heliocentrism from philosophical speculation to predictive geometrical astronomy. In reality, Copernicus's system did not predict the planets' positions any better than the Ptolemaic system.[76] This theory resolved the issue of planetary retrograde motion by arguing that such motion was only perceived and apparent, rather than real: it was a parallax effect, as an object that one is passing seems to move backwards against the horizon. This issue was also resolved in the geocentric Tychonic system; the latter, however, while eliminating the major epicycles, retained as a physical reality the irregular back-and-forth motion of the planets, which Kepler characterized as a "pretzel".[77]
+
+Copernicus cited Aristarchus in an early (unpublished) manuscript of De Revolutionibus (which still survives), stating: "Philolaus believed in the mobility of the earth, and some even say that Aristarchus of Samos was of that opinion."[78] However, in the published version he restricts himself to noting that in works by Cicero he had found an account of the theories of Hicetas and that Plutarch had provided him with an account of the Pythagoreans, Heraclides Ponticus, Philolaus, and Ecphantus. These authors had proposed a moving Earth, which did not, however, revolve around a central sun
+
+The first information about the heliocentric views of Nicolaus Copernicus was circulated in manuscript completed some time before May 1, 1514.[79]  Although only in manuscript, Copernicus' ideas were well known among astronomers and others. His ideas contradicted the then-prevailing understanding of the Bible. In the King James Bible (first published in 1611), First Chronicles 16:30 states that "the world also shall be stable, that it be not moved." Psalm 104:5 says, "[the Lord] Who laid the foundations of the earth, that it should not be removed for ever." Ecclesiastes 1:5 states that "The sun also ariseth, and the sun goeth down, and hasteth to his place where he arose."
+
+Nonetheless, in 1533, Johann Albrecht Widmannstetter delivered in Rome a series of lectures outlining Copernicus' theory. The lectures were heard with interest by Pope Clement VII and several Catholic cardinals.[80] On November 1, 1536, Archbishop of Capua Nikolaus von Schönberg wrote a letter to Copernicus from Rome encouraging him to publish a full version of his theory.
+
+However, in 1539, Martin Luther said:
+
+"There is talk of a new astrologer who wants to prove that the earth moves and goes around instead of the sky, the sun, the moon, just as if somebody were moving in a carriage or ship might hold that he was sitting still and at rest while the earth and the trees walked and moved. But that is how things are nowadays: when a man wishes to be clever he must . . . invent something special, and the way he does it must needs be the best! The fool wants to turn the whole art of astronomy upside-down. However, as Holy Scripture tells us, so did Joshua bid the sun to stand still and not the earth."[81]
+
+This was reported in the context of a conversation at the dinner table and not a formal statement of faith. Melanchthon, however, opposed the doctrine over a period of years.[82][83]
+
+Nicolaus Copernicus published the definitive statement of his system in De Revolutionibus in 1543. Copernicus began to write it in 1506 and finished it in 1530, but did not publish it until the year of his death. Although he was in good standing with the Church and had dedicated the book to Pope Paul III, the published form contained an unsigned preface by Osiander defending the system and arguing that it was useful for computation even if its hypotheses were not necessarily true. Possibly because of that preface, the work of Copernicus inspired very little debate on whether it might be heretical during the next 60 years. There was an early suggestion among Dominicans that the teaching of heliocentrism should be banned, but nothing came of it at the time.
+
+Some years after the publication of De Revolutionibus John Calvin preached a sermon in which he denounced those who "pervert the order of nature" by saying that "the sun does not move and that it is the earth that revolves and that it turns".[84][d]
+
+Prior to the publication of De Revolutionibus, the most widely accepted system had been proposed by Ptolemy, in which the Earth was the center of the universe and all celestial bodies orbited it. Tycho Brahe, arguably the most accomplished astronomer of his time, advocated against Copernicus's heliocentric system and for an alternative to the Ptolemaic geocentric system:  a geo-heliocentric system now known as the Tychonic system in which the five then known planets orbit the Sun, while the Sun and the Moon orbit the Earth.
+
+Tycho appreciated the Copernican system, but objected to the idea of a moving Earth on the basis of physics, astronomy, and religion. The Aristotelian physics of the time (modern Newtonian physics was still a century away) offered no physical explanation for the motion of a massive body like Earth, whereas it could easily explain the motion of heavenly bodies by postulating that they were made of a different sort substance called aether that moved naturally.  So Tycho said that the Copernican system "... expertly and completely circumvents all that is superfluous or discordant in the system of Ptolemy. On no point does it offend the principle of mathematics. Yet it ascribes to the Earth, that hulking, lazy body, unfit for motion, a motion as quick as that of the aethereal torches, and a triple motion at that."[89] Likewise, Tycho took issue with the vast distances to the stars that Aristarchus and Copernicus had assumed in order to explain the lack of any visible parallax. Tycho had measured the apparent sizes of stars (now known to be illusory), and used geometry to calculate that in order to both have those apparent sizes and be as far away as heliocentrism required, stars would have to be huge (much larger than the sun; the size of Earth's orbit or larger). Regarding this Tycho wrote, "Deduce these things geometrically if you like, and you will see how many absurdities (not to mention others) accompany this assumption [of the motion of the earth] by inference."[90]  He also cited the Copernican system's "opposition to the authority of Sacred Scripture in more than one place" as a reason why one might wish to reject it, and observed that his own geo-heliocentric alternative "offended neither the principles of physics nor Holy Scripture".[91]
+
+The Jesuit astronomers in Rome were at first unreceptive to Tycho's system; the most prominent, Clavius, commented that Tycho was "confusing all of astronomy, because he wants to have Mars lower than the Sun."[92] However, after the advent of the telescope showed problems with some geocentric models (by demonstrating that Venus circles the Sun, for example), the Tychonic system and variations on that system became popular among geocentrists, and the Jesuit astronomer Giovanni Battista Riccioli would continue Tycho's use of physics, stellar astronomy (now with a telescope), and religion to argue against heliocentrism and for Tycho's system well into the seventeenth century.
+
+Giordano Bruno (d. 1600) is the only known person to defend Copernicus's heliocentrism in his time.[93] Using measurements made at Tycho's observatory, Johannes Kepler developed his laws of planetary motion between 1609 and 1619.[94] In Astronomia nova (1609), Kepler made a diagram of the movement of Mars in relation to Earth if Earth were at the center of its orbit, which shows that Mars' orbit would be completely imperfect and never follow along the same path. To solve the apparent derivation of Mars' orbit from a perfect circle, Kepler derived both a mathematical definition and, independently, a matching ellipse around the Sun to explain the motion of the red planet.[95]
+
+Between 1617 and 1621, Kepler developed a heliocentric model of the Solar System in Epitome astronomiae Copernicanae, in which all the planets have elliptical orbits. This provided significantly increased accuracy in predicting the position of the planets. Kepler's ideas were not immediately accepted, and Galileo for example ignored them. In 1621, Epitome astronomia Copernicanae was placed on the Catholic Church's index of prohibited books despite Kepler being a Protestant.
+
+Galileo was able to look at the night sky with the newly invented telescope. He published his discoveries that Jupiter is orbited by moons and that the Sun rotates in his Sidereus Nuncius (1610)[93] and Letters on Sunspots (1613), respectively. Around this time, he also announced that Venus exhibits a full range of phases (satisfying an argument that had been made against Copernicus).[93] As the Jesuit astronomers confirmed Galileo's observations, the Jesuits moved away from the Ptolemaic model and toward Tycho's teachings.[96]
+
+In his 1615 "Letter to the Grand Duchess Christina", Galileo defended heliocentrism, and claimed it was not contrary to Holy Scripture. He took Augustine's position on Scripture: not to take every passage literally when the scripture in question is in a Bible book of poetry and songs, not a book of instructions or history. The writers of the Scripture wrote from the perspective of the terrestrial world, and from that vantage point the Sun does rise and set. In fact, it is the Earth's rotation which gives the impression of the Sun in motion across the sky.
+In February 1615, prominent Dominicans including Thomaso Caccini and Niccolò Lorini brought Galileo's writings on heliocentrism to the attention of the Inquisition, because they appeared to violate Holy Scripture and the decrees of the Council of Trent.[97][98] Cardinal and Inquisitor Robert Bellarmine was called upon to adjudicate, and wrote in April that treating heliocentrism as a real phenomenon would be "a very dangerous thing," irritating philosophers and theologians, and harming "the Holy Faith by rendering Holy Scripture as false."[99]
+
+In January 1616, Msgr. Francesco Ingoli addressed an essay to Galileo disputing the Copernican system. Galileo later stated that he believed this essay to have been instrumental in the ban against Copernicanism that followed in February.[100] According to Maurice Finocchiaro, Ingoli had probably been commissioned by the Inquisition to write an expert opinion on the controversy, and the essay provided the "chief direct basis" for the ban.[101] The essay focused on eighteen physical and mathematical arguments against heliocentrism.  It borrowed primarily from the arguments of Tycho Brahe, and it notedly mentioned the problem that heliocentrism requires the stars to be much larger than the Sun.  Ingoli wrote that the great distance to the stars in the heliocentric theory "clearly proves ... the fixed stars to be of such size, as they may surpass or equal the size of the orbit circle of the Earth itself."[102]  Ingoli included four theological arguments in the essay, but suggested to Galileo that he focus on the physical and mathematical arguments. Galileo did not write a response to Ingoli until 1624.[103]
+
+In February 1616, the Inquisition assembled a committee of theologians, known as qualifiers, who delivered their unanimous report condemning heliocentrism as "foolish and absurd in philosophy, and formally heretical since it explicitly contradicts in many places the sense of Holy Scripture." The Inquisition also determined that the Earth's motion "receives the same judgement in philosophy and ... in regard to theological truth it is at least erroneous in faith."[104] Bellarmine personally ordered Galileo
+
+to abstain completely from teaching or defending this doctrine and opinion or from discussing it... to abandon completely... the opinion that the sun stands still at the center of the world and the earth moves, and henceforth not to hold, teach, or defend it in any way whatever, either orally or in writing.
+
+In March 1616, after the Inquisition's injunction against Galileo, the papal Master of the Sacred Palace, Congregation of the Index, and the Pope banned all books and letters advocating the Copernican system, which they called "the false Pythagorean doctrine, altogether contrary to Holy Scripture."[105][106] In 1618, the Holy Office recommended that a modified version of Copernicus' De Revolutionibus be allowed for use in calendric calculations, though the original publication remained forbidden until 1758.[106]
+
+Pope Urban VIII encouraged Galileo to publish the pros and cons of heliocentrism. Galileo's response, Dialogue concerning the two chief world systems (1632), clearly advocated heliocentrism, despite his declaration in the preface that,
+
+I will endeavour to show that all experiments that can be made upon the Earth are insufficient means to conclude for its mobility but are indifferently applicable to the Earth, movable or immovable...[107]
+
+and his straightforward statement,
+
+I might very rationally put it in dispute, whether there be any such centre in nature, or no; being that neither you nor any one else hath ever proved, whether the World be finite and figurate, or else infinite and interminate; yet nevertheless granting you, for the present, that it is finite, and of a terminate Spherical Figure, and that thereupon it hath its centre...[107]
+
+Some ecclesiastics also interpreted the book as characterizing the Pope as a simpleton, since his viewpoint in the dialogue was advocated by the character Simplicio. Urban VIII became hostile to Galileo and he was again summoned to Rome.[108] Galileo's trial in 1633 involved making fine distinctions between "teaching" and "holding and defending as true". For advancing heliocentric theory Galileo was forced to recant Copernicanism and was put under house arrest for the last few years of his life. According to J. L. Heilbron, informed contemporaries of Galileo's "appreciated that the reference to heresy in connection with Galileo or Copernicus had no general or theological significance."[109]
+
+In 1664, Pope Alexander VII published his Index Librorum Prohibitorum Alexandri VII Pontificis Maximi jussu editus (Index of Prohibited Books, published by order of Alexander VII, P.M.) which included all previous condemnations of heliocentric books.[110]
+
+René Descartes postponed, and ultimately never finished, his treatise The World, which included a heliocentric model,[111] but the Galileo affair did little to slow the spread of heliocentrism across Europe, as Kepler's Epitome of Copernican Astronomy became increasingly influential in the coming decades.[112]  By 1686 the model was well enough established that the general public was reading about it in Conversations on the Plurality of Worlds, published in France by Bernard le Bovier de Fontenelle and translated into English and other languages in the coming years.  It has been called "one of the first great popularizations of science."[111]
+
+In 1687, Isaac Newton published Philosophiæ Naturalis Principia Mathematica, which provided an explanation for Kepler's laws in terms of universal gravitation and what came to be known as Newton's laws of motion. This placed heliocentrism on a firm theoretical foundation, although Newton's heliocentrism was of a somewhat modern kind. Already in the mid-1680s he recognized the "deviation of the Sun" from the center of gravity of the Solar System.[113] For Newton it was not precisely the center of the Sun or any other body that could be considered at rest, but "the common centre of gravity of the Earth, the Sun and all the Planets is to be esteem'd the Centre of the World", and this center of gravity "either is at rest or moves uniformly forward in a right line". Newton adopted the "at rest" alternative in view of common consent that the center, wherever it was, was at rest.[114]
+
+Meanwhile, the Catholic Church remained opposed to heliocentrism as a literal description, but this did not by any means imply opposition to all astronomy; indeed, it needed observational data to maintain its calendar. In support of this effort it allowed the cathedrals themselves to be used as solar observatories called meridiane; i.e., they were turned into "reverse sundials", or gigantic pinhole cameras, where the Sun's image was projected from a hole in a window in the cathedral's lantern onto a meridian line.[citation needed]
+
+In the mid-eighteenth century the Catholic Church's opposition began to fade. An annotated copy of Newton's Principia was published in 1742 by Fathers le Seur and Jacquier of the Franciscan Minims, two Catholic mathematicians, with a preface stating that the author's work assumed heliocentrism and could not be explained without the theory. In 1758 the Catholic Church dropped the general prohibition of books advocating heliocentrism from the Index of Forbidden Books.[115] The Observatory of the Roman College was established by Pope Clement XIV in 1774 (nationalized in 1878, but re-founded by Pope Leo XIII as the Vatican Observatory in 1891). In spite of dropping its active resistance to heliocentrism, the Catholic Church did not lift the prohibition of uncensored versions of Copernicus's De Revolutionibus or Galileo's Dialogue. The affair was revived in 1820, when the Master of the Sacred Palace (the Catholic Church's chief censor), Filippo Anfossi, refused to license a book by a Catholic canon, Giuseppe Settele, because it openly treated heliocentrism as a physical fact.[116] Settele appealed to pope Pius VII.  After the matter had been reconsidered by the Congregation of the Index and the Holy Office, Anfossi's decision was overturned.[116] Pius VII approved a decree in 1822 by the Sacred Congregation of the Inquisition to allow the printing of heliocentric books in Rome. Copernicus's De Revolutionibus and Galileo's Dialogue were then subsequently omitted from the next edition of the Index when it appeared in 1835.
+
+Three apparent proofs of the heliocentric hypothesis were provided in 1727 by James Bradley, in 1838 by Friedrich Wilhelm Bessel, and in 1851 by Léon Foucault. Bradley discovered the stellar aberration, proving the relative motion of the Earth. Bessel proved that the parallax of a star was greater than zero by measuring the parallax of 0.314 arcseconds of a star named 61 Cygni. In the same year Friedrich Georg Wilhelm Struve and Thomas Henderson measured the parallaxes of other stars, Vega and Alpha Centauri. Experiments like those of Foucault were performed by V. Viviani in 1661 in Florence and by Bartolini in 1833 in Rimini.[117]
+
+Already in the Talmud, Greek philosophy and science under general name "Greek wisdom" were considered dangerous. They were put under ban then and later for some periods.
+The first Jewish scholar to describe the Copernican system, albeit without mentioning Copernicus by name,  was Maharal of Prague,  his book "Be'er ha-Golah" (1593). Maharal makes an argument of radical skepticism, arguing that no scientific theory can be reliable, which he illustrates by the new-fangled theory of heliocentrism upsetting even the most fundamental views on the cosmos.[118]
+
+Copernicus is mentioned in the books of David Gans (1541–1613), who worked with Tycho Brahe and Johannes Kepler. Gans wrote two books on astronomy in Hebrew: a short one "Magen David" (1612) and a full one "Nehmad veNaim" (published only in 1743). He described objectively three systems: Ptolemy, Copernicus and of Tycho Brahe without taking sides.
+Joseph Solomon Delmedigo (1591–1655) in his "Elim" (1629) says that the arguments of Copernicus are so strong, that only an imbecile will not accept them.[119] Delmedigo studied at Padua and was acquainted with Galileo.[120]
+
+An actual controversy on the Copernican model within Judaism arises only in the early 18th century. Most authors in this period accept Copernican heliocentrism, with opposition from   David Nieto and Tobias Cohn.  Both of these authors argued against heliocentrism on grounds of contradictions to scripture. Nieto merely rejected the new system on those grounds without much passion, whereas Cohn went so far as to call Copernicus "a first-born of Satan", though he also acknowledged[121] that he would have found it difficult to counter one particular objection based on a passage from the Talmud.
+
+In the 19th century two students of the Hatam sofer wrote books that were given approbations by him even though one supported heliocentrism and the other geocentrism. The one, a commentary on Genesis Yafe’ah le-Ketz[122] written by R. Israel David Schlesinger resisted a heliocentric model and supported geocentrism.[123] The other, Mei Menuchot[124] written by R. Eliezer Lipmann Neusatz encouraged acceptance of the heliocentric model and other modern scientific thinking.[125]
+
+Since the 20th century most Jews have not questioned the science of heliocentrism. Exceptions include Shlomo Benizri[126] and R. M.M. Schneerson of Chabad who argued that the question of heliocentrism vs. geocentrism is obsolete because of the relativity of motion.[127] Schneerson's followers in Chabad continue to deny the heliocentric model.[128]
+
+Over the course of the 18th and 19th centuries, the status of the Sun as merely one star among many became increasingly obvious. By the 20th century, even before the discovery that there are many galaxies, it was no longer an issue.[citation needed]
+
+The concept of an absolute velocity, including being "at rest" as a particular case, is ruled out by the principle of relativity, also eliminating any obvious "center" of the universe as a natural origin of coordinates. Even if the discussion is limited to the Solar System, the Sun is not at the geometric center of any planet's orbit, but rather approximately at one focus of the elliptical orbit. Furthermore, to the extent that a planet's mass cannot be neglected in comparison to the Sun's mass, the center of gravity of the Solar System is displaced slightly away from the center of the Sun.[114] (The masses of the planets, mostly Jupiter, amount to 0.14% of that of the Sun.)  Therefore, a hypothetical astronomer on an extrasolar planet would observe a small "wobble" in the Sun's motion.[citation needed]
+
+In modern calculations, the terms "geocentric" and "heliocentric" are often used to refer to reference frames.[129]  In such systems the origin in the center of mass of the Earth, of the Earth–Moon system, of the Sun, of the Sun plus the major planets, or of the entire Solar System, can be selected.[130] Right ascension and declination are examples of geocentric coordinates, used in Earth-based observations, while the heliocentric latitude and longitude are used for orbital calculations.  This leads to such terms as "heliocentric velocity" and "heliocentric angular momentum". In this heliocentric picture, any planet of the Solar System can be used as a source of mechanical energy because it moves relatively to the Sun. A smaller body (either artificial or natural) may gain heliocentric velocity due to gravity assist – this effect can change the body's mechanical energy in heliocentric reference frame (although it will not changed in the planetary one). However, such selection of "geocentric" or "heliocentric" frames is merely a matter of computation. It does not have philosophical implications and does not constitute a distinct physical or scientific model. From the point of view of general relativity, inertial reference frames do not exist at all, and any practical reference frame is only an approximation to the actual space-time, which can have higher or lower precision. Some forms of Mach's principle consider the frame at rest with respect to the distant masses in the universe to have special properties.[citation needed]
+
+Footnotes
+
+Citations
+
+All Islamic astronomers from Thabit ibn Qurra in the ninth century to Ibn al-Shatir in the fourteenth, and all natural philosophers from al-Kindi to Averroes and later, are known to have accepted ... the Greek picture of the world as consisting of two spheres of which one, the celestial sphere ... concentrically envelops the other.
+

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+
+
+Helium (from Greek: ἥλιος, romanized: Helios, lit. 'Sun') is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas, the first in the noble gas group in the periodic table.[a] Its boiling point is the lowest among all the elements. Helium is the second lightest and second most abundant element in the observable universe (hydrogen is the lightest and most abundant). It is present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements combined. Its abundance is similar to this in both the Sun and in Jupiter. This is due to the very high nuclear binding energy (per nucleon) of helium-4, with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, the vast majority of which was formed during the Big Bang. Large amounts of new helium are being created by nuclear fusion of hydrogen in stars.
+
+Helium is named for the Greek Titan of the Sun, Helios. It was first detected as an unknown, yellow spectral line signature in sunlight, during a solar eclipse in 1868 by Georges Rayet,[11] Captain C. T. Haig,[12] Norman R. Pogson,[13] and Lieutenant John Herschel,[14] and was subsequently confirmed by French astronomer, Jules Janssen.[15] Janssen is often jointly credited with detecting the element, along with Norman Lockyer. Janssen recorded the helium spectral line during the solar eclipse of 1868, while Lockyer observed it from Britain. Lockyer was the first to propose that the line was due to a new element, which he named. The formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore, cleveite, which is now not regarded as a separate mineral species but as a variety of uraninite.[16][17] In 1903, large reserves of helium were found in natural gas fields in parts of the United States, which is by far the largest supplier of the gas today.
+
+Liquid helium is used in cryogenics (its largest single use, absorbing about a quarter of production), particularly in the cooling of superconducting magnets, with the main commercial application being in MRI scanners. Helium's other industrial uses—as a pressurizing and purge gas, as a protective atmosphere for arc welding, and in processes such as growing crystals to make silicon wafers—account for half of the gas produced. A well-known but minor use is as a lifting gas in balloons and airships.[18] As with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice. In scientific research, the behavior of the two fluid phases of helium-4 (helium I and helium II) is important to researchers studying quantum mechanics (in particular the property of superfluidity) and to those looking at the phenomena, such as superconductivity, produced in matter near absolute zero.
+
+On Earth, it is relatively rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium, although there are other examples), as the alpha particles emitted by such decays consist of helium-4 nuclei. This radiogenic helium is trapped with natural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. Previously, terrestrial helium—a non-renewable resource because once released into the atmosphere, it readily escapes into space—was thought to be in increasingly short supply.[19][20] However, recent studies suggest that helium produced deep in the earth by radioactive decay can collect in natural gas reserves in larger than expected quantities,[21] in some cases, having been released by volcanic activity.[22]
+
+The first evidence of helium was observed on August 18, 1868, as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India.[23][24] This line was initially assumed to be sodium. On October 20 of the same year, English astronomer, Norman Lockyer, observed a yellow line in the solar spectrum, which, he named the D3 because it was near the known D1 and D2 Fraunhofer line lines of sodium.[25][26] He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος (helios).[27][28]
+
+In 1881, Italian physicist Luigi Palmieri detected helium on Earth for the first time through its D3 spectral line, when he analyzed a material that had been sublimated during a recent eruption of Mount Vesuvius.[29]
+
+On March 26, 1895, Scottish chemist, Sir William Ramsay, isolated helium on Earth by treating the mineral cleveite (a variety of uraninite with at least 10% rare earth elements) with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas, liberated by sulfuric acid, he noticed a bright yellow line that matched the D3 line observed in the spectrum of the Sun.[26][31][32][33] These samples were identified as helium, by Lockyer, and British physicist William Crookes.[34][35] It was independently isolated from cleveite, in the same year, by chemists, Per Teodor Cleve and Abraham Langlet, in Uppsala, Sweden, who collected enough of the gas to accurately determine its atomic weight.[24][36][37] Helium was also isolated by the American geochemist, William Francis Hillebrand, prior to Ramsay's discovery, when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, however, attributed the lines to nitrogen.[38] His letter of congratulations to Ramsay offers an interesting case of discovery, and near-discovery, in science.[39]
+
+In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei, by allowing the particles to penetrate the thin, glass wall of an evacuated tube, then creating a discharge in the tube, to study the spectrum of the new gas inside.[40] In 1908, helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than five Kelvin.[41][42] He tried to solidify it, by further reducing the temperature, but failed, because helium does not solidify at atmospheric pressure. Onnes' student Willem Hendrik Keesom was eventually able to solidify 1 cm3 of helium in 1926 by applying additional external pressure.[43][44]
+
+In 1913, Niels Bohr published his "trilogy"[45][46] on atomic structure that included a reconsideration of the Pickering–Fowler series as central evidence in support of his model of the atom.[47][48] This series is named for Edward Charles Pickering, who in 1896 published observations of previously unknown lines in the spectrum of the star ζ Puppis[49] (these are now known to occur with Wolf–Rayet and other hot stars).[50] Pickering attributed the observation (lines at 4551, 5411, and 10123 Å) to a new form of hydrogen with half-integer transition levels.[51][52] In 1912, Alfred Fowler[53] managed to produce similar lines from a hydrogen-helium mixture, and supported Pickering's conclusion as to their origin.[54] Bohr's model does not allow for half-integer transitions (nor does quantum mechanics) and Bohr concluded that Pickering and Fowler were wrong, and instead assigned these spectral lines to ionised helium, He+.[55] Fowler was initially skeptical[56] but was ultimately convinced[57] that Bohr was correct,[45] and by 1915 "spectroscopists had transferred [the Pickering–Fowler series] definitively [from hydrogen] to helium."[48][58] Bohr's theoretical work on the Pickering series had demonstrated the need for "a re-examination of problems that seemed already to have been solved within classical theories" and provided important confirmation for his atomic theory.[48]
+
+In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity.[59] This phenomenon is related to Bose–Einstein condensation. In 1972, the same phenomenon was observed in helium-3, but at temperatures much closer to absolute zero, by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson. The phenomenon in helium-3 is thought to be related to pairing of helium-3 fermions to make bosons, in analogy to Cooper pairs of electrons producing superconductivity.[60]
+
+After an oil drilling operation in 1903 in Dexter, Kansas produced a gas geyser that would not burn, Kansas state geologist Erasmus Haworth collected samples of the escaping gas and took them back to the University of Kansas at Lawrence where, with the help of chemists Hamilton Cady and David McFarland, he discovered that the gas consisted of, by volume, 72% nitrogen, 15% methane (a combustible percentage only with sufficient oxygen), 1% hydrogen, and 12% an unidentifiable gas.[24][61] With further analysis, Cady and McFarland discovered that 1.84% of the gas sample was helium.[62][63] This showed that despite its overall rarity on Earth, helium was concentrated in large quantities under the American Great Plains, available for extraction as a byproduct of natural gas.[64]
+
+This enabled the United States to become the world's leading supplier of helium. Following a suggestion by Sir Richard Threlfall, the United States Navy sponsored three small experimental helium plants during World War I. The goal was to supply barrage balloons with the non-flammable, lighter-than-air gas. A total of 5,700 m3 (200,000 cu ft) of 92% helium was produced in the program even though less than a cubic meter of the gas had previously been obtained.[26] Some of this gas was used in the world's first helium-filled airship, the U.S. Navy's C-class blimp C-7, which flew its maiden voyage from Hampton Roads, Virginia, to Bolling Field in Washington, D.C., on December 1, 1921,[65] nearly two years before the Navy's first rigid helium-filled airship, the Naval Aircraft Factory-built USS Shenandoah, flew in September 1923.
+
+Although the extraction process using low-temperature gas liquefaction was not developed in time to be significant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air craft. During World War II, the demand increased for helium for lifting gas and for shielded arc welding. The helium mass spectrometer was also vital in the atomic bomb Manhattan Project.[66]
+
+The government of the United States set up the National Helium Reserve in 1925 at Amarillo, Texas, with the goal of supplying military airships in time of war and commercial airships in peacetime.[26] Because of the Helium Act of 1925, which banned the export of scarce helium on which the US then had a production monopoly, together with the prohibitive cost of the gas, the Hindenburg, like all German Zeppelins, was forced to use hydrogen as the lift gas. The helium market after World War II was depressed but the reserve was expanded in the 1950s to ensure a supply of liquid helium as a coolant to create oxygen/hydrogen rocket fuel (among other uses) during the Space Race and Cold War. Helium use in the United States in 1965 was more than eight times the peak wartime consumption.[67]
+
+After the "Helium Acts Amendments of 1960" (Public Law 86–777), the U.S. Bureau of Mines arranged for five private plants to recover helium from natural gas. For this helium conservation program, the Bureau built a 425-mile (684 km) pipeline from Bushton, Kansas, to connect those plants with the government's partially depleted Cliffside gas field near Amarillo, Texas. This helium-nitrogen mixture was injected and stored in the Cliffside gas field until needed, at which time it was further purified.[68]
+
+By 1995, a billion cubic meters of the gas had been collected and the reserve was US$1.4 billion in debt, prompting the Congress of the United States in 1996 to phase out the reserve.[24][69] The resulting Helium Privatization Act of 1996[70] (Public Law 104–273) directed the United States Department of the Interior to empty the reserve, with sales starting by 2005.[71]
+
+Helium produced between 1930 and 1945 was about 98.3% pure (2% nitrogen), which was adequate for airships. In 1945, a small amount of 99.9% helium was produced for welding use. By 1949, commercial quantities of Grade A 99.95% helium were available.[72]
+
+For many years, the United States produced more than 90% of commercially usable helium in the world, while extraction plants in Canada, Poland, Russia, and other nations produced the remainder. In the mid-1990s, a new plant in Arzew, Algeria, producing 17 million cubic meters (600 million cubic feet) began operation, with enough production to cover all of Europe's demand. Meanwhile, by 2000, the consumption of helium within the U.S. had risen to more than 15 million kg per year.[73] In 2004–2006, additional plants in Ras Laffan, Qatar, and Skikda, Algeria were built. Algeria quickly became the second leading producer of helium.[74] Through this time, both helium consumption and the costs of producing helium increased.[75] From 2002 to 2007 helium prices doubled.[76]
+
+As of 2012[update], the United States National Helium Reserve accounted for 30 percent of the world's helium.[77] The reserve was expected to run out of helium in 2018.[77] Despite that, a proposed bill in the United States Senate would allow the reserve to continue to sell the gas. Other large reserves were in the Hugoton in Kansas, United States, and nearby gas fields of Kansas and the panhandles of Texas and Oklahoma. New helium plants were scheduled to open in 2012 in Qatar, Russia, and the US state of Wyoming, but they were not expected to ease the shortage.[77]
+
+In 2013, Qatar started up the world's largest helium unit,[78] although the 2017 Qatar diplomatic crisis severely affected helium production there.[79] 2014 was widely acknowledged to be a year of over-supply in the helium business, following years of renowned shortages.[80] Nasdaq reported (2015) that for Air Products, an international corporation that sells gases for industrial use, helium volumes remain under economic pressure due to feedstock supply constraints.[81]
+
+In the perspective of quantum mechanics, helium is the second simplest atom to model, following the hydrogen atom. Helium is composed of two electrons in atomic orbitals surrounding a nucleus containing two protons and (usually) two neutrons. As in Newtonian mechanics, no system that consists of more than two particles can be solved with an exact analytical mathematical approach (see 3-body problem) and helium is no exception. Thus, numerical mathematical methods are required, even to solve the system of one nucleus and two electrons. Such computational chemistry methods have been used to create a quantum mechanical picture of helium electron binding which is accurate to within < 2% of the correct value, in a few computational steps.[82] Such models show that each electron in helium partly screens the nucleus from the other, so that the effective nuclear charge Z which each electron sees, is about 1.69 units, not the 2 charges of a classic "bare" helium nucleus.
+
+The nucleus of the helium-4 atom is identical with an alpha particle. High-energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's own electron cloud. This symmetry reflects similar underlying physics: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all these fermions fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each cancelling the other's intrinsic spin. Adding another of any of these particles would require angular momentum and would release substantially less energy (in fact, no nucleus with five nucleons is stable). This arrangement is thus energetically extremely stable for all these particles, and this stability accounts for many crucial facts regarding helium in nature.
+
+For example, the stability and low energy of the electron cloud state in helium accounts for the element's chemical inertness, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements.
+
+In a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions that involve either heavy-particle emission or fusion. Some stable helium-3 (2 protons and 1 neutron) is produced in fusion reactions from hydrogen, but it is a very small fraction compared to the highly favorable helium-4.
+
+The unusual stability of the helium-4 nucleus is also important cosmologically: it explains the fact that in the first few minutes after the Big Bang, as the "soup" of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible, almost all first compound atomic nuclei to form were helium-4 nuclei. So tight was helium-4 binding that helium-4 production consumed nearly all of the free neutrons in a few minutes, before they could beta-decay, and also leaving few to form heavier atoms such as lithium, beryllium, or boron. Helium-4 nuclear binding per nucleon is stronger than in any of these elements (see nucleogenesis and binding energy) and thus, once helium had been formed, no energetic drive was available to make elements 3, 4 and 5. It was barely energetically favorable for helium to fuse into the next element with a lower energy per nucleon, carbon. However, due to lack of intermediate elements, this process requires three helium nuclei striking each other nearly simultaneously (see triple alpha process). There was thus no time for significant carbon to be formed in the few minutes after the Big Bang, before the early expanding universe cooled to the temperature and pressure point where helium fusion to carbon was no longer possible. This left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4.
+
+All heavier elements (including those necessary for rocky planets like the Earth, and for carbon-based or other life) have thus been created since the Big Bang in stars which were hot enough to fuse helium itself. All elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe. Helium-4, by contrast, makes up about 23% of the universe's ordinary matter—nearly all the ordinary matter that is not hydrogen.
+
+Helium is the second least reactive noble gas after neon, and thus the second least reactive of all elements.[83] It is chemically inert and monatomic in all standard conditions. Because of helium's relatively low molar (atomic) mass, its thermal conductivity, specific heat, and sound speed in the gas phase are all greater than any other gas except hydrogen. For these reasons and the small size of helium monatomic molecules, helium diffuses through solids at a rate three times that of air and around 65% that of hydrogen.[26]
+
+Helium is the least water-soluble monatomic gas,[84] and one of the least water-soluble of any gas (CF4, SF6, and C4F8 have lower mole fraction solubilities: 0.3802, 0.4394, and 0.2372 x2/10−5, respectively, versus helium's 0.70797 x2/10−5),[85] and helium's index of refraction is closer to unity than that of any other gas.[86] Helium has a negative Joule–Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its Joule–Thomson inversion temperature (of about 32 to 50 K at 1 atmosphere) does it cool upon free expansion.[26] Once precooled below this temperature, helium can be liquefied through expansion cooling.
+
+Most extraterrestrial helium is found in a plasma state, with properties quite different from those of atomic helium. In a plasma, helium's electrons are not bound to its nucleus, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind together with ionized hydrogen, the particles interact with the Earth's magnetosphere, giving rise to Birkeland currents and the aurora.[87]
+
+Unlike any other element, helium will remain liquid down to absolute zero at normal pressures. This is a direct effect of quantum mechanics: specifically, the zero point energy of the system is too high to allow freezing. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) at about 25 bar (2.5 MPa) of pressure.[88] It is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same. The solid has a sharp melting point and has a crystalline structure, but it is highly compressible; applying pressure in a laboratory can decrease its volume by more than 30%.[89] With a bulk modulus of about 27 MPa[90] it is ~100 times more compressible than water. Solid helium has a density of 0.214±0.006 g/cm3 at 1.15 K and 66 atm; the projected density at 0 K and 25 bar (2.5 MPa) is 0.187±0.009 g/cm3.[91] At higher temperatures, helium will solidify with sufficient pressure. At room temperature, this requires about 114,000 atm.[92]
+
+Below its boiling point of 4.22 kelvins and above the lambda point of 2.1768 kelvins, the isotope helium-4 exists in a normal colorless liquid state, called helium I.[26] Like other cryogenic liquids, helium I boils when it is heated and contracts when its temperature is lowered. Below the lambda point, however, helium does not boil, and it expands as the temperature is lowered further.
+
+Helium I has a gas-like index of refraction of 1.026 which makes its surface so hard to see that floats of Styrofoam are often used to show where the surface is.[26] This colorless liquid has a very low viscosity and a density of 0.145–0.125 g/mL (between about 0 and 4 K),[93] which is only one-fourth the value expected from classical physics.[26] Quantum mechanics is needed to explain this property and thus both states of liquid helium (helium I and helium II) are called quantum fluids, meaning they display atomic properties on a macroscopic scale. This may be an effect of its boiling point being so close to absolute zero, preventing random molecular motion (thermal energy) from masking the atomic properties.[26]
+
+Liquid helium below its lambda point (called helium II) exhibits very unusual characteristics. Due to its high thermal conductivity, when it boils, it does not bubble but rather evaporates directly from its surface. Helium-3 also has a superfluid phase, but only at much lower temperatures; as a result, less is known about the properties of the isotope.[26]
+
+Helium II is a superfluid, a quantum mechanical state (see: macroscopic quantum phenomena) of matter with strange properties. For example, when it flows through capillaries as thin as 10−7 to 10−8 m it has no measurable viscosity.[24] However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Current theory explains this using the two-fluid model for helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a ground state, which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.[94]
+
+In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a sintered disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid helium, superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container.[95]
+
+The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of copper.[26] This is because heat conduction occurs by an exceptional quantum mechanism. Most materials that conduct heat well have a valence band of free electrons which serve to transfer the heat. Helium II has no such valence band but nevertheless conducts heat well. The flow of heat is governed by equations that are similar to the wave equation used to characterize sound propagation in air. When heat is introduced, it moves at 20 meters per second at 1.8 K through helium II as waves in a phenomenon known as second sound.[26]
+
+Helium II also exhibits a creeping effect. When a surface extends past the level of helium II, the helium II moves along the surface, against the force of gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30 nm-thick film regardless of surface material. This film is called a Rollin film and is named after the man who first characterized this trait, Bernard V. Rollin.[26][96][97] As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate. Waves propagating across a Rollin film are governed by the same equation as gravity waves in shallow water, but rather than gravity, the restoring force is the van der Waals force.[98] These waves are known as third sound.[99]
+
+There are nine known isotopes of helium, but only helium-3 and helium-4 are stable. In the Earth's atmosphere, one atom is 3He for every million that are 4He.[24] Unlike most elements, helium's isotopic abundance varies greatly by origin, due to the different formation processes. The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.[100]
+
+Helium-3 is present on Earth only in trace amounts. Most of it has been present since Earth's formation, though some falls to Earth trapped in cosmic dust.[101] Trace amounts are also produced by the beta decay of tritium.[102] Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten, and these ratios can be used to investigate the origin of rocks and the composition of the Earth's mantle.[101] 3He is much more abundant in stars as a product of nuclear fusion. Thus in the interstellar medium, the proportion of 3He to 4He is about 100 times higher than on Earth.[103] Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds. The Moon's surface contains helium-3 at concentrations on the order of 10 ppb, much higher than the approximately 5 ppt found in the Earth's atmosphere.[104][105] A number of people, starting with Gerald Kulcinski in 1986,[106] have proposed to explore the moon, mine lunar regolith, and use the helium-3 for fusion.
+
+Liquid helium-4 can be cooled to about 1 kelvin using evaporative cooling in a 1-K pot. Similar cooling of helium-3, which has a lower boiling point, can achieve about 0.2 kelvin in a helium-3 refrigerator. Equal mixtures of liquid 3He and 4He below 0.8 K separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: helium-4 atoms are bosons while helium-3 atoms are fermions).[26] Dilution refrigerators use this immiscibility to achieve temperatures of a few millikelvins.
+
+It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived heavy helium isotope is helium-5 with a half-life of 7.6×10−22 s. Helium-6 decays by emitting a beta particle and has a half-life of 0.8 second. Helium-7 also emits a beta particle as well as a gamma ray. Helium-7 and helium-8 are created in certain nuclear reactions.[26] Helium-6 and helium-8 are known to exhibit a nuclear halo.[26]
+
+Helium has a valence of zero and is chemically unreactive under all normal conditions.[89] It is an electrical insulator unless ionized. As with the other noble gases, helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential.[26] Helium can form unstable compounds, known as excimers, with tungsten, iodine, fluorine, sulfur, and phosphorus when it is subjected to a glow discharge, to electron bombardment, or reduced to plasma by other means. The molecular compounds HeNe, HgHe10, and WHe2, and the molecular ions He+2, He2+2, HeH+, and HeD+ have been created this way.[107] HeH+ is also stable in its ground state, but is extremely reactive—it is the strongest Brønsted acid known, and therefore can exist only in isolation, as it will protonate any molecule or counteranion it contacts. This technique has also produced the neutral molecule He2, which has a large number of band systems, and HgHe, which is apparently held together only by polarization forces.[26]
+
+Van der Waals compounds of helium can also be formed with cryogenic helium gas and atoms of some other substance, such as LiHe and He2.[108]
+
+Theoretically, other true compounds may be possible, such as helium fluorohydride (HHeF) which would be analogous to HArF, discovered in 2000.[109] Calculations show that two new compounds containing a helium-oxygen bond could be stable.[110] Two new molecular species, predicted using theory, CsFHeO and N(CH3)4FHeO, are derivatives of a metastable FHeO− anion first theorized in 2005 by a group from Taiwan. If confirmed by experiment, the only remaining element with no known stable compounds would be neon.[111]
+
+Helium atoms have been inserted into the hollow carbon cage molecules (the fullerenes) by heating under high pressure. The endohedral fullerene molecules formed are stable at high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside.[112] If helium-3 is used, it can be readily observed by helium nuclear magnetic resonance spectroscopy.[113] Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances have distinct properties and a definite composition, like all stoichiometric chemical compounds.
+
+Under high pressures helium can form compounds with various other elements. Helium-nitrogen clathrate (He(N2)11) crystals have been grown at room temperature at pressures ca. 10 GPa in a diamond anvil cell.[114] The insulating electride Na2He has been shown to be thermodynamically stable at pressures above 113 GPa. It has a fluorite structure.[115]
+
+Although it is rare on Earth, helium is the second most abundant element in the known Universe, constituting 23% of its baryonic mass. Only hydrogen is more abundant.[24] The vast majority of helium was formed by Big Bang nucleosynthesis one to three minutes after the Big Bang. As such, measurements of its abundance contribute to cosmological models. In stars, it is formed by the nuclear fusion of hydrogen in proton-proton chain reactions and the CNO cycle, part of stellar nucleosynthesis.[100]
+
+In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million.[116][117] The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth's atmosphere escapes into space by several processes.[118][119][120] In the Earth's heterosphere, a part of the upper atmosphere, helium and other lighter gases are the most abundant elements.
+
+Most helium on Earth is a result of radioactive decay. Helium is found in large amounts in minerals of uranium and thorium, including uraninite and its varieties cleveite and pitchblende,[16][121] carnotite and monazite (a group name; "monazite" usually refers to monazite-(Ce)),[122][123] because they emit alpha particles (helium nuclei, He2+) to which electrons immediately combine as soon as the particle is stopped by the rock. In this way an estimated 3000 metric tons of helium are generated per year throughout the lithosphere.[124][125][126] In the Earth's crust, the concentration of helium is 8 parts per billion. In seawater, the concentration is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. Because helium is trapped in the subsurface under conditions that also trap natural gas, the greatest natural concentrations of helium on the planet are found in natural gas, from which most commercial helium is extracted. The concentration varies in a broad range from a few ppm to more than 7% in a small gas field in San Juan County, New Mexico.[127][128]
+
+As of 2011[update] the world's helium reserves were estimated at 40 billion cubic meters, with a quarter of that being in the South Pars / North Dome Gas-Condensate field owned jointly by Qatar and Iran.[129] In 2015 and 2016 additional probable reserves were announced to be under the Rocky Mountains in North America[130] and in the East African Rift.[131]
+
+For large-scale use, helium is extracted by fractional distillation from natural gas, which can contain as much as 7% helium.[132] Since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and methane). The resulting crude helium gas is purified by successive exposures to lowering temperatures, in which almost all of the remaining nitrogen and other gases are precipitated out of the gaseous mixture. Activated charcoal is used as a final purification step, usually resulting in 99.995% pure Grade-A helium.[26] The principal impurity in Grade-A helium is neon. In a final production step, most of the helium that is produced is liquefied via a cryogenic process. This is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long distance transportation, as the largest liquid helium containers have more than five times the capacity of the largest gaseous helium tube trailers.[74][133]
+
+In 2008, approximately 169 million standard cubic meters (SCM) of helium were extracted from natural gas or withdrawn from helium reserves with approximately 78% from the United States, 10% from Algeria, and most of the remainder from Russia, Poland and Qatar.[134] By 2013, increases in helium production in Qatar (under the company RasGas managed by Air Liquide) had increased Qatar's fraction of world helium production to 25%, and made it the second largest exporter after the United States.[135]
+An estimated 54 billion cubic feet (1.5×109 m3) deposit of helium was found in Tanzania in 2016.[136]A large-scale helium plant was opened in Ningxia, China in 2020.[137]
+
+In the United States, most helium is extracted from natural gas of the Hugoton and nearby gas fields in Kansas, Oklahoma, and the Panhandle Field in Texas.[74][138] Much of this gas was once sent by pipeline to the National Helium Reserve, but since 2005 this reserve is being depleted and sold off, and is expected to be largely depleted by 2021,[135] under the October 2013 Responsible Helium Administration and Stewardship Act (H.R. 527).[139]
+
+Diffusion of crude natural gas through special semipermeable membranes and other barriers is another method to recover and purify helium.[140] In 1996, the U.S. had proven helium reserves, in such gas well complexes, of about 147 billion standard cubic feet (4.2 billion SCM).[141] At rates of use at that time (72 million SCM per year in the U.S.; see pie chart below) this would have been enough helium for about 58 years of U.S. use, and less than this (perhaps 80% of the time) at world use rates, although factors in saving and processing impact effective reserve numbers.
+
+Helium must be extracted from natural gas because it is present in air at only a fraction of that of neon, yet the demand for it is far higher. It is estimated that if all neon production were retooled to save helium, 0.1% of the world's helium demands would be satisfied. Similarly, only 1% of the world's helium demands could be satisfied by re-tooling all air distillation plants.[142] Helium can be synthesized by bombardment of lithium or boron with high-velocity protons, or by bombardment of lithium with deuterons, but these processes are a completely uneconomical method of production.[143]
+
+Helium is commercially available in either liquid or gaseous form. As a liquid, it can be supplied in small insulated containers called dewars which hold as much as 1,000 liters of helium, or in large ISO containers which have nominal capacities as large as 42 m3 (around 11,000 U.S. gallons). In gaseous form, small quantities of helium are supplied in high-pressure cylinders holding as much as 8 m3 (approx. 282 standard cubic feet), while large quantities of high-pressure gas are supplied in tube trailers which have capacities of as much as 4,860 m3 (approx. 172,000 standard cubic feet).
+
+According to helium conservationists like Nobel laureate physicist Robert Coleman Richardson, writing in 2010, the free market price of helium has contributed to "wasteful" usage (e.g. for helium balloons). Prices in the 2000s had been lowered by the decision of the U.S. Congress to sell off the country's large helium stockpile by 2015.[144] According to Richardson, the price needed to be multiplied by 20 to eliminate the excessive wasting of helium. In their book, the Future of helium as a natural resource (Routledge, 2012), Nuttall, Clarke & Glowacki (2012) also proposed to create an International Helium Agency (IHA) to build a sustainable market for this precious commodity.[145]
+
+Estimated 2014 U.S. fractional helium use by category. Total use is 34 million cubic meters.[146]
+
+While balloons are perhaps the best known use of helium, they are a minor part of all helium use.[69] Helium is used for many purposes that require some of its unique properties, such as its low boiling point, low density, low solubility, high thermal conductivity, or inertness. Of the 2014 world helium total production of about 32 million kg (180 million standard cubic meters) helium per year, the largest use (about 32% of the total in 2014) is in cryogenic applications, most of which involves cooling the superconducting magnets in medical MRI scanners and NMR spectrometers.[147] Other major uses were pressurizing and purging systems, welding, maintenance of controlled atmospheres, and leak detection. Other uses by category were relatively minor fractions.[146]
+
+Helium is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, and in gas chromatography,[89] because it is inert. Because of its inertness, thermally and calorically perfect nature, high speed of sound, and high value of the heat capacity ratio, it is also useful in supersonic wind tunnels[148] and impulse facilities.[149]
+
+Helium is used as a shielding gas in arc welding processes on materials that at welding temperatures are contaminated and weakened by air or nitrogen.[24] A number of inert shielding gases are used in gas tungsten arc welding, but helium is used instead of cheaper argon especially for welding materials that have higher heat conductivity, like aluminium or copper.
+
+One industrial application for helium is leak detection. Because helium diffuses through solids three times faster than air, it is used as a tracer gas to detect leaks in high-vacuum equipment (such as cryogenic tanks) and high-pressure containers.[150] The tested object is placed in a chamber, which is then evacuated and filled with helium. The helium that escapes through the leaks is detected by a sensitive device (helium mass spectrometer), even at the leak rates as small as 10−9 mbar·L/s (10−10 Pa·m3/s). The measurement procedure is normally automatic and is called helium integral test. A simpler procedure is to fill the tested object with helium and to manually search for leaks with a hand-held device.[151]
+
+Helium leaks through cracks should not be confused with gas permeation through a bulk material. While helium has documented permeation constants (thus a calculable permeation rate) through glasses, ceramics, and synthetic materials, inert gases such as helium will not permeate most bulk metals.[152]
+
+Because it is lighter than air, airships and balloons are inflated with helium for lift. While hydrogen gas is more buoyant, and escapes permeating through a membrane at a lower rate, helium has the advantage of being non-flammable, and indeed fire-retardant. Another minor use is in rocketry, where helium is used as an ullage medium to displace fuel and oxidizers in storage tanks and to condense hydrogen and oxygen to make rocket fuel. It is also used to purge fuel and oxidizer from ground support equipment prior to launch and to pre-cool liquid hydrogen in space vehicles. For example, the Saturn V rocket used in the Apollo program needed about 370,000 m3 (13 million cubic feet) of helium to launch.[89]
+
+Helium as a breathing gas has no narcotic properties, so helium mixtures such as trimix, heliox and heliair are used for deep diving to reduce the effects of narcosis, which worsen with increasing depth.[153][154] As pressure increases with depth, the density of the breathing gas also increases, and the low molecular weight of helium is found to considerably reduce the effort of breathing by lowering the density of the mixture. This reduces the Reynolds number of flow, leading to a reduction of turbulent flow and an increase in laminar flow, which requires less work of breathing.[155][156] At depths below 150 metres (490 ft) divers breathing helium–oxygen mixtures begin to experience tremors and a decrease in psychomotor function, symptoms of high-pressure nervous syndrome.[157] This effect may be countered to some extent by adding an amount of narcotic gas such as hydrogen or nitrogen to a helium–oxygen mixture.[158]
+
+Helium–neon lasers, a type of low-powered gas laser producing a red beam, had various practical applications which included barcode readers and laser pointers, before they were almost universally replaced by cheaper diode lasers.[24]
+
+For its inertness and high thermal conductivity, neutron transparency, and because it does not form radioactive isotopes under reactor conditions, helium is used as a heat-transfer medium in some gas-cooled nuclear reactors.[150]
+
+Helium, mixed with a heavier gas such as xenon, is useful for thermoacoustic refrigeration due to the resulting high heat capacity ratio and low Prandtl number.[159] The inertness of helium has environmental advantages over conventional refrigeration systems which contribute to ozone depletion or global warming.[160]
+
+Helium is also used in some hard disk drives.[161]
+
+The use of helium reduces the distorting effects of temperature variations in the space between lenses in some telescopes, due to its extremely low index of refraction.[26] This method is especially used in solar telescopes where a vacuum tight telescope tube would be too heavy.[162][163]
+
+Helium is a commonly used carrier gas for gas chromatography.
+
+The age of rocks and minerals that contain uranium and thorium can be estimated by measuring the level of helium with a process known as helium dating.[24][26]
+
+Helium at low temperatures is used in cryogenics, and in certain cryogenics applications. As examples of applications, liquid helium is used to cool certain metals to the extremely low temperatures required for superconductivity, such as in superconducting magnets for magnetic resonance imaging. The Large Hadron Collider at CERN uses 96 metric tons of liquid helium to maintain the temperature at 1.9 kelvins.[164]
+
+Helium was approved for medical use in the United States in April 2020 for humans and animals.[165][166]
+
+While chemically inert, helium contamination will impair the operation of microelectromechanical systems such that iPhones may fail.[167]
+
+Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood.
+
+The speed of sound in helium is nearly three times the speed of sound in air. Because the natural resonance frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled, a corresponding increase occurs in the resonant frequencies of the vocal tract, which is the amplifier of vocal sound.[24][168] This increase in the resonant frequency of the amplifier (the vocal tract) gives an increased amplification to the high-frequency components of the sound wave produced by the direct vibration of the vocal folds, compared to the case when the voice box is filled with air. When a person speaks after inhaling helium gas, the muscles that control the voice box still move in the same way as when the voice box is filled with air, therefore the fundamental frequency (sometimes called pitch) produced by direct vibration of the vocal folds does not change[169]. However, the high-frequency-preferred amplification causes a change in timbre of the amplified sound, resulting in a reedy, duck-like vocal quality. The opposite effect, lowering resonant frequencies, can be obtained by inhaling a dense gas such as sulfur hexafluoride or xenon.
+
+Inhaling helium can be dangerous if done to excess, since helium is a simple asphyxiant and so displaces oxygen needed for normal respiration.[24][170] Fatalities have been recorded, including a youth who suffocated in Vancouver in 2003 and two adults who suffocated in South Florida in 2006.[171][172] In 1998, an Australian girl from Victoria fell unconscious and temporarily turned blue after inhaling the entire contents of a party balloon.[173][174][175]
+Inhaling helium directly from pressurized cylinders or even balloon filling valves is extremely dangerous, as high flow rate and pressure can result in barotrauma, fatally rupturing lung tissue.[170][176]
+
+Death caused by helium is rare. The first media-recorded case was that of a 15-year-old girl from Texas who died in 1998 from helium inhalation at a friend's party; the exact type of helium death is unidentified.[173][174][175]
+
+In the United States only two fatalities were reported between 2000 and 2004, including a man who died in North Carolina of barotrauma in 2002.[171][176] A youth asphyxiated in Vancouver during 2003, and a 27-year-old man in Australia had an embolism after breathing from a cylinder in 2000.[171] Since then two adults asphyxiated in South Florida in 2006,[171][172][177] and there were cases in 2009 and 2010, one a Californian youth who was found with a bag over his head, attached to a helium tank,[178] and another teenager in Northern Ireland died of asphyxiation.[179] At Eagle Point, Oregon a teenage girl died in 2012 from barotrauma at a party.[180][181][182] A girl from Michigan died from hypoxia later in the year.[183]
+
+On February 4, 2015 it was revealed that, during the recording of their main TV show on January 28, a 12-year-old member (name withheld) of Japanese all-girl singing group 3B Junior suffered from air embolism, losing consciousness and falling into a coma as a result of air bubbles blocking the flow of blood to the brain, after inhaling huge quantities of helium as part of a game. The incident was not made public until a week later.[184][185] The staff of TV Asahi held an emergency press conference to communicate that the member had been taken to the hospital and is showing signs of rehabilitation such as moving eyes and limbs, but her consciousness has not yet been sufficiently recovered. Police have launched an investigation due to a neglect of safety measures.[186][187]
+
+On July 13, 2017 CBS News reported that a political operative who reportedly attempted to recover e-mails missing from the Clinton server, Peter W. Smith, "apparently" committed suicide in May at a hotel room in Rochester, Minnesota and that his death was recorded as "asphyxiation due to displacement of oxygen in confined space with helium".[188] More details followed in the Chicago Tribune.[189]
+
+The safety issues for cryogenic helium are similar to those of liquid nitrogen; its extremely low temperatures can result in cold burns, and the liquid-to-gas expansion ratio can cause explosions if no pressure-relief devices are installed. Containers of helium gas at 5 to 10 K should be handled as if they contain liquid helium due to the rapid and significant thermal expansion that occurs when helium gas at less than 10 K is warmed to room temperature.[89]
+
+At high pressures (more than about 20 atm or two MPa), a mixture of helium and oxygen (heliox) can lead to high-pressure nervous syndrome, a sort of reverse-anesthetic effect; adding a small amount of nitrogen to the mixture can alleviate the problem.[190][157]
+
+General
+
+More detail
+
+Miscellaneous
+
+Helium shortage
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+Animals (also called Metazoa) are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 micrometres (0.00033 in) to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The kingdom Animalia includes humans but in colloquial use the term animal often refers only to non-human animals. The scientific study of animals is known as zoology.
+
+Most living animal species are in Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing both the echinoderms as well as the chordates, the latter containing the vertebrates. Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago.
+
+Historically, Aristotle divided animals into those with blood and those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (synonymous for Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa.
+
+Humans make use of many other animal species, such as for food (including meat, milk, and eggs), for materials (such as leather and wool), and also as pets, and for transports, as working animals. Dogs have been used in hunting, while many terrestrial and aquatic animals were hunted for sports. Non-human animals have appeared in art from the earliest times and are featured in mythology and religion.
+
+The word "animal" comes from the Latin animalis, meaning having breath, having soul or living being.[1] The biological definition includes all members of the kingdom Animalia.[2] In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals.[3][4][5][6]
+
+Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular,[7][8] unlike bacteria, which are prokaryotic, and unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients[9] animals are heterotrophic,[8][10] feeding on organic material and digesting it internally.[11] With very few exceptions, animals respire aerobically.[12] All animals are motile[13] (able to spontaneously move their bodies) during at least part of their life cycle, but some animals, such as sponges, corals, mussels, and barnacles, later become sessile. The blastula is a stage in embryonic development that is unique to most animals,[14] allowing cells to be differentiated into specialised tissues and organs.
+
+All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins.[15] During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as shells, bones, and spicules.[16] In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth.[17] Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes.[18]
+
+With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues.[19] These include muscles, which enable locomotion, and nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).[20]
+
+Nearly all animals make use of some form of sexual reproduction.[21] They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova.[22] These fuse to form zygotes,[23] which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge.[24] In most other groups, the blastula  undergoes more complicated rearrangement.[25] It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm.[26] In most cases, a third germ layer, the mesoderm, also develops between them.[27] These germ layers then differentiate to form tissues and organs.[28]
+
+Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits.[29][30] Animals have evolved numerous mechanisms for avoiding close inbreeding.[31]
+
+Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding, such as in Hydra and other cnidarians; or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.[32][33]
+
+Animals are categorised into ecological groups depending on how they obtain or consume organic material, including carnivores, herbivores, omnivores, detritivores,[34] and parasites.[35] Interactions between animals form complex food webs. In carnivorous or omnivorous species, predation is a consumer-resource interaction where a predator feeds on another organism (called its prey).[36] Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various anti-predator adaptations.[37][38] Almost all multicellular predators are animals.[39] Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts' living tissues, killing them in the process,[40] but the adults primarily consume nectar from flowers.[41] Other animals may have very specific feeding behaviours, such as hawksbill sea turtles primarily eating sponges.[42]
+
+Most animals rely on the biomass and energy produced by plants through photosynthesis. Herbivores eat plant material directly, while carnivores, and other animals on higher trophic levels typically acquire it indirectly by eating other animals. Animals oxidize carbohydrates, lipids, proteins, and other biomolecules to unlock the chemical energy of molecular oxygen,[43] which allows the animal to grow and to sustain biological processes such as locomotion.[44][45][46] Animals living close to hydrothermal vents and cold seeps on the dark sea floor consume organic matter of archaea and bacteria produced in these locations through chemosynthesis (by oxidizing inorganic compounds, such as hydrogen sulfide).[47]
+
+Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as land plants, probably between 510–471 million years ago during the Late Cambrian or Early Ordovician.[48] Vertebrates such as the lobe-finned fish Tiktaalik started to move on to land in the late Devonian, about 375 million years ago.[49][50] Animals occupy virtually all of earth's habitats and microhabitats, including salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of animals, plants, fungi and rocks.[51] Animals are however not particularly heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F).[52] Only very few species of animals (mostly nematodes) inhabit the most extreme cold deserts of continental Antarctica.[53]
+
+The blue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to at least 190 tonnes and measuring up to 33.6 metres (110 ft) long.[54][55][56] The largest extant terrestrial animal is the African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes[54] and measuring up to 10.67 metres (35.0 ft) long.[54] The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus, which may have weighed as much as 73 tonnes.[57] Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 µm,[58] and one of the smallest species (Myxobolus shekel) is no more than 8.5 µm when fully grown.[59]
+
+The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species,[60] along with their principal habitats (terrestrial, fresh water,[61] and marine),[62] and free-living or parasitic ways of life.[63] Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.[64] Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.[65][66][a]
+
+3,000–6,500[74]
+
+4,000–25,000[74]
+
+The first fossils that might represent animals appear in the 665-million-year-old rocks of the Trezona Formation of South Australia. These fossils are interpreted as most probably being early sponges.[78]
+
+The oldest animals are found in the Ediacaran biota, towards the end of the Precambrian, around 610 million years ago. It had long been doubtful whether these included animals,[79][80][81] but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes that these were indeed animals.[77] Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by anaerobic respiration, but as they became specialized for aerobic metabolism they became fully dependent on oxygen in their environments.[82]
+
+Many animal phyla first appear in the fossil record during the Cambrian explosion, starting about 542 million years ago, in beds such as the Burgess shale. Extant phyla in these rocks include molluscs, brachiopods, onychophorans, tardigrades, arthropods, echinoderms and hemichordates, along with numerous now-extinct forms such as the predatory Anomalocaris. The apparent suddenness of the event may however be an artefact of the fossil record, rather than showing that all these animals appeared simultaneously.[83][84][85][86]
+
+Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.[87] Trace fossils such as tracks and burrows found in the Tonian period may indicate the presence of triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms.[88] However, similar tracks are produced today by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution.[89][90] Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly-evolved animals.[91]
+
+Animals are monophyletic, meaning they are derived from a common ancestor. Animals are sister to the Choanoflagellata, with which they form the Choanozoa.[92] The most basal animals, the Porifera, Ctenophora, Cnidaria, and Placozoa, have body plans that lack bilateral symmetry. Their relationships are still disputed; the sister group to all other animals could be the Porifera or the Ctenophora, both of which lack hox genes, important in body plan development.[93]
+
+These genes are found in the Placozoa[94][95] and the higher animals, the Bilateria.[96][97] 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian. 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of the Wnt and TGF-beta signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body's system of axes (in three dimensions), and another 7 are for transcription factors including homeodomain proteins involved in the control of development.[98][99]
+
+The phylogenetic tree (of major lineages only) indicates approximately how many millions of years ago (mya) the lineages split.[100][101][102][103][104]
+
+Choanoflagellata
+
+Porifera
+
+Ctenophora
+
+Placozoa
+
+Cnidaria
+
+Xenacoelomorpha
+
+Chordata
+
+Ambulacraria
+
+Scalidophora
+
+Arthropoda and allies
+
+Nematoda and allies
+
+Rotifera and allies
+
+Chaetognatha
+
+Platyhelminthes and allies
+
+Mollusca and allies
+
+Annelida and allies
+
+
+
+Several animal phyla lack bilateral symmetry. Among these, the sponges (Porifera) probably diverged first, representing the oldest animal phylum.[105] Sponges lack the complex organization found in most other animal phyla;[106] their cells are differentiated, but in most cases not organised into distinct tissues.[107] They typically feed by drawing in water through pores.[108]
+
+The Ctenophora (comb jellies) and Cnidaria (which includes jellyfish, sea anemones, and corals) are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus.[109] Animals in both phyla have distinct tissues, but these are not organised into organs.[110] They are diploblastic, having only two main germ layers, ectoderm and endoderm.[111] The tiny placozoans are similar, but they do not have a permanent digestive chamber.[112][113]
+
+The remaining animals, the great majority—comprising some 29 phyla and over a million species—form a clade, the Bilateria. The body is triploblastic, with three well-developed germ layers, and their tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is an internal body cavity, a coelom or pseudocoelom. Animals with this bilaterally symmetric body plan and a tendency to move in one direction have a head end (anterior) and a tail end (posterior) as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.[114][115]
+
+Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth. Many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;[115] these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis.[116] They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), while some parasitic worms have extremely simplified body structures.[114][115]
+
+Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, the  protostomes and the deuterostomes.[117] The basalmost bilaterians are the Xenacoelomorpha.[118][119][120]
+
+Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the Spiralia) undergo spiral cleavage.[121]
+Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily.[122][123] Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm.[124]
+
+The main deuterostome phyla are the Echinodermata and the Chordata.[125] Echinoderms are exclusively marine and include starfish, sea urchins, and sea cucumbers.[126] The chordates are dominated by the vertebrates (animals with backbones),[127] which consist of fishes, amphibians, reptiles, birds, and mammals.[128] The deuterostomes also include the Hemichordata (acorn worms).[129][130]
+
+The Ecdysozoa are protostomes, named after their shared trait of ecdysis, growth by moulting.[131] They include the largest animal phylum, the Arthropoda, which contains insects, spiders, crabs, and their kin. All of these have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits. The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water;[132] some are important parasites.[133] Smaller phyla related to them are the Nematomorpha or horsehair worms, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom.[134]
+
+The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo.[135] The Spiralia's phylogeny has been disputed, but it contains a large clade, the superphylum Lophotrochozoa, and smaller groups of phyla such as the Rouphozoa which includes the gastrotrichs and the flatworms. All of these are grouped as the Platytrochozoa, which has a sister group, the Gnathifera, which includes the rotifers.[136][137]
+
+The Lophotrochozoa includes the molluscs, annelids, brachiopods, nemerteans, bryozoa and entoprocts.[136][138][139] The molluscs, the second-largest animal phylum by number of described species, includes snails, clams, and squids, while the annelids are the segmented worms, such as earthworms, lugworms, and leeches. These two groups have long been considered close relatives because they share trochophore larvae.[140][141]
+
+In the classical era, Aristotle divided animals,[d] based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then arranged on a scale from man (with blood, 2 legs, rational soul) down through the live-bearing tetrapods (with blood, 4 legs, sensitive soul) and other groups such as crustaceans (no blood, many legs, sensitive soul) down to spontaneously-generating creatures like sponges (no blood, no legs, vegetable soul). Aristotle was uncertain whether sponges were animals, which in his system ought to have sensation, appetite, and locomotion, or plants, which did not: he knew that sponges could sense touch, and would contract if about to be pulled off their rocks, but that they were rooted like plants and never moved about.[143]
+
+In 1758, Carl Linnaeus created the first hierarchical classification in his Systema Naturae.[144] In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Jean-Baptiste de Lamarck, who called the Vermes une espèce de chaos (a chaotic mess)[e] and split the group into three new phyla, worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his Philosophie Zoologique, Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely cirripedes, annelids, crustaceans, arachnids, insects, worms, radiates, polyps, and infusorians.[142]
+
+In his 1817 Le Règne Animal, Georges Cuvier used comparative anatomy to group the animals into four embranchements ("branches" with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), and zoophytes (radiata) (echinoderms, cnidaria and other forms).[146] This division into four was followed by the embryologist Karl Ernst von Baer in 1828, the zoologist Louis Agassiz in 1857, and the comparative anatomist Richard Owen in 1860.[147]
+
+In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals, with five phyla: coelenterates, echinoderms, articulates, molluscs, and vertebrates) and Protozoa (single-celled animals), including a sixth animal phylum, sponges.[148][147] The protozoa were later moved to the former kingdom Protista, leaving only the Metazoa as a synonym of Animalia.[149]
+
+The human population exploits a large number of other animal species for food, both of domesticated livestock species in animal husbandry and, mainly at sea, by hunting wild species.[150][151] Marine fish of many species are caught commercially for food. A smaller number of species are farmed commercially.[150][152][153]
+Invertebrates including cephalopods, crustaceans, and bivalve or gastropod molluscs are hunted or farmed for food.[154]
+Chickens, cattle, sheep, pigs and other animals are raised as livestock for meat across the world.[151][155][156]
+Animal fibres such as wool are used to make textiles, while animal sinews have been used as lashings and bindings, and leather is widely used to make shoes and other items. Animals have been hunted and farmed for their fur to make items such as coats and hats.[157][158] Dyestuffs including carmine (cochineal),[159][160] shellac,[161][162] and kermes[163][164] have been made from the bodies of insects. Working animals including cattle and horses have been used for work and transport from the first days of agriculture.[165]
+
+Animals such as the fruit fly Drosophila melanogaster serve a major role in science as experimental models.[166][167][168][169] Animals have been used to create vaccines since their discovery in the 18th century.[170] Some medicines such as the cancer drug Yondelis are based on toxins or other molecules of animal origin.[171]
+
+People have used hunting dogs to help chase down and retrieve animals,[172] and birds of prey to catch birds and mammals,[173] while tethered cormorants have been used to catch fish.[174] Poison dart frogs have been used to poison the tips of blowpipe darts.[175][176]
+A wide variety of animals are kept as pets, from invertebrates such as tarantulas and octopuses, insects including praying mantises,[177] reptiles such as snakes and chameleons,[178] and birds including canaries, parakeets, and parrots[179] all finding a place. However, the most kept pet species are mammals, namely dogs, cats, and rabbits.[180][181][182] There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own.[183]
+A wide variety of terrestrial and aquatic animals are hunted for sport.[184]
+
+Animals have been the subjects of art from the earliest times, both historical, as in Ancient Egypt, and prehistoric, as in the cave paintings at Lascaux. Major animal paintings include Albrecht Dürer's 1515 The Rhinoceros, and George Stubbs's c. 1762 horse portrait Whistlejacket.[185]
+Insects, birds and mammals play roles in literature and film,[186] such as in giant bug movies.[187][188][189] Animals including insects[190] and mammals[191] feature in mythology and religion. In both Japan and Europe, a butterfly was seen as the personification of a person's soul,[190][192][193] while the scarab beetle was sacred in ancient Egypt.[194] Among the mammals, cattle,[195] deer,[191] horses,[196] lions,[197] bats,[198] bears,[199] and wolves[200] are the subjects of myths and worship. The signs of the Western and Chinese zodiacs are based on animals.[201][202]

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+Hello Kitty (Japanese: ハロー・キティ, Hepburn: Harō Kiti),[6] also known by her full name Kitty White (キティ・ホワイト, Kiti Howaito),[5] is a fictional character produced by the Japanese company Sanrio,[7] created by Yuko Shimizu and currently designed by Yuko Yamaguchi. Kitty is, contrary to popular belief, a little girl and not a cat.[citation needed] Kitty's non-cat status is emphasised by her owning a pet cat: Charmmy Kitty.[original research?] Sanrio depicts Hello Kitty as a young female gijinka (anthropomorphization) of Japanese Bobtail with a red bow and, notably, no mouth.[8] According to her backstory, she is a perpetual 3rd-grade student who lives outside of London.[9] Kitty and her twin sister, Mimi, have their birthday on November 1.[10][11] Since the cartoon character's creation, Hello Kitty has become a media franchise including a product line, clothing apparel, toy-line, manga comics, anime series, popular music, games, and other media.
+
+Shortly after her creation in 1974, the Hello Kitty vinyl coin purse was introduced by Sanrio on March 5, 1975. Sanrio brought the character to the United States in 1976.[12][13] Hello Kitty is a staple of the kawaii segment of Japanese popular culture.[14] By 2010, Sanrio had groomed Hello Kitty into a global marketing phenomenon,[15] worth US$6 billion a year.[16] By 2014, when Hello Kitty was 40 years old, she was worth about $8 billion a year.[17] As of 2019, Hello Kitty is the second highest-grossing media franchise of all time (behind Pokémon), having generated $80 billion in lifetime retail sales.[18]
+
+First aimed at pre-teenage girls, Hello Kitty's market included adolescent and adult consumers. A variety of products ranging from school supplies to fashion accessories feature this character. Several Hello Kitty TV series, targeted towards children, have been produced, as well as several manga comics and anime films. There are Sanrio theme parks based on Hello Kitty: Harmonyland in Hiji, Ōita, Japan,[19] Sanrio Puroland in Tama New Town, Tokyo, Japan, and a former one, Sanrio Hello Kitty Town in Iskandar Puteri, Johor, Malaysia.[20][21]
+
+In 1962, Shintaro Tsuji, founder of Sanrio, began selling rubber sandals with flowers painted on them.[22] Tsuji noted the profits gained by adding a cute design to the shoes and hired cartoonists to design cute characters for his merchandise.[22] The company produced a line of character merchandise around gift-giving occasions.[23] Hello Kitty was designed by Yuko Shimizu and was added to the lineup of early Sanrio characters in 1974.[13] The character's first appearance on an item was a vinyl coin purse in Japan where she was pictured sitting between a bottle of milk and a goldfish bowl.[24] She first appeared in the United States in 1976.[12]
+
+Sanrio decided to make Hello Kitty British because at the time of her creation foreign countries, in particular, Britain, were trendy in Japan. Sanrio already had several characters set in the US, and it wanted Hello Kitty to be different.[14][25] Shimizu got the name Kitty from Lewis Carroll's Through the Looking-Glass; during a scene early in the book, Alice plays with a cat she calls Kitty.[26] Sanrio's motto is "social communication," and Tsuji wanted the brand name to reflect that. He first considered "Hi Kitty" before settling on "Hello" for the greeting.[27]
+
+Spokespeople for Sanrio have said that Hello Kitty has no mouth, as they want people to "project their feelings onto the character" and "be happy or sad together with Hello Kitty."[14][28] Another explanation Sanrio has given for her lack of a mouth is that she "speaks from the heart. She's Sanrio's ambassador to the world and isn't bound to any particular language".[25] Representatives for Sanrio have said they see Hello Kitty as a symbol of friendship, and they hope she will foster between people across the world.[14] There has been speculation[29][30] that Hello Kitty has its origins in Maneki Neko—the name "Hello Kitty" itself is a back-translation of Maneki Neko, meaning beckoning cat in English. Despite this, no definitive statement supports that speculation.[31]
+
+Hello Kitty sold well immediately after the 1974 launch, and Sanrio's sales increased seven times up until they slumped temporarily in 1978.[14][32] New series with Hello Kitty in different themed designs are released regularly, following current trends. Yuko Yamaguchi, the main designer for most of Hello Kitty's history, has said that fashion, movies, and TV inspire her in creating new designs.[14][32]
+
+Hello Kitty was originally marketed only towards a child and preteen audience. In the 1990s, the target market for Hello Kitty was broadened to include teens and adults as a retro brand.[14][25] Marketed to those who could not get Hello Kitty merchandise as children, or who fondly remember items they had, Sanrio began selling Hello Kitty branded products such as purses and laptops.[14][25][32] The 1994–1996 Face series was the first to be designed for a more mature appeal.[14]
+
+According to Sanrio, in 1999, Hello Kitty appeared on 12,000 different products yearly.[27] By 2008, Hello Kitty was responsible for half of Sanrio's $1 billion net income, and there were over 50,000 different Hello Kitty branded products in more than 60 countries.[25] Beginning in 2007, following trends in Japan, Sanrio began using darker designs for Hello Kitty with more black and less pink and pulling away from kawaii styles.[32]
+
+Hello Kitty and Mimmy celebrated their 40th Anniversary on 1 November 2014. The "Arigato Everyone Birthday Celebration" took place in Sanrio Puroland in Tokyo for several days.[33]
+
+On January 1, 2020, the Sanrio Hello Kitty Town in Iskandar Puteri, Malaysia permanently closed down due to lack of attendance.[20] On February 21, 2020, the Sanrio Puroland theme in Tokyo closed due to the COVID-19 pandemic.[34] Park officials hoped to have it reopened early April 2020.[34] The theme park also goes by the name Hello Kitty Land.[34] In June 2020, the parent company of Hello Kitty, [[Sanrio]] issued a statement that its founder, Shintaro Tsuji will retire as Sanrio chief executive on July 1, and his grandson, Tomokuni Tsuji, 31, would take over to "ensure efficient decision making,".[35]
+
+Originally aimed at the pre-adolescent female market, the Hello Kitty product range has expanded from dolls, stickers, greeting cards, clothes, accessories, school supplies and stationery to purses, toasters, televisions, other home appliances, massagers, and computer equipment. These products range from mass market items to high-end consumer products and rare collectibles.[36] As of 2014[update] more than 50,000 Hello Kitty product lines were available in over 130 countries.[37]
+
+Sanrio and various corporate partners have released Hello Kitty-branded products, including the Hello Kitty Stratocaster electric guitar (since 2006, with Fender in the US) [38] and an Airbus A330-200 commercial passenger jet airliner, dubbed the Hello Kitty Jet (2005–2009, with EVA Airways in Taiwan).[39] In late 2011 and early 2012, EVA Air revived their "Hello Kitty Jets" with their three new A330-300s. However, due to high demand,[40] the airline added two more onto their existing A330-200s in mid-2012. A year after, EVA Air introduced another Hello Kitty Jet onto one of their 777-300ERs, which featured other Sanrio characters as well as Hello Kitty.
+
+In 2009 Hello Kitty entered the wine market with a collection of four wines available for purchase online, continuing the expansion of products targeted at older audiences.[41]
+
+In Spring 2005, Simmons Jewelry Co. and Sanrio announced a partnership. "Kimora Lee Simmons for Hello Kitty" was launched exclusively at Neiman Marcus, with prices ranging from $300 to $5000. Designed by Kimora Lee Simmons and launched as the first series of collections, the jewelry is all hand-made, consisting of diamonds, gemstones, semi-precious stones, 18K gold, sterling silver, enamel, and ceramic.[42]
+
+In Fall 2008, Simmons Jewelry Co. and Sanrio introduced a collection of fine jewelry and watches named "Hello Kitty® by Simmons Jewelry Co." The collection launched with Zales Corporation to further expand the reach of the brand, and it developed accessories to satisfy every Hello Kitty fan. The designs incorporate colorful gemstones and sterling silver to attract a youthful audience.[43]
+
+There is a themed restaurant named Hello Kitty Sweets in Taipei, Taiwan, which opened in 2008. The restaurant's decor and many of its dishes are patterned after the Hello Kitty character.[44][45] A Hello Kitty Diner opened in the Chatswood area of Sydney, Australia,[46] and a Hello Kitty dim sum restaurant opened in Kowloon, Hong Kong.[47]
+
+Hello Kitty cafés have opened around the world, including in Seoul and other locations in South Korea,[48] Bangkok, Thailand,[49] Adelaide, Australia,[50] Irvine, California, US[51] and the Santa Anita Mall in California.[52]
+
+In 2008, a Hello Kitty-themed maternity hospital opened in Yuanlin, Taiwan.  Hello Kitty is featured on the receiving blankets, room decor, bed linens, birth certificate covers, and nurses' uniforms.  The hospital's owner explained that he hoped that the theme would help ease the stress of childbirth.[53][54]
+
+Hello Kitty is included as part of the Sanrio livery at the Japanese theme parks Harmonyland and Sanrio Puroland.
+
+In January 2018, Puma collaborated with Hello Kitty to create the new Puma X Hello Kitty For All Time collection, which features the company's signature sneakers for both children and adults.[55]
+
+Fender Musical Instruments Inc. partnered with Hello Kitty to create the Hello Kitty Stratocaster under its squire sun-brand. While initially aimed at pre-teen girls, the Hello Kitty Stratocaster has proven its worth in the hands of guitarists including Zakk Wylde, John5, and Slash. The guitar's cult following has caused prices to rise, with second hand guitars going for over £660 in 2020.
+
+There have been several different Hello Kitty TV series. The first animated television series was Hello Kitty's Furry Tale Theater, an anime series that was 13 episodes long and aired in 1987.[56] The next, an OVA titled Hello Kitty and Friends, came out in 1993 and was also 13 episodes long. Hello Kitty's Paradise came out in 1999 and was 16 episodes long. Hello Kitty's Stump Village came out in 2005, and The Adventures of Hello Kitty & Friends came out in 2006 and has aired 52 episodes. A crossover series under the name Kiss Hello Kitty (that paired animated versions of the members of the rock band KISS with Hello Kitty) was announced in March 2013. Produced by Gene Simmons, this show was supposed to air on The Hub Network (now Discovery Family),[57] but it never came to fruition.
+
+Hello Kitty's Paradise [ja] was a long-running live-action children's program that aired on TXN from January 1999 to March 2011. It was the longest-running weekly kids' television program in the network's history. In January 2011, the show's creators mutually agreed to end the series after twelve seasons, with the final episode being broadcast on 29 March 2011.
+
+In August 2018, Sanrio began streaming a CGI animated series on YouTube. It features Hello Kitty talking to the camera about her life in the style of vlogging virtual YouTubers.[58][59]
+
+Hello Kitty had two manga comics serialized in Ribon, a shōjo manga magazine - Hello Kitty Doki (ran from May 2007 to April 2008)[60] and Hello Kitty Peace (released in June 2008).[61]
+
+In March 2016, Sanrio launched a webcomic featuring Hello Kitty as a strawberry-themed superhero called "Ichigoman" (ichigo meaning strawberry), who fights monsters with the help of her giant robot. The webcomic is created by Toshiki Inoue and Shakua Sinkai and updates once a month.[62] The Ichigoman alter-ego originates from a 2011 exhibition of Yuko Yamaguchi's artwork.[63]
+
+Hello Kitty has her own branded album, Hello World, featuring Hello Kitty-inspired songs performed by a collection of artists, including Keke Palmer, Cori Yarckin, and Ainjel Emme.[64] Hello Kitty was also chosen by AH-Software to be the basis of the new Vocaloid Nekomura Iroha (猫村いろは, Nekomura Iroha)[65] to celebrate the 50th anniversary of Sanrio.[66]
+
+Hello Kitty was mentioned in the parody song "Another Tattoo (parody of Nothin' On You by B.o.B and Bruno Mars)" from "Weird Al" Yankovic's 2011 album Alpocalypse.
+
+Canadian singer-songwriter Avril Lavigne has written and recorded a song called "Hello Kitty" for her fifth studio album, Avril Lavigne, released in 2013.
+
+Musician Yoshiki unveiled the Hello Kitty theme song "Hello Hello" in November 2014 at the first Hello Kitty Con. Yoshiki, who was the first celebrity to have his own Hello Kitty doll, "Yoshikitty", was approached by Yamaguchi to compose the song seven years prior. Yoshiki dedicated the anthem to Tsuji Kunihiko, the son of Sanrio founder Shintaro Kunihiko.[67]
+
+There are numerous Hello Kitty games starting with the release of the first title for Famicom in 1992; however, the majority of these games never released outside Japan. Hello Kitty also has made cameo appearances in games featuring other Sanrio characters, such as the Keroppi game, Kero Kero Keroppi no Bōken Nikki: Nemureru Mori no Keroleen. Special-edition consoles such as the Hello Kitty Dreamcast, Hello Kitty Game Boy Pocket, and Hello Kitty Crystal Xbox have also been released exclusively in Japan.
+
+Hello Kitty also appeared as a guest character in Sega's Sonic Dash in 2016, as part of Sega's partnership with Sanrio. Hello Kitty (as well as  My Melody, another Sanrio character) also appeared in Super Mario Maker as unlockable Mystery Mushroom costumes.
+
+Three Hello Kitty anime films were released in Japan. Hello Kitty: Cinderella released in 1987, Hello Kitty no Oyayubi Hime released in 1990 and Hello Kitty no Mahō no Mori no Ohime-sama released in 1991.[72]
+
+On 3 July 2015, Sanrio announced a full-length Hello Kitty theatrical feature initially planned for 2019.[73] In early 2019, it was revealed that New Line Cinema will be teaming up alongside Sanrio and Flynn Picture Company for an “English language film based on the venerable kid brand.”[74]
+
+The Hello Kitty brand rose to greater prominence internationally during the late 1990s. At that time, several celebrities, such as Mariah Carey, had adopted Hello Kitty as a fashion statement.[25] Newer products featuring the character can be found in a large variety of American department stores.
+
+In May 2008, Japan named Hello Kitty the ambassador of Japanese tourism in both China and Hong Kong (where the character is exceptionally popular among children and young women), marking it the first time Japan's tourism ministry had appointed a fictional character to the role.[75] Dr. Sharon Kinsella, a lecturer at Oxford University on Japanese sociology, called the selection of Hello Kitty "a bit farcical"; "as if a dumbed-down cultural icon ... can somehow do something significant to alter the gnarly and difficult state of China-Japan relations."[25]
+
+UNICEF has also awarded Hello Kitty the exclusive title of UNICEF Special Friend of Children.[76][77]
+
+Hello Kitty's popularity in Japan peaked in the late 1990s when she was the country's top-grossing character. In 2002, Hello Kitty lost her place as the top-grossing character in Japan in the Character Databank popularity chart. In a 2010 survey, she was in third place behind Anpanman and Pikachu from Pokémon.[32] In 2010, The New York Times attributed the character's relative decline in Japan to her biography not being "compelling enough to draw many fans." The newspaper later wrote that analysts called the characterization "weak",[32] and that Hello Kitty not having a mouth has dampened her success as an animated TV character.[32] Hello Kitty has nevertheless remained one of the top three highest-grossing characters in Japan as of 2013.[78] Overseas, her global popularity has increased over the years, with worldwide annual sales reaching $8 billion in 2013.[17] She has been particularly popular in other Asian countries for decades, such as in China, where her cultural impact is comparable to that of Barbie in the Western world.[79]
+
+In July 2008, the Dutch artist Dick Bruna, creator of Miffy, alleged that Hello Kitty is a copy of Miffy (in Dutch: Nijntje), being rendered in a similar style. He stated disapprovingly in an interview for the British newspaper The Daily Telegraph:
+
+Mercis, the firm that managed copyrights for Bruna, took Sanrio to court over their Hello Kitty-associated character Cathy, a rabbit which made her first appearance in 1976 and which Mercis argued infringed the copyright for Miffy. A court in Amsterdam ruled in favour of Mercis in November 2010 and ordered Sanrio to stop the production and sale of merchandise featuring Cathy in the Benelux countries. However, in June 2011, the two companies announced that they had reached a settlement agreement to end their legal dispute. Sanrio stopped using the Cathy character, and the two firms jointly donating €150,000 for reconstruction after the 2011 Tōhoku earthquake and tsunami.[81]
+
+Musti, a cat character created by Belgian cartoonist Ray Goossens, was also cited as an inspiration for Hello Kitty.[82][83]
+
+In 1994, artist Tom Sachs was invited to create a scene for Barneys New York Christmas displays and titled it Hello Kitty Nativity. For this scene, the Virgin Mary was replaced by Madonna with an open Chanel bra, the three Kings were all Bart Simpson, the stable was marked by a McDonald's logo, and the Christ Child was replaced by Hello Kitty. This contemporary revision of the nativity scene received mostly great attention,[84] and demonstrated Sachs' interest in the phenomena of consumerism, branding, and the cultural fetishization of products.
+
+In 2009, Tom Sachs' Bronze Collection was shown at the Public art space in Manhattan's Lever House, as well as in the Baldwin Gallery in Aspen, Colorado, and the Trocadéro in Paris. The collection featured white bronze casts of a foam core Hello Kitty sculpture – a style distinctive to the artist. As of April 2010, the Wind-Up Hello Kitty sculpture is still on display at Lever House.[85]  Although Sachs did not seek permission to use the character in his work, a brand marketing manager for Sanrio was quoted as saying "You know, there was Marilyn Monroe and Andy Warhol, and then Michael Jackson and Jeff Koons. When you're an icon, that's what happens."[86]
+
+In 2015, a 9-foot tall pearlescent Hello Kitty sculpture by artist Sebastian Masuda, was exhibited at the Dag Hammarskjold Plaza, New York City, as part of the Japan Society's exhibition: Life of Cats: Selections from the Hiraki Ukiyo-e Collection.[87]

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+Helsinki (/ˈhɛlsɪŋki/ HEL-sink-ee or /hɛlˈsɪŋki/ (listen) hel-SINK-ee;[7][8] Finnish: [ˈhelsiŋki] (listen); Swedish: Helsingfors [hɛlsɪŋˈfɔʂː] (listen), Finland Swedish: [helsiŋˈforsː] (listen)) is the capital, primate and most populous city of Finland. Located on the shore of the Gulf of Finland, it is the seat of the region of Uusimaa in southern Finland, and has a population of 650,058.[9] The city's urban area has a population of 1,268,296,[10] making it by far the most populous urban area in Finland as well as the country's most important center for politics, education, finance, culture, and research; while Tampere in the Pirkanmaa region, located 179 kilometres (111 mi) to the north from Helsinki, is the second largest urban area in Finland. Helsinki is located 80 kilometres (50 mi) north of Tallinn, Estonia, 400 km (250 mi) east of Stockholm, Sweden, and 300 km (190 mi) west of Saint Petersburg, Russia. It has close historical ties with these three cities.
+
+Together with the cities of Espoo, Vantaa, and Kauniainen, and surrounding commuter towns,[11] Helsinki forms the Greater Helsinki metropolitan area, which has a population of nearly 1.5 million. Often considered to be Finland's only metropolis, it is the world's northernmost metro area with over one million people as well as the northernmost capital of an EU member state. After Stockholm and Oslo, Helsinki is the third largest municipality in the Nordic countries. Finnish and Swedish are both official languages. The city is served by the international Helsinki Airport, located in the neighboring city of Vantaa, with frequent service to many destinations in Europe and Asia.
+
+Helsinki was the World Design Capital for 2012,[12] the venue for the 1952 Summer Olympics, and the host of the 52nd Eurovision Song Contest in 2007.
+
+Helsinki has one of the world's highest urban standards of living. In 2011, the British magazine Monocle ranked Helsinki the world's most liveable city in its liveable cities index.[13] In the Economist Intelligence Unit's 2016 liveability survey, Helsinki was ranked ninth among 140 cities.[14]
+
+According to a theory presented in the 1630s, at the time of Swedish colonisation of coastal areas of Finland, colonists from Hälsingland in central Sweden had arrived at what is now known as the Vantaa River and called it Helsingå ("Helsinge River"), which gave rise to the names of Helsinge village and church in the 1300s.[15] This theory is questionable, because dialect research suggests that the settlers arrived from Uppland and nearby areas.[16] Others have proposed the name as having been derived from the Swedish word helsing, an archaic form of the word hals (neck), referring to the narrowest part of a river, the rapids.[17] Other Scandinavian cities at similar geographic locations were given similar names at the time, e.g. Helsingør in Denmark and Helsingborg in Sweden.
+
+When a town was founded in Forsby village in 1548, it was named Helsinge fors, "Helsinge rapids". The name refers to the Vanhankaupunginkoski rapids at the mouth of the river.[18] The town was commonly known as Helsinge or Helsing, from which the contemporary Finnish name arose.[19]
+
+Official Finnish Government documents and Finnish language newspapers have used the name Helsinki since 1819, when the Senate of Finland moved itself into the city from Turku, the former capital of Finland. The decrees issued in Helsinki were dated with Helsinki as the place of issue. This is how the form Helsinki came to be used in written Finnish.[20] As part of the Grand Duchy of Finland in the Russian Empire, Helsinki was known as Gelsingfors in Russian.
+
+In Helsinki slang, the city is called Stadi (from the Swedish word stad, meaning "city") or Hesa (short for Helsinki).[1][21] Helsset is the Northern Sami name of Helsinki.[22]
+
+In the Iron Age the area occupied by present-day Helsinki was inhabited by Tavastians. They used the area for fishing and hunting, but due to a lack of archeological finds it is difficult to say how extensive their settlements were. Pollen analysis has shown that there were cultivating settlements in the area in the 10th century and surviving historical records from the 14th century describe Tavastian settlements in the area.[23]
+
+Swedes colonized the coastline of the Helsinki region in the late 13th century after the successful Second Crusade to Finland, which led to the defeat of the Tavastians.[24][23]
+
+Helsinki was established as a trading town by King Gustav I of Sweden in 1550 as the town of Helsingfors, which he intended to be a rival to the Hanseatic city of Reval (today known as Tallinn).[25] In order to populate his newly founded town, the King issued an order to resettle the bourgeoisie of Porvoo, Ekenäs, Rauma and Ulvila into the town.[26] Little came of the plans as Helsinki remained a tiny town plagued by poverty, wars, and diseases. The plague of 1710 killed the greater part of the inhabitants of Helsinki.[25] The construction of the naval fortress Sveaborg (in Finnish Viapori, today also Suomenlinna) in the 18th century helped improve Helsinki's status, but it was not until Russia defeated Sweden in the Finnish War and annexed Finland as the autonomous Grand Duchy of Finland in 1809 that the town began to develop into a substantial city. Russians besieged the Sveaborg fortress during the war, and about one quarter of the town was destroyed in an 1808 fire.[27]
+
+Russian Emperor Alexander I of Russia moved the Finnish capital from Turku to Helsinki in 1812[28] to reduce Swedish influence in Finland, and to bring the capital closer to Saint Petersburg. Following the Great Fire of Turku in 1827, the Royal Academy of Turku, which at the time was the country's only university, was also relocated to Helsinki and eventually became the modern University of Helsinki. The move consolidated the city's new role and helped set it on a path of continuous growth. This transformation is highly apparent in the downtown core, which was rebuilt in the neoclassical style to resemble Saint Petersburg, mostly to a plan by the German-born architect C. L. Engel. As elsewhere, technological advancements such as railroads and industrialization were key factors behind the city's growth.
+
+Despite the tumultuous nature of Finnish history during the first half of the 20th century (including the Finnish Civil War and the Winter War which both left marks on the city), Helsinki continued its steady development. A landmark event was the 1952 Olympic Games, held in Helsinki. Finland's rapid urbanization in the 1970s, occurring late relative to the rest of Europe, tripled the population in the metropolitan area, and the Helsinki Metro subway system was built. The relatively sparse population density of Helsinki and its peculiar structure have often been attributed to the lateness of its growth.[citation needed]
+
+Called the "Daughter of the Baltic", Helsinki is on the tip of a peninsula and on 315 islands. The inner city is located on a southern peninsula, Helsinginniemi ("Cape of Helsinki), which is rarely referred to by its actual name, Vironniemi ("Cape of Estonia"). Population density in certain parts of Helsinki's inner city area is comparatively higher, reaching 16,494 inhabitants per square kilometre (42,720/sq mi) in the district of Kallio, but as a whole Helsinki's population density of 3,050 per square kilometre (7,900/sq mi) ranks the city as rather sparsely populated in comparison to other European capital cities.[29][30] Outside of the inner city, much of Helsinki consists of postwar suburbs separated by patches of forest. A narrow, 10 kilometres (6.2 mi) long Helsinki Central Park, stretching from the inner city to Helsinki's northern border, is an important recreational area for residents. The City of Helsinki has about 11,000 boat berths and possesses over 14,000 hectares (34,595 acres; 54.1 sq mi) of marine fishing waters adjacent to the Capital Region. Some 60 fish species are found in this area and recreational fishing is popular.
+
+Major islands in Helsinki include Seurasaari, Vallisaari, Lauttasaari, and Korkeasaari – the lattermost being the site of Finland's largest zoo. Other noteworthy islands are the fortress island of Suomenlinna (Sveaborg), the military island of Santahamina, and Isosaari. Pihlajasaari island is a favorite summer spot for gay men and naturists, comparable to Fire Island in New York City.
+
+The Helsinki metropolitan area, also known as the Capital Region (Finnish: Pääkaupunkiseutu, Swedish: Huvudstadsregionen) comprises four municipalities: Helsinki, Espoo, Vantaa, and Kauniainen.[31] The Helsinki urban area is considered to be the only metropolis in Finland.[32] It has a population of over 1.1 million, and is the most densely populated area of Finland. The Capital Region spreads over a land area of 770 square kilometres (300 sq mi) and has a population density of 1,418 inhabitants per square kilometre (3,670/sq mi). With over 20 percent of the country's population in just 0.2 percent of its surface area, the area's housing density is high by Finnish standards.
+
+The Helsinki Metropolitan Area (Greater Helsinki) consists of the cities of Helsinki Capital Region and ten surrounding municipalities. The Metropolitan Area covers 3,697 square kilometres (1,427 sq mi) and has a population of over 1.4 million, or about a fourth of the total population of Finland. The metropolitan area has a high concentration of employment: approximately 750,000 jobs.[33] Despite the intensity of land use, the region also has large recreational areas and green spaces. The Greater Helsinki area is the world's northernmost urban area with a population of over one million people, and the northernmost EU capital city.
+
+The Helsinki urban area is an officially recognized urban area in Finland, defined by its population density. The area stretches throughout 11 municipalities, and is the largest such area in Finland, with a land area of 669.31 square kilometres (258.42 sq mi) and approximately 1,2 million inhabitants.
+
+Helsinki has a humid continental climate (Köppen: Dfb) similar to that of Hokkaido or Nova Scotia coastal.[34] Owing to the mitigating influence of the Baltic Sea and North Atlantic Current (see also Extratropical cyclone), temperatures during the winter are higher than the northern location might suggest, with the average in January and February around −5 °C (23 °F).[35]
+
+Winters in Helsinki are notably warmer than in the north of Finland, and the snow season is much shorter in the capital, due to it being in extreme Southern Finland and the urban heat island effect. Temperatures below −20 °C (−4 °F) occur a few times a year at most. However, because of the latitude, days last 5 hours and 48 minutes around the winter solstice with very low sun (at noon, the sun is a little bit over 6 degrees in the sky), and the cloudy weather at this time of year exacerbates darkness. Conversely, Helsinki enjoys long daylight during the summer; during the summer solstice, days last 18 hours and 57 minutes.[36]
+
+The average maximum temperature from June to August is around 19 to 22 °C (66 to 72 °F). Due to the marine effect, especially during hot summer days, daily temperatures are a little cooler and night temperatures higher than further inland. The highest temperature ever recorded in the city was 33.2 °C (91.8 °F), on 28 July 2019 at Kaisaniemi weather station,[37] breaking the previous record of 33.1 °C (91.6 °F) that was observed in July 1945 at Ilmala weather station.[38] The lowest temperature ever recorded in the city was −34.4 °C (−30 °F), on 10 January 1987 although an unofficial low of -35 was recorded in December 1876.[39] Helsinki Airport (in Vantaa, 17 kilometres (11 mi) north of the Helsinki city centre) recorded a temperature of 33.7 °C (92.7 °F), on 29 July 2010, and a low of −35.9 °C (−33 °F), on 9 January 1987. Precipitation is received from frontal passages and thunderstorms. Thunderstorms are most common in the summer.
+
+Helsinki is divided into three major areas: Helsinki Downtown (Finnish: Helsingin kantakaupunki, Swedish: Helsingfors innerstad), North Helsinki (Finnish: Pohjois-Helsinki, Swedish: Norra Helsingfors) and East Helsinki (Finnish: Itä-Helsinki, Swedish: Östra Helsingfors).
+
+Carl Ludvig Engel, appointed to plan a new city centre on his own, designed several neoclassical buildings in Helsinki. The focal point of Engel's city plan was the Senate Square. It is surrounded by the Government Palace (to the east), the main building of Helsinki University (to the west), and (to the north) the large Helsinki Cathedral, which was finished in 1852, twelve years after Engel's death. Helsinki's epithet, "The White City of the North", derives from this construction era.
+
+Helsinki is also home to numerous Art Nouveau-influenced (Jugend in Finnish) buildings belonging to the Kansallisromantiikka = romantic nationalism trend, designed in the early 20th century and strongly influenced by Kalevala, which was a common theme of the era. Helsinki's Art Nouveau style is also featured in central residential districts, such as Katajanokka and Ullanlinna. An important architect of the Finnish Art Nouveau style was Eliel Saarinen, whose architectural masterpiece was the Helsinki Central Station.
+
+Helsinki also features several buildings by Finnish architect Alvar Aalto, recognized as one of the pioneers of architectural functionalism. However, some of his works, such as the headquarters of the paper company Stora Enso and the concert venue Finlandia Hall, have been subject to divided opinions from the citizens.[44][45][46]
+
+Functionalist buildings in Helsinki by other architects include the Olympic Stadium, the Tennis Palace, the Rowing Stadium, the Swimming Stadium, the Velodrome, the Glass Palace, the Töölö Sports Hall, and Helsinki-Malmi Airport. The sports venues were built to serve the 1940 Helsinki Olympic Games; the games were initially cancelled due to the Second World War, but the venues fulfilled their purpose in the 1952 Olympic Games. Many of them are listed by DoCoMoMo as significant examples of modern architecture. The Olympic Stadium and Helsinki-Malmi Airport are also catalogued by the Finnish National Board of Antiquities as cultural-historical environments of national significance.[citation needed]
+
+Helsinki's neoclassical buildings were often used as a backdrop for scenes set to take place in the Soviet Union in many Cold War era Hollywood movies, when filming in the USSR was not possible. Some of them include The Kremlin Letter (1970), Reds (1981), and Gorky Park (1983).[47] Because some streetscapes were reminiscent of Leningrad's and Moscow's old buildings, they too were used in movie productions. At the same time the government secretly instructed Finnish officials not to extend assistance to such film projects.[48] Rarely has Helsinki been represented on its own in films, most notably the 1967 British-American espionage thriller Billion Dollar Brain, starring Michael Caine.[49][50]
+
+The start of the 21st century marked the beginning of highrise construction in Helsinki, when the city decided to allow the construction of skyscrapers. As of April 2017 there are no skyscrapers taller than 100 meters in the Helsinki area, but there are several projects under construction or planning, mainly in Pasila and Kalasatama. An international architecture competition for at least 10 high-rises to be built in Pasila is being held. Construction of the towers will start before 2020.[51] In Kalasatama, the first 35-story (130 m, 427 ft) and 32-story (122 m, 400 ft) residential towers are already under construction. Later they will be joined by a 37-story (140 metres, 459 ft), two 32-story (122 metres, 400 feet), 31-story (120 metres, 394 ft), and 27-story (100 metres, 328 ft) residential buildings. In the Kalasatama area, there will be about 15 high-rises within 10 years.[52]
+
+As is the case with all Finnish municipalities, Helsinki's city council is the main decision-making organ in local politics, dealing with issues such as urban planning, schools, health care, and public transport. The council is chosen in the nationally held municipal elections, which are held every four years.
+
+Helsinki's city council consists of eighty-five members. Following the most recent municipal elections in 2017, the three largest parties are the National Coalition Party (25), the Green League (21), and the Social Democratic Party (12).[53]
+
+The Mayor of Helsinki is Jan Vapaavuori.
+
+At 53 percent of the population, women form a greater proportion of Helsinki residents than the national average of 51 percent. Helsinki's population density of 2,739.36 people per square kilometre makes Helsinki the most densely-populated city in Finland. The life expectancy for men and women is slightly below the national averages: 75.1 years for men as compared to 75.7 years, 81.7 years for women as compared to 82.5 years.[54][55]
+
+Helsinki has experienced strong growth since the 1810s, when it replaced Turku as the capital of the Grand Duchy of Finland, which later became the sovereign Republic of Finland. The city continued its growth from that time on, with an exception during the Finnish Civil War. From the end of World War II up until the 1970s there was a massive exodus of people from the countryside to the cities of Finland, in particular Helsinki. Between 1944 and 1969 the population of the city nearly doubled from 275,000[56] to 525,600.[57]
+
+In the 1960s, the population growth of Helsinki began to decrease, mainly due to a lack of housing.[58] Some residents began to move to the neighbouring cities of Espoo and Vantaa, resulting in increased population growth in both municipalities. Espoo's population increased ninefold in sixty years, from 22,874 people in 1950 to 244,353 in 2009.[citation needed] Vantaa saw an even more dramatic change in the same time span: from 14,976 in 1950 to 197,663 in 2009, a thirteenfold increase. These population changes prompted the municipalities of Greater Helsinki into more intense cooperation in areas such as public transportation[59] – resulting in the foundation of HSL – and waste management.[60] The increasing scarcity of housing and the higher costs of living in the capital region have pushed many daily commuters to find housing in formerly rural areas, and even further, to cities such as Lohja, Hämeenlinna, Lahti, and Porvoo.
+
+Finnish and Swedish are the official languages of Helsinki. 79.1%[62] of the citizens speak Finnish as their native language. 5.7% speak Swedish. The remaining 15.3% of the population speaks a native language other than Finnish or Swedish.
+
+Helsinki slang is a regional dialect of the city. It combines influences mainly from Finnish and English, and has traditionally had strong Russian and Swedish influences. Finnish today is the common language of communication between Finnish speakers, Swedish speakers, and speakers of other languages (New Finns) in day-to-day affairs in the public sphere between unknown persons.[citation needed] Swedish is commonly spoken in city or national agencies specifically aimed at Finland-Swedish speakers, such as the Social Services Department on Hämeentie or the Luckan Cultural centre in Kamppi. Knowledge of Finnish is also essential in business and is usually a basic requirement in the employment market.[63]
+
+Finnish speakers surpassed Swedish speakers in 1890 to become the majority of the city's population.[64] At the time, the population of Helsinki was 61,530.[65]
+
+As the crossroads of many international ports and Finland's largest airport, Helsinki is the global gateway to and from Finland. The city has Finland's largest immigrant population in both absolute and relative terms. There are over 140 nationalities represented in Helsinki. It is home to the world's largest Estonian community outside of Estonia.
+
+Foreign citizens make up 9.6% of the population, while the total immigrant population makes up 16%.[67][68] In 2018, 101,825[69] residents spoke a native language other than Finnish, Swedish, or one of the three Sami languages spoken in Finland, and 103,499 had a foreign background. The largest groups of residents not of Finnish background come from Russia (14,532), Estonia (9,065), and Somalia (6,845).[67] One third of Finland's immigrant population lives in the city of Helsinki.[70]
+
+The number of people with a foreign mother tongue is expected to be 196,500 in 2035, or 26% of the population. 114,000 will speak non-European languages, which will be 15% of the population.[71]
+
+Greater Helsinki generates approximately one third of Finland's GDP. GDP per capita is roughly 1.3 times the national average.[72] Helsinki profits on serviced-related IT and public sectors. Having moved from heavy industrial works, shipping companies also employ a substantial number of people.[73]
+
+The metropolitan area's gross value added per capita is 200% of the mean of 27 European metropolitan areas, equalling those of Stockholm and Paris. The gross value added annual growth has been around 4%.[74]
+
+83 of the 100 largest Finnish companies have their headquarters in Greater Helsinki. Two-thirds of the 200 highest-paid Finnish executives live in Greater Helsinki and 42% in Helsinki. The average income of the top 50 earners was 1.65 million euro.[75]
+
+The tap water is of excellent quality and it is supplied by 120 km (75 mi) long Päijänne Water Tunnel, one of the world's longest continuous rock tunnels.[76]
+
+The Temppeliaukio Church is a Lutheran church in the Töölö neighborhood of the city. The church was designed by architects and brothers Timo and Tuomo Suomalainen and opened in 1969. Built directly into solid rock, it is also known as the Church of the Rock and Rock Church.[77][78] The Cathedral of the Diocese of Helsinki is the Helsinki Cathedral, completed in 1852. It is a major landmark in the city and has 1,300 seats.
+
+The largest Orthodox congregation is the Orthodox Church of Helsinki. It has 20,000 members. Its main church is the Uspenski Cathedral.[79] The two largest Catholic congregations are Saint Henry's Cathedral Parish, with 4,552 members, established in 1860 and St. Mary Catholic Parish, with 4,107 members, established in 1854.[80] The main Catholic churches are the Cathedral of Saint Henry and St. Mary's Church.
+
+At the end of 2018, 52.4% of the population were affiliated to the Evangelical Lutheran Church of Finland.[81] Helsinki is the least Lutheran municipality in Finland.[82]
+
+There are around 30 mosques in the Helsinki region. Many linguistic and ethnic groups such as Bangladeshis, Kosovars, Kurds and Bosniaks have established their own mosques.[83] The largest congregation in both Helsinki and Finland is the Helsinki Islamic Center, established in 1995. It has over 2,800 members as of 2017, and it received €24,131 in government assistance.[84][85]
+
+In 2015, imam Anas Hajar estimated that on big celebrations around 10,000 Muslims visit mosques.[86] In 2004, it was estimated that there were 8,000 Muslims in Helsinki, 1.5% of the population at the time.[87]
+
+The main synagogue of Helsinki is the Helsinki Synagogue, located in Kamppi. It has over 1,200 members, out of the 1,800 Jews in Finland. The congregation includes a synagogue, Jewish kindergarten, school, library, Jewish meat shop, two Jewish cemeteries and an retirement home. Many Jewish organizations and societies are based there, and the synagogue publishes the main Jewish magazine in Finland, HaKehila.[88]
+
+Helsinki has 190 comprehensive schools, 41 upper secondary schools, and 15 vocational institutes. Half of the 41 upper secondary schools are private or state-owned, the other half municipal. There are two major research universities in Helsinki, the University of Helsinki and Aalto University, and a number of other higher level institutions and polytechnics which focus on higher-level professional education.
+
+Helsinki is one of the co-location centres of the Knowledge and Innovation Community (Future information and communication society) of The European Institute of Innovation and Technology (EIT).[89]
+
+The biggest historical museum in Helsinki is the National Museum of Finland, which displays a vast historical collection from prehistoric times to the 21st century. The museum building itself, a national romantic style neomedieval castle, is a tourist attraction. Another major historical museum is the Helsinki City Museum, which introduces visitors to Helsinki's 500-year history. The University of Helsinki also has many significant museums, including the Helsinki University Museum "Arppeanum" and the Finnish Museum of Natural History.
+
+The Finnish National Gallery consists of three museums: Ateneum Art Museum for classical Finnish art, Sinebrychoff Art Museum for classical European art, and Kiasma Art Museum for modern art, in a building by architect Steven Holl. The old Ateneum, a neo-Renaissance palace from the 19th century, is one of the city's major historical buildings. All three museum buildings are state-owned through Senate Properties.
+
+The city of Helsinki hosts its own art collection in the Helsinki Art Museum (HAM), primarily located in its Tennispalatsi gallery. Pieces outside of Tennispalatsi include about 200 public art pieces and all art held in property owned by the city.
+
+Helsinki Art Museum will in 2020 launch the Helsinki Biennial, which will bring art to maritime Helsinki – in its first year to the island of Vallisaari.[90]
+
+The Design Museum is devoted to the exhibition of both Finnish and foreign design, including industrial design, fashion, and graphic design. Other museums in Helsinki include the Military Museum of Finland, Didrichsen Art Museum, Amos Rex Art Museum, and the Tram Museum.
+
+Sinebrychoff Art Museum (1842)
+
+Helsinki University Museum "Arppeanum" (1869)
+
+The Cygnaeus Gallery Museum (1870)
+
+The Military Museum of Finland (1881)
+
+Classical art museum Ateneum (1887)
+
+The Design Museum (1894)
+
+Tram museum (Ratikkamuseo) (1900)
+
+The National Museum of Finland (1910)
+
+The Helsinki City Museum (1911)
+
+The Finnish Museum of Natural History (1913)
+
+Kunsthalle Helsinki art venue (1928)
+
+Didrichsen Art Museum (1964)
+
+Helsinki Art Museum (1968)
+
+Kiasma museum of contemporary art (1998)
+
+Amos Rex art museum (2018)
+
+Helsinki has three major theatres: The Finnish National Theatre, the Helsinki City Theatre, and the Swedish Theatre (Svenska Teatern). Other notable theatres in the city include the Alexander Theatre, Q-teatteri, Savoy Theatre, KOM-theatre, and Teatteri Jurkka.
+
+Helsinki is home to two full-size symphony orchestras, the Helsinki Philharmonic Orchestra and the Finnish Radio Symphony Orchestra, both of which perform at the Helsinki Music Centre concert hall. Acclaimed contemporary composers Kaija Saariaho, Magnus Lindberg, Esa-Pekka Salonen, and Einojuhani Rautavaara, among others, were born and raised in Helsinki, and studied at the Sibelius Academy. The Finnish National Opera, the only full-time, professional opera company in Finland, is located in Helsinki. The opera singer Martti Wallén, one of the company's long-time soloists, was born and raised in Helsinki, as was mezzo-soprano Monica Groop.
+
+Many widely renowned and acclaimed bands have originated in Helsinki, including Nightwish, Children of Bodom, Hanoi Rocks, HIM, Stratovarius, The 69 Eyes, Finntroll, Ensiferum, Wintersun, The Rasmus, Poets of the Fall, and Apocalyptica.
+
+The city's main musical venues are the Finnish National Opera, the Finlandia concert hall, and the Helsinki Music Centre. The Music Centre also houses a part of the Sibelius Academy. Bigger concerts and events are usually held at one of the city's two big ice hockey arenas: the Hartwall Arena or the Helsinki Ice Hall. Helsinki has Finland's largest fairgrounds, the Messukeskus Helsinki.
+
+Helsinki Arena hosted the Eurovision Song Contest 2007, the first Eurovision Song Contest arranged in Finland, following Lordi's win in 2006.[91]
+
+The Helsinki Festival is an annual arts and culture festival, which takes place every August (including the Night of the Arts).[92]
+
+Vappu is an annual carnival for students and workers.
+
+At the Senate Square in fall 2010, Finland's largest open-air art exhibition to date took place: About 1.4 million people saw the international exhibition of United Buddy Bears.[93]
+
+Helsinki was the 2012 World Design Capital, in recognition of the use of design as an effective tool for social, cultural, and economic development in the city. In choosing Helsinki, the World Design Capital selection jury highlighted Helsinki's use of 'Embedded Design', which has tied design in the city to innovation, "creating global brands, such as Nokia, Kone, and Marimekko, popular events, like the annual Helsinki Design Week, outstanding education and research institutions, such as the Aalto University School of Arts, Design and Architecture, and exemplary architects and designers such as Eliel Saarinen and Alvar Aalto".[12]
+
+Helsinki hosts many film festivals. Most of them are small venues, while some have generated interest internationally. The most prolific of these is the Love & Anarchy film festival, also known as Helsinki International Film Festival, which features films on a wide spectrum. Night Visions, on the other hand, focuses on genre cinema, screening horror, fantasy, and science fiction films in very popular movie marathons that last the entire night. Another popular film festival is DocPoint, a festival that focuses solely on documentary cinema.[94][95][96]
+
+Today,[when?] there are around 200 newspapers, 320 popular magazines, 2,100 professional magazines, 67 commercial radio stations, three digital radio channels, and one nationwide and five national public service radio channels.[citation needed]
+
+Sanoma publishes Finland's journal of record, Helsingin Sanomat, the tabloid Ilta-Sanomat, the commerce-oriented Taloussanomat, and the television channel Nelonen. Another Helsinki-based media house, Alma Media, publishes over thirty magazines, including the newspaper Aamulehti, the tabloid Iltalehti, and the commerce-oriented Kauppalehti.
+
+Finland's national public-broadcasting institution Yle operates five television channels and thirteen radio channels in both national languages. Yle is headquartered in the neighbourhood of Pasila. All TV channels are broadcast digitally, both terrestrially and on cable.
+
+The commercial television channel MTV3 and commercial radio channel Radio Nova are owned by Nordic Broadcasting (Bonnier and Proventus Industrier).
+
+Helsinki has a long tradition of sports: the city gained much of its initial international recognition during the 1952 Summer Olympics, and the city has arranged sporting events such as the first World Championships in Athletics 1983 and 2005, and the European Championships in Athletics 1971, 1994, and 2012. Helsinki hosts successful local teams in both of the most popular team sports in Finland: football and ice hockey. Helsinki houses HJK Helsinki, Finland's largest and most successful football club, and IFK Helsingfors, their local rivals with 7 championship titles. The fixtures between the two are commonly known as Stadin derby. Helsinki's track and field club Helsingin Kisa-Veikot is also dominant within Finland. Ice hockey is popular among many Helsinki residents, who usually support either of the local clubs IFK Helsingfors (HIFK) or Jokerit. HIFK, with 14 Finnish championships titles, also plays in the highest bandy division,[97] along with Botnia-69. The Olympic stadium hosted the first ever Bandy World Championship in 1957.[98]
+
+Helsinki was elected host-city of the 1940 Summer Olympics, but due to World War II they were canceled. Instead Helsinki was the host of the 1952 Summer Olympics. The Olympics were a landmark event symbolically and economically for Helsinki and Finland as a whole that was recovering from the winter war and the continuation war fought with the Soviet Union. Helsinki was also in 1983 the first ever city to host the World Championships in Athletics. Helsinki also hosted the event in 2005, thus also becoming the first city to ever host the Championships for a second time. The Helsinki City Marathon has been held in the city every year since 1980, usually in August. A Formula 3000 race through the city streets was held on 25 May 1997. In 2009 Helsinki was host of the European Figure Skating Championships, and in 2017 it hosted World Figure Skating Championships.[99] The city will host the 2021 FIBA Under-19 Basketball World Cup.
+
+The backbone of Helsinki's motorway network consists of three semicircular beltways, Ring I, Ring II, and Ring III, which connect expressways heading to other parts of Finland, and the western and eastern arteries of Länsiväylä and Itäväylä respectively. While variants of a Keskustatunneli tunnel under the city centre have been repeatedly proposed, as of 2017[update] the plan remains on the drawing board.
+
+Many important Finnish highways leave Helsinki for various parts of Finland, most of them in the form of motorways. The most significant highways are:
+
+Helsinki has some 390 cars per 1000 inhabitants.[100] This is less than in cities of similar population and construction density, such as Brussels' 483 per 1000, Stockholm's 401, and Oslo's 413.[101][102]
+
+The Helsinki Central Railway Station is the main terminus of the rail network in Finland. Two rail corridors lead out of Helsinki, the Main Line to the north (to Tampere, Oulu, Rovaniemi), and the Coastal Line to the west (to Turku). The railway connection to the east branches from the Main Line outside of Helsinki at Kerava, and leads via Lahti to eastern parts of Finland and to Russia.
+
+A majority of intercity passenger services in Finland originate or terminate at the Helsinki Central Railway Station. All major cities in Finland are connected to Helsinki by rail service, with departures several times a day. The most frequent service is to Tampere, with more than 25 intercity departures per day as of 2017. There are international services from Helsinki to Saint Petersburg and to Moscow in Russia. The Saint Petersburg to Helsinki route is operated with the Allegro high-speed trains.
+
+A Helsinki to Tallinn Tunnel has been proposed[103] and agreed upon by representatives of the cities.[104] The rail tunnel would connect Helsinki to the Estonian capital Tallinn, further linking Helsinki to the rest of continental Europe by Rail Baltica.
+
+Air traffic is handled primarily from Helsinki Airport, located approximately 17 kilometres (11 mi) north of Helsinki's downtown area, in the neighbouring city of Vantaa. Helsinki's own airport, Helsinki-Malmi Airport, is mainly used for general and private aviation. Charter flights are available from Hernesaari Heliport.
+
+Like many other cities, Helsinki was deliberately founded at a location on the sea in order to take advantage of shipping. The freezing of the sea imposed limitations on sea traffic up to the end of the 19th century. But for the last hundred years, the routes leading to Helsinki have been kept open even in winter with the aid of icebreakers, many of them built in the Helsinki Hietalahti shipyard. The arrival and departure of ships has also been a part of everyday life in Helsinki. Regular route traffic from Helsinki to Stockholm, Tallinn, and Saint Petersburg began as far back as 1837. Over 300 cruise ships and 360,000 cruise passengers visit Helsinki annually. There are international cruise ship docks in South Harbour, Katajanokka, West Harbour, and Hernesaari. Helsinki is the second busiest passenger port in Europe with approximately 11 million passengers in 2013.[105] Ferry connections to Tallinn, Mariehamn, and Stockholm are serviced by various companies. Finnlines passenger-freight ferries to Gdynia, Poland; Travemünde, Germany; and Rostock, Germany are also available. St. Peter Line offers passenger ferry service to Saint Petersburg several times a week.
+
+In the Helsinki metropolitan area, public transportation is managed by the Helsinki Regional Transport Authority, the metropolitan area transportation authority. The diverse public transport system consists of trams, commuter rail, the metro, bus lines, two ferry lines and a public bike system.
+
+Helsinki's tram system has been in operation with electric drive continuously since 1900. 13 routes that cover the inner part of the city are operated. As of 2017, the city is expanding the tram network, with several major tram line construction projects under way. These include the Jokeri light rail (replacing the 550 bus line), roughly along Ring I around the city center, and a new tramway to the island of Laajasalo.
+
+The Helsinki Metro, opened in 1982, is the only metro system in Finland, albeit the Helsinki commuter rail trains operate at metro-like frequencies. In 2006, the construction of the long debated extension of the metro into Western Helsinki and Espoo was approved.[106] The extension finally opened after delays in November 2017.[107] An eastern extension into the planned new district of Östersundom and neighboring Sipoo has also been seriously debated. Helsinki's metro system consists of 25 stations, with 14 of them underground.[108]
+
+The commuter rail system includes purpose-built double track for local services in two rail corridors along intercity railways, and the Ring Rail Line, an urban double-track railway with a station at the Helsinki Airport in Vantaa. Electric operation of commuter trains was first begun in 1969, and the system has been gradually expanded since. 15 different services are operated as of 2017, some extending outside of the Helsinki region. The frequent services run at a 10-minute headway in peak traffic.
+
+Helsinki is officially the sister city of Beijing, China (since 2006).[109][110][111] In addition, the city [109] has a special partnership relation with:

en/2522.html.txt

@@ -0,0 +1,118 @@
+Red blood cells (RBCs), also referred to as red cells,[1] red blood corpuscles, haematids, erythroid cells  or erythrocytes (from Greek erythros for "red" and kytos for "hollow vessel", with -cyte translated as "cell" in modern usage), are the most common type of blood cell and the vertebrate's principal means of delivering oxygen (O2) to the body tissues—via blood flow through the circulatory system.[2]  RBCs take up oxygen in the lungs, or gills of fish, and release it into tissues while squeezing through the body's capillaries.
+
+The cytoplasm of erythrocytes is rich in hemoglobin, an iron-containing biomolecule that can bind oxygen and is responsible for the red color of the cells and the blood. Each human red blood cell contains approximately 270 million[3] of these hemoglobin molecules. The cell membrane is composed of proteins and lipids, and this structure provides properties essential for physiological cell function such as deformability and stability while traversing the circulatory system and specifically the capillary network.
+
+In humans, mature red blood cells are flexible and oval biconcave disks. They lack a cell nucleus and most organelles, in order to accommodate maximum space for hemoglobin; they can be viewed as sacks of hemoglobin, with a plasma membrane as the sack. Approximately 2.4 million new erythrocytes are produced per second in human adults.[4] The cells develop in the bone marrow and circulate for about 100–120 days in the body before their components are recycled by macrophages.  Each circulation takes about 60 seconds (one minute).[5] Approximately 84% of the cells in the human body are 20–30 trillion red blood cells.[6][7][8] Nearly half of the blood's volume (40% to 45%) is red blood cells.
+
+Packed red blood cells (pRBC) are red blood cells that have been donated, processed, and stored in a blood bank for blood transfusion.
+
+Almost all vertebrates, including all mammals and humans, have red blood cells. Red blood cells are cells present in blood in order to transport oxygen. The only known vertebrates without red blood cells are the crocodile icefish (family Channichthyidae); they live in very oxygen-rich cold water and transport oxygen freely dissolved in their blood.[10] While they no longer use hemoglobin, remnants of hemoglobin genes can be found in their genome.[11]
+
+Vertebrate red blood cells consist mainly of hemoglobin, a complex metalloprotein containing heme groups whose iron atoms temporarily bind to oxygen molecules (O2) in the lungs or gills and release them throughout the body. Oxygen can easily diffuse through the red blood cell's cell membrane. Hemoglobin in the red blood cells also carries some of the waste product carbon dioxide back from the tissues; most waste carbon dioxide, however, is transported back to the pulmonary capillaries of the lungs as bicarbonate (HCO3−) dissolved in the blood plasma. Myoglobin, a compound related to hemoglobin, acts to store oxygen in muscle cells.[12]
+
+The color of red blood cells is due to the heme group of hemoglobin. The blood plasma alone is straw-colored, but the red blood cells change color depending on the state of the hemoglobin: when combined with oxygen the resulting oxyhemoglobin is scarlet, and when oxygen has been released the resulting deoxyhemoglobin is of a dark red burgundy color. However, blood can appear bluish when seen through the vessel wall and skin.[13] Pulse oximetry takes advantage of the hemoglobin color change to directly measure the arterial blood oxygen saturation using colorimetric techniques. Hemoglobin also has a very high affinity for carbon monoxide, forming carboxyhemoglobin which is a very bright red in color. Flushed, confused patients with a saturation reading of 100% on pulse oximetry are sometimes found to be suffering from carbon monoxide poisoning.
+
+Having oxygen-carrying proteins inside specialized cells (as opposed to oxygen carriers being dissolved in body fluid) was an important step in the evolution of vertebrates as it allows for less viscous blood, higher concentrations of oxygen, and better diffusion of oxygen from the blood to the tissues. The size of red blood cells varies widely among vertebrate species; red blood cell width is on average about 25% larger than capillary diameter, and it has been hypothesized that this improves the oxygen transfer from red blood cells to tissues.[14]
+
+The red blood cells of mammals are typically shaped as biconcave disks: flattened and depressed in the center, with a dumbbell-shaped cross section, and a torus-shaped rim on the edge of the disk. This shape allows for a high surface-area-to-volume (SA/V) ratio to facilitate diffusion of gases.[15] However, there are some exceptions concerning shape in the artiodactyl order (even-toed ungulates including cattle, deer, and their relatives), which displays a wide variety of bizarre red blood cell morphologies: small and highly ovaloid cells in llamas and camels (family Camelidae), tiny spherical cells in mouse deer (family Tragulidae), and cells which assume fusiform, lanceolate, crescentic, and irregularly polygonal and other angular forms in red deer and wapiti (family Cervidae). Members of this order have clearly evolved a mode of red blood cell development substantially different from the mammalian norm.[9][16] Overall, mammalian red blood cells are remarkably flexible and deformable so as to squeeze through tiny capillaries, as well as to maximize their apposing surface by assuming a cigar shape, where they efficiently release their oxygen load.[17]
+
+Red blood cells in mammals are unique amongst vertebrates as they do not have nuclei when mature. They do have nuclei during early phases of erythropoiesis, but extrude them during development as they mature; this provides more space for hemoglobin.  The red blood cells without nuclei, called reticulocytes, subsequently lose all other cellular organelles such as their mitochondria, Golgi apparatus and endoplasmic reticulum.
+
+The spleen acts as a reservoir of red blood cells, but this effect is somewhat limited in humans. In some other mammals such as dogs and horses, the spleen sequesters large numbers of red blood cells, which are dumped into the blood during times of exertion stress, yielding a higher oxygen transport capacity.
+
+A typical human red blood cell has a disk diameter of approximately 6.2–8.2 µm[18] and a thickness at the thickest point of 2–2.5 µm and a minimum thickness in the centre of 0.8–1 µm, being much smaller than most other human cells. These cells have an average volume of about 90 fL[19] with a surface area of about 136 μm2, and can swell up to a sphere shape containing 150 fL, without membrane distension.
+
+Adult humans have roughly 20–30 trillion red blood cells at any given time, constituting approximately 70% of all cells by number.[20] Women have about 4–5 million red blood cells per microliter (cubic millimeter) of blood and men about 5–6 million; people living at high altitudes with low oxygen tension will have more. Red blood cells are thus much more common than the other blood particles: there are about 4,000–11,000 white blood cells and about 150,000–400,000 platelets per microliter.
+
+Human red blood cells take on average 60 seconds to complete one cycle of circulation.[5][8][21]
+
+The blood's red color is due to the spectral properties of the hemic iron ions in hemoglobin. Each hemoglobin molecule carries four heme groups; hemoglobin constitutes about a third of the total cell volume. Hemoglobin is responsible for the transport of more than 98% of the oxygen in the body (the remaining oxygen is carried dissolved in the blood plasma). The red blood cells of an average adult human male store collectively about 2.5 grams of iron, representing about 65% of the total iron contained in the body.[22][23]
+
+Red blood cells in mammals anucleate when mature, meaning that they lack a cell nucleus. In comparison, the red blood cells of other vertebrates have nuclei; the only known exceptions are salamanders of the genus Batrachoseps and fish of the genus Maurolicus.[24][25]
+
+The elimination of the nucleus in vertebrate red blood cells has been offered as an explanation for the subsequent accumulation of non-coding DNA in the genome.[26] The argument runs as follows: Efficient gas transport requires red blood cells to pass through very narrow capillaries, and this constrains their size. In the absence of nuclear elimination, the accumulation of repeat sequences is constrained by the volume occupied by the nucleus, which increases with genome size.
+
+Nucleated red blood cells in mammals consist of two forms: normoblasts, which are normal erythropoietic precursors to mature red blood cells, and megaloblasts, which are abnormally large precursors that occur in megaloblastic anemias.
+
+Red blood cells are deformable, flexible, are able to adhere to other cells, and are able to interface with immune cells. Their membrane plays many roles in this. These functions are highly dependent on the membrane composition. The red blood cell membrane is composed of 3 layers: the glycocalyx on the exterior, which is rich in carbohydrates; the lipid bilayer which contains many transmembrane proteins, besides its lipidic main constituents; and the membrane skeleton, a structural network of proteins located on the inner surface of the lipid bilayer. Half of the membrane mass in human and most mammalian red blood cells are proteins.  The other half are lipids, namely phospholipids and cholesterol.[27]
+
+The red blood cell membrane comprises a typical lipid bilayer, similar to what can be found in virtually all human cells. Simply put, this lipid bilayer is composed of cholesterol and phospholipids in equal proportions by weight. The lipid composition is important as it defines many physical properties such as membrane permeability and fluidity. Additionally, the activity of many membrane proteins is regulated by interactions with lipids in the bilayer.
+
+Unlike cholesterol, which is evenly distributed between the inner and outer leaflets, the 5 major phospholipids are asymmetrically disposed, as shown below:
+
+Outer monolayer
+
+Inner monolayer
+
+This asymmetric phospholipid distribution among the bilayer is the result of the function of several energy-dependent and energy-independent phospholipid transport proteins. Proteins called “Flippases” move phospholipids from the outer to the inner monolayer, while others called “floppases” do the opposite operation, against a concentration gradient in an energy-dependent manner. Additionally, there are also “scramblase” proteins that move phospholipids in both directions at the same time, down their concentration gradients in an energy-independent manner. There is still considerable debate ongoing regarding the identity of these membrane maintenance proteins in the red cell membrane.
+
+The maintenance of an asymmetric phospholipid distribution in the bilayer (such as an exclusive localization of PS and PIs in the inner monolayer) is critical for the cell integrity and function due to several reasons:
+
+The presence of specialized structures named "lipid rafts" in the red blood cell membrane have been described by recent studies. These are structures enriched in cholesterol and sphingolipids associated with specific membrane proteins, namely flotillins, stomatins (band 7), G-proteins, and β-adrenergic receptors. Lipid rafts that have been implicated in cell signaling events in nonerythroid cells have been shown in erythroid cells to mediate β2-adregenic receptor signaling and increase cAMP levels, and thus regulating entry of malarial parasites into normal red cells.[28][29]
+
+The proteins of the membrane skeleton are responsible for the deformability, flexibility and durability of the red blood cell, enabling it to squeeze through capillaries less than half the diameter of the red blood cell (7–8 μm) and recovering the discoid shape as soon as these cells stop receiving compressive forces, in a similar fashion to an object made of rubber.
+
+There are currently more than 50 known membrane proteins, which can exist in a few hundred up to a million copies per red blood cell. Approximately 25 of these membrane proteins carry the various blood group antigens, such as the A, B and Rh antigens, among many others. These membrane proteins can perform a wide diversity of functions, such as transporting ions and molecules across the red cell membrane, adhesion and interaction with other cells such as endothelial cells, as signaling receptors, as well as other currently unknown functions. The blood types of humans are due to variations in surface glycoproteins of red blood cells. Disorders of the proteins in these membranes are associated with many disorders, such as hereditary spherocytosis, hereditary elliptocytosis, hereditary stomatocytosis, and paroxysmal nocturnal hemoglobinuria.[27][28]
+
+The red blood cell membrane proteins organized according to their function:
+
+Transport
+
+Cell adhesion
+
+Structural role – The following membrane proteins establish linkages with skeletal proteins and may play an important role in regulating cohesion between the lipid bilayer and membrane skeleton, likely enabling the red cell to maintain its favorable membrane surface area by preventing the membrane from collapsing (vesiculating).
+
+[27][28]
+
+The zeta potential is an electrochemical property of cell surfaces that is determined by the net electrical charge of molecules exposed at the surface of cell membranes of the cell. The normal zeta potential of the red blood cell is −15.7 millivolts (mV).[33] Much of this potential appears to be contributed by the exposed sialic acid residues in the membrane: their removal results in zeta potential of −6.06 mV.
+
+When red blood cells undergo shear stress in constricted vessels, they release ATP, which causes the vessel walls to relax and dilate so as to promote normal blood flow.[34]
+
+When their hemoglobin molecules are deoxygenated, red blood cells release S-nitrosothiols, which also act to dilate blood vessels,[35] thus directing more blood to areas of the body depleted of oxygen.
+
+Red blood cells can also synthesize nitric oxide enzymatically, using L-arginine as substrate, as do endothelial cells.[36] Exposure of red blood cells to physiological levels of shear stress activates nitric oxide synthase and export of nitric oxide,[37] which may contribute to the regulation of vascular tonus.
+
+Red blood cells can also produce hydrogen sulfide, a signalling gas that acts to relax vessel walls. It is believed that the cardioprotective effects of garlic are due to red blood cells converting its sulfur compounds into hydrogen sulfide.[38]
+
+Red blood cells also play a part in the body's immune response: when lysed by pathogens such as bacteria, their hemoglobin releases free radicals, which break down the pathogen's cell wall and membrane, killing it.[39][40]
+
+As a result of not containing mitochondria, red blood cells use none of the oxygen they transport; instead they produce the energy carrier ATP by the glycolysis of glucose and lactic acid fermentation on the resulting pyruvate.[41][42] Furthermore, the pentose phosphate pathway plays an important role in red blood cells; see glucose-6-phosphate dehydrogenase deficiency for more information.
+
+As red blood cells contain no nucleus, protein biosynthesis is currently assumed to be absent in these cells.
+
+Because of the lack of nuclei and organelles, mature red blood cells do not contain DNA and cannot synthesize any RNA, and consequently cannot divide and have limited repair capabilities.[43]  The inability to carry out protein synthesis means that no virus can evolve to target mammalian red blood cells.[44]  However, infection with parvoviruses (such as human parvovirus B19) can affect erythroid precursors while they still have DNA, as recognized by the presence of giant pronormoblasts with viral particles and inclusion bodies, thus temporarily depleting the blood of reticulocytes and causing anemia.[45]
+
+Human red blood cells are produced through a process named erythropoiesis, developing from committed stem cells to mature red blood cells in about 7 days. When matured, in a healthy individual these cells live in blood circulation for about 100 to 120 days (and 80 to 90 days in a full term infant).[46] At the end of their lifespan, they are removed from circulation. In many chronic diseases, the lifespan of the red blood cells is reduced.
+
+Erythropoiesis is the process by which new red blood cells are produced; it lasts about 7 days. Through this process red blood cells are continuously produced in the red bone marrow of large bones. (In the embryo, the liver is the main site of red blood cell production.) The production can be stimulated by the hormone erythropoietin (EPO), synthesised by the kidney. Just before and after leaving the bone marrow, the developing cells are known as reticulocytes; these constitute about 1% of circulating red blood cells.
+
+The functional lifetime of a red blood cell is about 100–120 days, during which time the red blood cells are continually moved by the blood flow push (in arteries), pull (in veins) and a combination of the two as they squeeze through microvessels such as capillaries. They are also recycled in the bone marrow.[47]
+
+The aging red blood cell undergoes changes in its plasma membrane, making it susceptible to selective recognition by macrophages and subsequent phagocytosis in the mononuclear phagocyte system (spleen, liver and lymph nodes), thus removing old and defective cells and continually purging the blood. This process is termed eryptosis, red blood cell programmed death.[48] This process normally occurs at the same rate of production by erythropoiesis, balancing the total circulating red blood cell count. Eryptosis is increased in a wide variety of diseases including sepsis, haemolytic uremic syndrome, malaria, sickle cell anemia, beta-thalassemia, glucose-6-phosphate dehydrogenase deficiency, phosphate depletion, iron deficiency and Wilson's disease. Eryptosis can be elicited by osmotic shock, oxidative stress, and energy depletion, as well as by a wide variety of endogenous mediators and xenobiotics. Excessive eryptosis is observed in red blood cells lacking the cGMP-dependent protein kinase type I or the AMP-activated protein kinase AMPK. Inhibitors of eryptosis include erythropoietin, nitric oxide, catecholamines and high concentrations of urea.
+
+Much of the resulting breakdown products are recirculated in the body. The heme constituent of hemoglobin are broken down into iron (Fe3+) and biliverdin. The biliverdin is reduced to bilirubin, which is released into the plasma and recirculated to the liver bound to albumin.  The iron is released into the plasma to be recirculated by a carrier protein called transferrin.  Almost all red blood cells are removed in this manner from the circulation before they are old enough to hemolyze. Hemolyzed hemoglobin is bound to a protein in plasma called haptoglobin, which is not excreted by the kidney.[49]
+
+Blood diseases involving the red blood cells include:
+
+Red blood cells may be given as part of a blood transfusion. Blood may be donated from another person, or stored by the recipient at an earlier date. Donated blood usually requires screening to ensure that donors do not contain risk factors for the presence of blood-borne diseases, or will not suffer themselves by giving blood. Blood is usually collected and tested for common or serious blood-borne diseases including Hepatitis B, Hepatitis C and HIV. The blood type (A, B, AB, or O) or the blood product is identified and matched with the recipient's blood to minimise the likelihood of acute hemolytic transfusion reaction, a type of transfusion reaction. This relates to the presence of antigens on the cell's surface. After this process, the blood is stored, and within a short duration is used. Blood can be given as a whole product or the red blood cells separated as packed red blood cells.
+
+Blood is often transfused when there is known anaemia, active bleeding, or when there is an expectation of serious blood loss, such as prior to an operation. Before blood is given, a small sample of the recipient's blood is tested with the transfusion in a process known as cross-matching.
+
+In 2008 it was reported that human embryonic stem cells had been successfully coaxed into becoming red blood cells in the lab. The difficult step was to induce the cells to eject their nucleus; this was achieved by growing the cells on stromal cells from the bone marrow. It is hoped that these artificial red blood cells can eventually be used for blood transfusions.[51]
+
+Several blood tests involve red blood cells. These include a RBC count (the number of red blood cells per volume of blood), calculation of the hematocrit (percentage of blood volume occupied by red blood cells), and the erythrocyte sedimentation rate. The blood type needs to be determined to prepare for a blood transfusion or an organ transplantation.
+
+Many diseases involving red blood cells are diagnosed with a blood film (or peripheral blood smear), where a thin layer of blood is smeared on a microscope slide. This may reveal abnormalities of red blood cell shape and form. When red blood cells sometimes occur as a stack, flat side next to flat side. This is known as rouleaux formation, and it occurs more often if the levels of certain serum proteins are elevated, as for instance during inflammation.
+
+Red blood cells can be obtained from whole blood by centrifugation, which separates the cells from the blood plasma in a process known as blood fractionation. Packed red blood cells, which are made in this way from whole blood with the plasma removed, are used in transfusion medicine.[52] During plasma donation, the red blood cells are pumped back into the body right away and only the plasma is collected.
+
+Some athletes have tried to improve their performance by blood doping: first about 1 litre of their blood is extracted, then the red blood cells are isolated, frozen and stored, to be reinjected shortly before the competition. (Red blood cells can be conserved for 5 weeks at −79 °C or −110 °F, or over 10 years using cryoprotectants[53]) This practice is hard to detect but may endanger the human cardiovascular system which is not equipped to deal with blood of the resulting higher viscosity. Another method of blood doping involves injection with erythropoietin in order to stimulate production of red blood cells.  Both practices are banned by the World Anti-Doping Agency.
+
+The first person to describe red blood cells was the young Dutch biologist Jan Swammerdam, who had used an early microscope in 1658 to study the blood of a frog.[54] Unaware of this work, Anton van Leeuwenhoek provided another microscopic description in 1674, this time providing a more precise description of red blood cells, even approximating their size, "25,000 times smaller than a fine grain of sand".
+
+In 1901, Karl Landsteiner published his discovery of the three main blood groups—A, B, and C (which he later renamed to O). Landsteiner described the regular patterns in which reactions occurred when serum was mixed with red blood cells, thus identifying compatible and conflicting combinations between these blood groups. A year later Alfred von Decastello and Adriano Sturli, two colleagues of Landsteiner, identified a fourth blood group—AB.
+
+In 1959, by use of X-ray crystallography, Dr. Max Perutz was able to unravel the structure of hemoglobin, the red blood cell protein that carries oxygen.[55]
+
+The oldest intact red blood cells ever discovered were found in Ötzi the Iceman, a natural mummy of a man who died around 3255 BCE. These cells were discovered in May 2012.[56]
+

en/2523.html.txt

@@ -0,0 +1,5 @@
+Hematology, also spelled haematology, is the branch of medicine concerned with the study of the cause, prognosis, treatment, and prevention of diseases related to blood.[1][2] It involves treating diseases that affect the production of blood and its components, such as blood cells, hemoglobin, blood proteins, bone marrow, platelets, blood vessels, spleen, and the mechanism of coagulation. Such diseases might include hemophilia, blood clots (thrombus), other bleeding disorders, and blood cancers such as leukemia, multiple myeloma, and lymphoma. The laboratory work that goes into the study of blood is frequently performed by a medical technologist or medical laboratory scientist.
+
+Physicians specialized in hematology are  known as hematologists or haematologists. Their routine work mainly includes the care and treatment of patients with hematological diseases, although some may also work at the hematology laboratory viewing blood films and bone marrow slides under the microscope, interpreting various hematological test results and blood clotting test results. In some institutions, hematologists also manage the hematology laboratory. Physicians who work in hematology laboratories, and most commonly manage them, are pathologists specialized in the diagnosis of hematological diseases, referred to as hematopathologists or haematopathologists. Hematologists and hematopathologists generally work in conjunction to formulate a diagnosis and deliver the most appropriate therapy if needed. Hematology is a distinct subspecialty of internal medicine, separate from but overlapping with the subspecialty of medical oncology. Hematologists may specialize further or have special interests, for example, in:
+
+Starting hematologists (in the US) complete a four-year medical degree followed by three or four more years in residency or internship programs. After completion, they further expand their knowledge by spending two or three more years learning how to experiment, diagnose, and treat blood disorders. When applying for this career, most job openings look for first-hand practical experience in a recognized training program that provides practice in the following: Cause of abnormalities in formation of blood and other disorders, diagnosis of numerous blood related conditions or cancers using experimentation, and the proper care and treatment of patients in the best manner.

en/2524.html.txt

@@ -0,0 +1,27 @@
+Coordinates: 90°0′0″N 0°0′0″E / 90.00000°N 0.00000°E / 90.00000; 0.00000
+
+The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's North Pole.[1]
+
+Owing to the Earth's axial tilt, winter in the Northern Hemisphere lasts from the December solstice (typically December 21 UTC) to the March equinox (typically March 20 UTC), while summer lasts from the June solstice through to the September equinox (typically on 23 September UTC). The dates vary each year due to the difference between the calendar year and the astronomical year.
+
+Its surface is 60.7% water, compared with 80.9% water in the case of the Southern Hemisphere, and it contains 67.3% of Earth's land.[2]
+
+The Arctic is a region around the North Pole (90° latitude). Its climate is characterized by cold winters and cool summers. Precipitation mostly comes in the form of snow. Areas inside the Arctic Circle (66°34′ latitude) experience some days in summer when the Sun never sets, and some days during the winter when it never rises. The duration of these phases varies from one day for locations right on the Arctic Circle to several months near the Pole, which is the middle of the Northern Hemisphere.
+
+Between the Arctic Circle and the Tropic of Cancer (23°26′ latitude) lies the Northern temperate zone. The changes in these regions between summer and winter are generally mild, rather than extreme hot or cold. However, a temperate climate can have very unpredictable weather.
+
+Tropical regions (between the Tropic of Cancer and the Equator, 0° latitude) are generally hot all year round and tend to experience a rainy season during the summer months, and a dry season during the winter months.
+
+In the Northern Hemisphere, objects moving across or above the surface of the Earth tend to turn to the right because of the Coriolis effect. As a result, large-scale horizontal flows of air or water tend to form clockwise-turning gyres. These are best seen in ocean circulation patterns in the North Atlantic and North Pacific oceans.[citation needed]
+
+For the same reason, flows of air down toward the northern surface of the Earth tend to spread across the surface in a clockwise pattern. Thus, clockwise air circulation is characteristic of high pressure weather cells in the Northern Hemisphere. Conversely, air rising from the northern surface of the Earth (creating a region of low pressure) tends to draw air toward it in a counter-clockwise pattern. Hurricanes and tropical storms (massive low-pressure systems) spin counter-clockwise in the Northern Hemisphere.[citation needed]
+
+The shadow of a sundial moves clockwise on latitudes north of the subsolar point. During the day on these latitudes, the Sun tends to rise to its maximum at a southerly position. Between the Tropic of Cancer and the Equator, the sun can be seen to the north, directly overhead, or to the south at noon dependent on the time of year. In the Southern Hemisphere the midday Sun is predominantly at north.
+
+When viewed from the Northern Hemisphere, the Moon appears inverted compared to a view from the Southern Hemisphere.[3][4] The North Pole faces away from the galactic center of the Milky Way. This results in the Milky Way being sparser and dimmer in the Northern Hemisphere compared to the Southern Hemisphere, making the Northern Hemisphere more suitable for deep-space observation, as it is not "blinded" by the Milky Way.[citation needed]
+
+The Northern Hemisphere is home to approximately 6.57 billion people which is around 90% of the earth's total human population of 7.3 billion people.[5][6]
+
+Earth's Northern Hemisphere comprises the following regions of continents:
+
+Media related to Northern Hemisphere at Wikimedia Commons

en/2525.html.txt

@@ -0,0 +1,27 @@
+Coordinates: 90°0′0″N 0°0′0″E / 90.00000°N 0.00000°E / 90.00000; 0.00000
+
+The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's North Pole.[1]
+
+Owing to the Earth's axial tilt, winter in the Northern Hemisphere lasts from the December solstice (typically December 21 UTC) to the March equinox (typically March 20 UTC), while summer lasts from the June solstice through to the September equinox (typically on 23 September UTC). The dates vary each year due to the difference between the calendar year and the astronomical year.
+
+Its surface is 60.7% water, compared with 80.9% water in the case of the Southern Hemisphere, and it contains 67.3% of Earth's land.[2]
+
+The Arctic is a region around the North Pole (90° latitude). Its climate is characterized by cold winters and cool summers. Precipitation mostly comes in the form of snow. Areas inside the Arctic Circle (66°34′ latitude) experience some days in summer when the Sun never sets, and some days during the winter when it never rises. The duration of these phases varies from one day for locations right on the Arctic Circle to several months near the Pole, which is the middle of the Northern Hemisphere.
+
+Between the Arctic Circle and the Tropic of Cancer (23°26′ latitude) lies the Northern temperate zone. The changes in these regions between summer and winter are generally mild, rather than extreme hot or cold. However, a temperate climate can have very unpredictable weather.
+
+Tropical regions (between the Tropic of Cancer and the Equator, 0° latitude) are generally hot all year round and tend to experience a rainy season during the summer months, and a dry season during the winter months.
+
+In the Northern Hemisphere, objects moving across or above the surface of the Earth tend to turn to the right because of the Coriolis effect. As a result, large-scale horizontal flows of air or water tend to form clockwise-turning gyres. These are best seen in ocean circulation patterns in the North Atlantic and North Pacific oceans.[citation needed]
+
+For the same reason, flows of air down toward the northern surface of the Earth tend to spread across the surface in a clockwise pattern. Thus, clockwise air circulation is characteristic of high pressure weather cells in the Northern Hemisphere. Conversely, air rising from the northern surface of the Earth (creating a region of low pressure) tends to draw air toward it in a counter-clockwise pattern. Hurricanes and tropical storms (massive low-pressure systems) spin counter-clockwise in the Northern Hemisphere.[citation needed]
+
+The shadow of a sundial moves clockwise on latitudes north of the subsolar point. During the day on these latitudes, the Sun tends to rise to its maximum at a southerly position. Between the Tropic of Cancer and the Equator, the sun can be seen to the north, directly overhead, or to the south at noon dependent on the time of year. In the Southern Hemisphere the midday Sun is predominantly at north.
+
+When viewed from the Northern Hemisphere, the Moon appears inverted compared to a view from the Southern Hemisphere.[3][4] The North Pole faces away from the galactic center of the Milky Way. This results in the Milky Way being sparser and dimmer in the Northern Hemisphere compared to the Southern Hemisphere, making the Northern Hemisphere more suitable for deep-space observation, as it is not "blinded" by the Milky Way.[citation needed]
+
+The Northern Hemisphere is home to approximately 6.57 billion people which is around 90% of the earth's total human population of 7.3 billion people.[5][6]
+
+Earth's Northern Hemisphere comprises the following regions of continents:
+
+Media related to Northern Hemisphere at Wikimedia Commons

en/2526.html.txt

@@ -0,0 +1,25 @@
+The Southern Hemisphere is the half of Earth that is south of the Equator. It contains all or parts of five continents[1] (Antarctica, Australia, about 90% of South America, one third of Africa, and several islands off the continental mainland of Asia), four oceans (Indian, South Atlantic, Southern, and South Pacific) and most of the Pacific Islands in Oceania. Its surface is 80.9% water, compared with 60.7% water in the case of the Northern Hemisphere, and it contains 32.7% of Earth's land.[2]
+
+Owing to the tilt of Earth's rotation relative to the Sun and the ecliptic plane, summer is from December to March and winter is from June to September. September 22 or 23 is the vernal equinox and March 20 or 21 is the autumnal equinox. The South Pole is in the center of the southern hemispherical region.
+
+Southern Hemisphere climates tend to be slightly milder than those at similar latitudes in the Northern Hemisphere, except in the Antarctic which is colder than the Arctic. This is because the Southern Hemisphere has significantly more ocean and much less land; water heats up and cools down more slowly than land.[3] The differences are also attributed to oceanic heat transfer  and differing extents of greenhouse trapping.[4]
+
+In the Southern Hemisphere, the sun passes from east to west through the north, although north of the Tropic of Capricorn the mean sun can be directly overhead or due north at midday. The Sun rotating through the north causes an apparent right-left trajectory through the sky unlike the left-right motion of the Sun when seen from the Northern Hemisphere as it passes through the southern sky. Sun-cast shadows turn anticlockwise throughout the day and sundials have the hours increasing in the anticlockwise direction. During solar eclipses viewed from a point to the south of the Tropic of Capricorn, the Moon moves from left to right on the disc of the Sun (see, for example, photos with timings of the solar eclipse of November 13, 2012), while viewed from a point to the north of the Tropic of Cancer (i.e., in the Northern Hemisphere), the Moon moves from right to left during solar eclipses.
+
+Cyclones and tropical storms spin clockwise in the Southern Hemisphere (as opposed to anticlockwise in the Northern Hemisphere) due to the Coriolis effect.[5]
+
+The southern temperate zone, a subsection of the Southern Hemisphere, is nearly all oceanic. This zone includes the southern tip of Uruguay and South Africa; the southern half of Chile and Argentina; parts of Australia, going south from Adelaide, and all of New Zealand.
+
+The Sagittarius constellation that includes the galactic centre is a southern constellation as well as both Magellanic Clouds. This, combined with clearer skies, makes for excellent viewing of the night sky from the Southern Hemisphere with brighter and more numerous stars.
+
+Forests in the Southern Hemisphere have special features which set them apart from those in the Northern Hemisphere. Both Chile and Australia share, for example, unique beech species or Nothofagus, and New Zealand has members of the closely related genera Lophozonia and Fuscospora. The eucalyptus is native to Australia but is now also planted in Southern Africa and Latin America for pulp production, and increasingly, biofuel uses.
+
+Around 800 million humans live in the Southern Hemisphere, representing only 10–12% of the total global human population of 7.3 billion.[6][7] Of those 800 million people, more than 200 million live in Brazil, the largest country by land area in the Southern Hemisphere, while 145 million live on the island of Java, the most populous island in the world. The most populous nation in the Southern Hemisphere is Indonesia, with 267 million people (roughly 30 million of whom live north of the Equator on the northern portions of the islands of Sumatra, Borneo, and Sulawesi, while the rest of the population lives in the Southern Hemisphere). Portuguese is the most spoken language in the Southern Hemisphere,[8] followed by Spanish and Javanese.
+
+The largest metropolitan areas in the Southern Hemisphere are Jakarta (32 million people), São Paulo (22 million people), Buenos Aires (16 million people), Rio de Janeiro (12 million people), Kinshasa (11 million people), Lima (10 million), Johannesburg (10 million), Santiago (7 million) and Sydney (5 million). The most important financial and commercial centers in the Southern Hemisphere are São Paulo, where the Bovespa Index is headquartered, along with Sydney, home to the Australian Securities Exchange, Johannesburg, home to the Johannesburg Stock Exchange, and Buenos Aires, headquarters of the Buenos Aires Stock Exchange, the oldest stock market in the Southern Hemisphere.
+
+Among the most developed nations in the Southern Hemisphere is Australia, with a nominal GDP per capita of US$53,825 and a human development index (HDI) of 0.938, the sixth-highest in the world as of the 2019 report. New Zealand is also well developed, with a nominal GDP per capita of US$41,616 and an HDI of 0.921, putting it at number 14 in the world in 2019. The least developed nations in the Southern Hemisphere cluster in Africa and Oceania, with Mozambique and Burundi at the lowest ends of the HDI, at 0.446 (number 180 in the world) and 0.423 (number 185 in the world), respectively. The nominal GDPs per capita of these two countries do not go above US$550, a tiny fraction of the incomes enjoyed by Australians and New Zealanders.
+
+The most widespread religions in the Southern Hemisphere are Christianity in South America, Southern Africa, Australia and New Zealand, followed by Islam in most of the islands of Indonesia and in parts of southeastern Africa, and Hinduism, which is mostly concentrated on the island of Bali and neighboring islands.
+
+The oldest continuously inhabited city in the Southern Hemisphere is Bogor, in western Java, which was founded in 669 CE. Ancient texts from the Hindu kingdoms prevalent in the area definitively record 669 CE as the year when Bogor was founded. However,  some evidence shows that Zanzibar, an ancient port with around 200,000 inhabitants on the coast of Tanzania, may be older than Bogor. A Greco-Roman text written between 1 and 100 CE, the Periplus of the Erythraean Sea, mentioned the island of Menuthias (Ancient Greek: Μενουθιάς) as a trading port on the east African coast, which is probably the small island of Unguja on which Zanzibar is located. The oldest monumental civilizations in the Southern Hemisphere are the Norte Chico civilization and Casma–Sechin culture from the northern coast of Peru. These civilizations built cities, pyramids, and plazas in the coastal river valleys of northern Peru with some ruins dated back to 3600 BCE.

en/2527.html.txt

@@ -0,0 +1,25 @@
+The Southern Hemisphere is the half of Earth that is south of the Equator. It contains all or parts of five continents[1] (Antarctica, Australia, about 90% of South America, one third of Africa, and several islands off the continental mainland of Asia), four oceans (Indian, South Atlantic, Southern, and South Pacific) and most of the Pacific Islands in Oceania. Its surface is 80.9% water, compared with 60.7% water in the case of the Northern Hemisphere, and it contains 32.7% of Earth's land.[2]
+
+Owing to the tilt of Earth's rotation relative to the Sun and the ecliptic plane, summer is from December to March and winter is from June to September. September 22 or 23 is the vernal equinox and March 20 or 21 is the autumnal equinox. The South Pole is in the center of the southern hemispherical region.
+
+Southern Hemisphere climates tend to be slightly milder than those at similar latitudes in the Northern Hemisphere, except in the Antarctic which is colder than the Arctic. This is because the Southern Hemisphere has significantly more ocean and much less land; water heats up and cools down more slowly than land.[3] The differences are also attributed to oceanic heat transfer  and differing extents of greenhouse trapping.[4]
+
+In the Southern Hemisphere, the sun passes from east to west through the north, although north of the Tropic of Capricorn the mean sun can be directly overhead or due north at midday. The Sun rotating through the north causes an apparent right-left trajectory through the sky unlike the left-right motion of the Sun when seen from the Northern Hemisphere as it passes through the southern sky. Sun-cast shadows turn anticlockwise throughout the day and sundials have the hours increasing in the anticlockwise direction. During solar eclipses viewed from a point to the south of the Tropic of Capricorn, the Moon moves from left to right on the disc of the Sun (see, for example, photos with timings of the solar eclipse of November 13, 2012), while viewed from a point to the north of the Tropic of Cancer (i.e., in the Northern Hemisphere), the Moon moves from right to left during solar eclipses.
+
+Cyclones and tropical storms spin clockwise in the Southern Hemisphere (as opposed to anticlockwise in the Northern Hemisphere) due to the Coriolis effect.[5]
+
+The southern temperate zone, a subsection of the Southern Hemisphere, is nearly all oceanic. This zone includes the southern tip of Uruguay and South Africa; the southern half of Chile and Argentina; parts of Australia, going south from Adelaide, and all of New Zealand.
+
+The Sagittarius constellation that includes the galactic centre is a southern constellation as well as both Magellanic Clouds. This, combined with clearer skies, makes for excellent viewing of the night sky from the Southern Hemisphere with brighter and more numerous stars.
+
+Forests in the Southern Hemisphere have special features which set them apart from those in the Northern Hemisphere. Both Chile and Australia share, for example, unique beech species or Nothofagus, and New Zealand has members of the closely related genera Lophozonia and Fuscospora. The eucalyptus is native to Australia but is now also planted in Southern Africa and Latin America for pulp production, and increasingly, biofuel uses.
+
+Around 800 million humans live in the Southern Hemisphere, representing only 10–12% of the total global human population of 7.3 billion.[6][7] Of those 800 million people, more than 200 million live in Brazil, the largest country by land area in the Southern Hemisphere, while 145 million live on the island of Java, the most populous island in the world. The most populous nation in the Southern Hemisphere is Indonesia, with 267 million people (roughly 30 million of whom live north of the Equator on the northern portions of the islands of Sumatra, Borneo, and Sulawesi, while the rest of the population lives in the Southern Hemisphere). Portuguese is the most spoken language in the Southern Hemisphere,[8] followed by Spanish and Javanese.
+
+The largest metropolitan areas in the Southern Hemisphere are Jakarta (32 million people), São Paulo (22 million people), Buenos Aires (16 million people), Rio de Janeiro (12 million people), Kinshasa (11 million people), Lima (10 million), Johannesburg (10 million), Santiago (7 million) and Sydney (5 million). The most important financial and commercial centers in the Southern Hemisphere are São Paulo, where the Bovespa Index is headquartered, along with Sydney, home to the Australian Securities Exchange, Johannesburg, home to the Johannesburg Stock Exchange, and Buenos Aires, headquarters of the Buenos Aires Stock Exchange, the oldest stock market in the Southern Hemisphere.
+
+Among the most developed nations in the Southern Hemisphere is Australia, with a nominal GDP per capita of US$53,825 and a human development index (HDI) of 0.938, the sixth-highest in the world as of the 2019 report. New Zealand is also well developed, with a nominal GDP per capita of US$41,616 and an HDI of 0.921, putting it at number 14 in the world in 2019. The least developed nations in the Southern Hemisphere cluster in Africa and Oceania, with Mozambique and Burundi at the lowest ends of the HDI, at 0.446 (number 180 in the world) and 0.423 (number 185 in the world), respectively. The nominal GDPs per capita of these two countries do not go above US$550, a tiny fraction of the incomes enjoyed by Australians and New Zealanders.
+
+The most widespread religions in the Southern Hemisphere are Christianity in South America, Southern Africa, Australia and New Zealand, followed by Islam in most of the islands of Indonesia and in parts of southeastern Africa, and Hinduism, which is mostly concentrated on the island of Bali and neighboring islands.
+
+The oldest continuously inhabited city in the Southern Hemisphere is Bogor, in western Java, which was founded in 669 CE. Ancient texts from the Hindu kingdoms prevalent in the area definitively record 669 CE as the year when Bogor was founded. However,  some evidence shows that Zanzibar, an ancient port with around 200,000 inhabitants on the coast of Tanzania, may be older than Bogor. A Greco-Roman text written between 1 and 100 CE, the Periplus of the Erythraean Sea, mentioned the island of Menuthias (Ancient Greek: Μενουθιάς) as a trading port on the east African coast, which is probably the small island of Unguja on which Zanzibar is located. The oldest monumental civilizations in the Southern Hemisphere are the Norte Chico civilization and Casma–Sechin culture from the northern coast of Peru. These civilizations built cities, pyramids, and plazas in the coastal river valleys of northern Peru with some ruins dated back to 3600 BCE.

en/2528.html.txt

@@ -0,0 +1,195 @@
+Hemoglobin (American English) or haemoglobin (British English) (Greek αἷμα (haîma, “blood”) + -in) + -o- + globulin (from Latin globus (“ball, sphere”) + -in) (/ˈhiːməˌɡloʊbɪn, ˈhɛ-, -moʊ-/[1][2][3]), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein in the red blood cells (erythrocytes) of almost all vertebrates[4] (the exception being the fish family Channichthyidae[5]) as well as the tissues of some invertebrates. Hemoglobin in blood carries oxygen from the lungs or gills to the rest of the body (i.e. the tissues).  There it releases the oxygen to permit aerobic respiration to provide energy to power the functions of the organism in the process called metabolism. A healthy individual has 12 to 20 grams of hemoglobin in every 100 ml of blood.
+
+In mammals, the protein makes up about 96% of the red blood cells' dry content (by weight), and around 35% of the total content (including water).[6] Hemoglobin has an oxygen-binding capacity of 1.34 mL O2 per gram,[7] which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood. The mammalian hemoglobin molecule can bind (carry) up to four oxygen molecules.[8]
+
+Hemoglobin is involved in the transport of other gases: It carries some of the body's respiratory carbon dioxide (about 20–25% of the total[9]) as carbaminohemoglobin, in which CO2 is bound to the heme protein. The molecule also carries the important regulatory molecule nitric oxide bound to a globin protein thiol group, releasing it at the same time as oxygen.[10]
+
+Hemoglobin is also found outside red blood cells and their progenitor lines. Other cells that contain hemoglobin include the A9 dopaminergic neurons in the substantia nigra, macrophages, alveolar cells, lungs, retinal pigment epithelium, hepatocytes, mesangial cells in the kidney, endometrial cells, cervical cells and vaginal epithelial cells.[11] In these tissues, hemoglobin has a non-oxygen-carrying function as an antioxidant and a regulator of iron metabolism.[12] Excessive glucose in one's blood can attach to hemoglobin and raise the level of hemoglobin A1c.[13]
+
+Hemoglobin and hemoglobin-like molecules are also found in many invertebrates, fungi, and plants.[14] In these organisms, hemoglobins may carry oxygen, or they may act to transport and regulate other small molecules and ions such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide. A variant of the molecule, called leghemoglobin, is used to scavenge oxygen away from anaerobic systems, such as the nitrogen-fixing nodules of leguminous plants, lest the oxygen poison (deactivate) the system.
+
+Hemoglobinemia is a medical condition in which there is an excess of hemoglobin in the blood plasma. This is an effect of intravascular hemolysis, in which hemoglobin separates from red blood cells, a form of anemia.
+
+In 1825 J. F. Engelhart discovered that the ratio of iron to protein is identical in the hemoglobins of several species.[16][17] From the known atomic mass of iron he calculated the molecular mass of hemoglobin to n × 16000 (n = number of iron atoms per hemoglobin, now known to be 4), the first determination of a protein's molecular mass. This "hasty conclusion" drew a lot of ridicule at the time from scientists who could not believe that any molecule could be that big. Gilbert Smithson Adair confirmed Engelhart's results in 1925 by measuring the osmotic pressure of hemoglobin solutions.[18]
+
+The oxygen-carrying property of hemoglobin was discovered by Hünefeld in 1840.[19] In 1851,[20] German physiologist Otto Funke published a series of articles in which he described growing hemoglobin crystals by successively diluting red blood cells with a solvent such as pure water, alcohol or ether, followed by slow evaporation of the solvent from the resulting protein solution.[21] Hemoglobin's reversible oxygenation was described a few years later by Felix Hoppe-Seyler.[22]
+
+In 1959, Max Perutz determined the molecular structure of hemoglobin by X-ray crystallography.[23][24] This work resulted in his sharing with John Kendrew the 1962 Nobel Prize in Chemistry for their studies of the structures of globular proteins.
+
+The role of hemoglobin in the blood was elucidated by French physiologist Claude Bernard.
+The name hemoglobin is derived from the words heme and globin, reflecting the fact that each subunit of hemoglobin is a globular protein with an embedded heme group. Each heme group contains one iron atom, that can bind one oxygen molecule through [ion]-induced dipole forces. The most common type of hemoglobin in mammal contains four such subunits.
+
+Hemoglobin consists of protein subunits (the "globin" molecules), and these proteins, in turn, are folded chains of a large number of different amino acids called polypeptides. The amino acid sequence of any polypeptide created by a cell is in turn determined by the stretches of DNA called genes. In all proteins, it is the amino acid sequence that determines the protein's chemical properties and function.
+
+There is more than one hemoglobin gene: in humans, hemoglobin A (the main form of hemoglobin present) is coded for by the genes, HBA1, HBA2, and HBB.[25] The amino acid sequences of the globin proteins in hemoglobins usually differ between species. These differences grow with evolutionary distance between species. For example, the most common hemoglobin sequences in humans, bonobos and chimpanzees are completely identical, without even single amino acid difference in either the alpha or the beta globin protein chains.[26] [27] [28]Where as the human & gorilla hemoglobin differ in one aminoacid in both alpha & beta chains. These differences grow larger between less closely related species.
+
+Even within a species, different variants of hemoglobin always exist, although one sequence is usually a "most common" one in each species. Mutations in the genes for the hemoglobin protein in a species result in hemoglobin variants.[29][30] Many of these mutant forms of hemoglobin cause no disease. Some of these mutant forms of hemoglobin, however, cause a group of hereditary diseases termed the hemoglobinopathies. The best known hemoglobinopathy is sickle-cell disease, which was the first human disease whose mechanism was understood at the molecular level. A (mostly) separate set of diseases called thalassemias involves underproduction of normal and sometimes abnormal hemoglobins, through problems and mutations in globin gene regulation. All these diseases produce anemia.[31]
+
+Variations in hemoglobin amino acid sequences, as with other proteins, may be adaptive. For example, hemoglobin has been found to adapt in different ways to high altitudes. Organisms living at high elevations experience lower partial pressures of oxygen compared to those at sea level. This presents a challenge to the organisms that inhabit such environments because hemoglobin, which normally binds oxygen at high partial pressures of oxygen, must be able to bind oxygen when it is present at a lower pressure. Different organisms have adapted to such a challenge. For example, recent studies have suggested genetic variants in deer mice that help explain how deer mice that live in the mountains are able to survive in the thin air that accompanies high altitudes. A researcher from the University of Nebraska-Lincoln found mutations in four different genes that can account for differences between deer mice that live in lowland prairies versus the mountains. After examining wild mice captured from both highlands and lowlands, it was found that: the genes of the two breeds are "virtually identical—except for those that govern the oxygen-carrying capacity of their hemoglobin". "The genetic difference enables highland mice to make more efficient use of their oxygen", since less is available at higher altitudes, such as those in the mountains.[32] Mammoth hemoglobin featured mutations that allowed for oxygen delivery at lower temperatures, thus enabling mammoths to migrate to higher latitudes during the Pleistocene.[33] This was also found in hummingbirds that inhabit the Andes. Hummingbirds already expend a lot of energy and thus have high oxygen demands and yet Andean hummingbirds have been found to thrive in high altitudes. Non-synonymous mutations in the hemoglobin gene of multiple species living at high elevations (Oreotrochilus, A. castelnaudii, C. violifer, P. gigas, and A. viridicuada) have caused the protein to have less of an affinity for inositol hexaphosphate (IHP), a molecule found in birds that has a similar role as 2,3-BPG in humans; this results in the ability to bind oxygen in lower partial pressures.[34]
+
+Birds' unique circulatory lungs also promote efficient use of oxygen at low partial pressures of O2. These two adaptations reinforce each other and account for birds' remarkable high-altitude performance.
+
+Hemoglobin adaptation extends to humans, as well. There is a higher offspring survival rate among Tibetan women with high oxygen saturation genotypes residing at 4,000 m.[35] Natural selection seems to be the main force working on this gene because the mortality rate of offspring is significantly lower for women with higher hemoglobin-oxygen affinity when compared to the mortality rate of offspring from women with low hemoglobin-oxygen affinity. While the exact genotype and mechanism by which this occurs is not yet clear, selection is acting on these women's ability to bind oxygen in low partial pressures, which overall allows them to better sustain crucial metabolic processes.
+
+Hemoglobin (Hb) is synthesized in a complex series of steps. The heme part is synthesized in a series of steps in the mitochondria and the cytosol of immature red blood cells, while the globin protein parts are synthesized by ribosomes in the cytosol.[36] Production of Hb continues in the cell throughout its early development from the proerythroblast to the reticulocyte in the bone marrow. At this point, the nucleus is lost in mammalian red blood cells, but not in birds and many other species. Even after the loss of the nucleus in mammals, residual ribosomal RNA allows further synthesis of Hb until the reticulocyte loses its RNA soon after entering the vasculature (this hemoglobin-synthetic RNA in fact gives the reticulocyte its reticulated appearance and name).[37]
+
+Hemoglobin has a quaternary structure characteristic of many multi-subunit globular proteins.[38] Most of the amino acids in hemoglobin form alpha helices, and these helices are connected by short non-helical segments. Hydrogen bonds stabilize the helical sections inside this protein, causing attractions within the molecule, which then causes each polypeptide chain to fold into a specific shape.[39] Hemoglobin's quaternary structure comes from its four subunits in roughly a tetrahedral arrangement.[38]
+
+In most vertebrates, the hemoglobin molecule is an assembly of four globular protein subunits. Each subunit is composed of a protein chain tightly associated with a non-protein prosthetic heme group. Each protein chain arranges into a set of alpha-helix structural segments connected together in a globin fold arrangement. Such a name is given because this arrangement is the same folding motif used in other heme/globin proteins such as myoglobin.[40][41] This folding pattern contains a pocket that strongly binds the heme group.
+
+A heme group consists of an iron (Fe) ion held in a heterocyclic ring, known as a porphyrin. This porphyrin ring consists of four pyrrole molecules cyclically linked together (by methine bridges) with the iron ion bound in the center.[42] The iron ion, which is the site of oxygen binding, coordinates with the four nitrogen atoms in the center of the ring, which all lie in one plane. The iron is bound strongly (covalently) to the globular protein via the N atoms of the imidazole ring of F8 histidine residue (also known as the proximal histidine) below the porphyrin ring. A sixth position can reversibly bind oxygen by a coordinate covalent bond,[43] completing the octahedral group of six ligands. This reversible bonding with oxygen is why hemoglobin is so useful for transporting oxygen around the body.[44] Oxygen binds in an "end-on bent" geometry where one oxygen atom binds to Fe and the other protrudes at an angle. When oxygen is not bound, a very weakly bonded water molecule fills the site, forming a distorted octahedron.
+
+Even though carbon dioxide is carried by hemoglobin, it does not compete with oxygen for the iron-binding positions but is bound to the amine groups of the protein chains attached to the heme groups.
+
+The iron ion may be either in the ferrous Fe2+ or in the ferric Fe3+ state, but ferrihemoglobin (methemoglobin) (Fe3+) cannot bind oxygen.[45] In binding, oxygen temporarily and reversibly oxidizes (Fe2+) to (Fe3+) while oxygen temporarily turns into the superoxide ion, thus iron must exist in the +2 oxidation state to bind oxygen. If superoxide ion associated to Fe3+ is protonated, the hemoglobin iron will remain oxidized and incapable of binding oxygen. In such cases, the enzyme methemoglobin reductase will be able to eventually reactivate methemoglobin by reducing the iron center.
+
+In adult humans, the most common hemoglobin type is a tetramer (which contains four subunit proteins) called hemoglobin A, consisting of two α and two β subunits non-covalently bound, each made of 141 and 146 amino acid residues, respectively. This is denoted as α2β2. The subunits are structurally similar and about the same size. Each subunit has a molecular weight of about 16,000 daltons,[46] for a total molecular weight of the tetramer of about 64,000 daltons (64,458 g/mol).[47] Thus, 1 g/dL = 0.1551 mmol/L. Hemoglobin A is the most intensively studied of the hemoglobin molecules.
+
+In human infants, the hemoglobin molecule is made up of 2 α chains and 2 γ chains. The gamma chains are gradually replaced by β chains as the infant grows.[48]
+
+The four polypeptide chains are bound to each other by salt bridges, hydrogen bonds, and the hydrophobic effect.
+
+In general, hemoglobin can be saturated with oxygen molecules (oxyhemoglobin), or desaturated with oxygen molecules (deoxyhemoglobin).[49]
+
+Oxyhemoglobin is formed during physiological respiration when oxygen binds to the heme component of the protein hemoglobin in red blood cells. This process occurs in the pulmonary capillaries adjacent to the alveoli of the lungs. The oxygen then travels through the blood stream to be dropped off at cells where it is utilized as a terminal electron acceptor in the production of ATP by the process of oxidative phosphorylation. It does not, however, help to counteract a decrease in blood pH. Ventilation, or breathing, may reverse this condition by removal of carbon dioxide, thus causing a shift up in pH.[50]
+
+Hemoglobin exists in two forms, a taut (tense) form (T) and a relaxed form (R). Various factors such as low pH, high CO2 and high 2,3 BPG at the level of the tissues favor the taut form, which has low oxygen affinity and releases oxygen in the tissues. Conversely, a high pH, low CO2, or low 2,3 BPG favors the relaxed form, which can better bind oxygen.[51] The partial pressure of the system also affects O2 affinity where, at high partial pressures of oxygen (such as those present in the alveoli), the relaxed (high affinity, R) state is favoured. Inversely, at low partial pressures (such as those present in respiring tissues), the (low affinity, T) tense state is favoured.[52] Additionally, the binding of oxygen to the iron(II) heme pulls the iron into the plane of the porphyrin ring, causing a slight conformational shift. The shift encourages oxygen to bind to the three remaining heme units within hemoglobin (thus, oxygen binding is cooperative).
+
+Deoxygenated hemoglobin is the form of hemoglobin without the bound oxygen. The absorption spectra of oxyhemoglobin and deoxyhemoglobin differ. The oxyhemoglobin has significantly lower absorption of the 660 nm wavelength than deoxyhemoglobin, while at 940 nm its absorption is slightly higher. This difference is used for the measurement of the amount of oxygen in a patient's blood by an instrument called a pulse oximeter. This difference also accounts for the presentation of cyanosis, the blue to purplish color that tissues develop during hypoxia.[53]
+
+Deoxygenated hemoglobin is paramagnetic; it is weakly attracted to magnetic fields.[54][55] In contrast, oxygenated hemoglobin exhibits diamagnetism, a weak repulsion from a magnetic field.[55]
+
+Scientists agree that the event that separated myoglobin from hemoglobin occurred after lampreys diverged from jawed vertebrates.[56] This separation of myoglobin and hemoglobin allowed for the different functions of the two molecules to arise and develop: myoglobin has more to do with oxygen storage while hemoglobin is tasked with oxygen transport.[57] The α- and β-like globin genes encode the individual subunits of the protein.[25] The predecessors of these genes arose through another duplication event also after the gnathosome common ancestor derived from jawless fish, approximately 450–500 million years ago.[56] The development of α and β genes created the potential for hemoglobin to be composed of multiple subunits, a physical composition central to hemoglobin's ability to transport oxygen. Having multiple subunits contributes to hemoglobin's ability to bind oxygen cooperatively as well as be regulated allosterically.[57] Subsequently, the α gene also underwent a duplication event to form the HBA1 and HBA2 genes.[58] These further duplications and divergences have created a diverse range of α- and β-like globin genes that are regulated so that certain forms occur at different stages of development.[57]
+
+Most ice fish of the family Channichthyidae have lost their hemoglobin genes as an adaptation to cold water.[5]
+
+Assigning oxygenated hemoglobin's oxidation state is difficult because oxyhemoglobin (Hb-O2), by experimental measurement, is diamagnetic (no net unpaired electrons), yet the lowest-energy (ground-state) electron configurations in both oxygen and iron are paramagnetic (suggesting at least one unpaired electron in the complex). The lowest-energy form of oxygen, and the lowest energy forms of the relevant oxidation states of iron, are these:
+
+All of these structures are paramagnetic (have unpaired electrons), not diamagnetic. Thus, a non-intuitive (e.g., a higher-energy for at least one species) distribution of electrons in the combination of iron and oxygen must exist, in order to explain the observed diamagnetism and no unpaired electrons.
+
+The two logical possibilities to produce diamagnetic (no net spin) Hb-O2 are:
+
+Another possible model in which low-spin Fe4+ binds to peroxide, O22−, can be ruled out by itself, because the iron is paramagnetic (although the peroxide ion is diamagnetic). Here, the iron has been oxidized by two electrons, and the oxygen reduced by two electrons.
+
+Direct experimental data:
+
+Thus, the nearest formal oxidation state of iron in Hb-O2 is the +3 state, with oxygen in the −1 state (as superoxide .O2−). The diamagnetism in this configuration arises from the single unpaired electron on superoxide aligning antiferromagnetically with the single unpaired electron on iron (in a low-spin d5 state), to give no net spin to the entire configuration, in accordance with diamagnetic oxyhemoglobin from experiment.[62][63]
+
+The second choice of the logical possibilities above for diamagnetic oxyhemoglobin being found correct by experiment, is not surprising: singlet oxygen (possibility #1) is an unrealistically high energy state.  Model 3 leads to unfavorable separation of charge (and does not agree with the magnetic data), although it could make a minor contribution as a resonance form. Iron's shift to a higher oxidation state in Hb-O2 decreases the atom's size, and allows it into the plane of the porphyrin ring, pulling on the coordinated histidine residue and initiating the allosteric changes seen in the globulins.
+
+Early postulates by bio-inorganic chemists claimed that possibility #1 (above) was correct and that iron should exist in oxidation state II. This conclusion seemed likely, since the iron oxidation state III as methemoglobin, when not accompanied by superoxide .O2− to "hold" the oxidation electron, was known to render hemoglobin incapable of binding normal triplet O2 as it occurs in the air. It was thus assumed that iron remained as Fe(II) when oxygen gas was bound in the lungs. The iron chemistry in this previous classical model was elegant, but the required presence of the diamagnetic, high-energy, singlet oxygen molecule was never explained. It was classically argued that the binding of an oxygen molecule placed high-spin iron(II) in an octahedral field of strong-field ligands; this change in field would increase the crystal field splitting energy, causing iron's electrons to pair into the low-spin configuration, which would be diamagnetic in Fe(II). This forced low-spin pairing is indeed thought to happen in iron when oxygen binds, but is not enough to explain iron's change in size. Extraction of an additional electron from iron by oxygen is required to explain both iron's smaller size and observed increased oxidation state, and oxygen's weaker bond.
+
+The assignment of a whole-number oxidation state is a formalism, as the covalent bonds are not required to have perfect bond orders involving whole electron transfer. Thus, all three models for paramagnetic Hb-O2 may contribute to some small degree (by resonance) to the actual electronic configuration of Hb-O2. However, the model of iron in Hb-O2 being Fe(III) is more correct than the classical idea that it remains Fe(II).
+
+When oxygen binds to the iron complex, it causes the iron atom to move back toward the center of the plane of the porphyrin ring (see moving diagram). At the same time, the imidazole side-chain of the histidine residue interacting at the other pole of the iron is pulled toward the porphyrin ring. This interaction forces the plane of the ring sideways toward the outside of the tetramer, and also induces a strain in the protein helix containing the histidine as it moves nearer to the iron atom. This strain is transmitted to the remaining three monomers in the tetramer, where it induces a similar conformational change in the other heme sites such that binding of oxygen to these sites becomes easier.
+
+As oxygen binds to one monomer of hemoglobin, the tetramer's conformation shifts from the T (tense) state to the R (relaxed) state. This shift promotes the binding of oxygen to the remaining three monomer's heme groups, thus saturating the hemoglobin molecule with oxygen.[64]
+
+In the tetrameric form of normal adult hemoglobin, the binding of oxygen is, thus, a cooperative process. The binding affinity of hemoglobin for oxygen is increased by the oxygen saturation of the molecule, with the first molecules of oxygen bound influencing the shape of the binding sites for the next ones, in a way favorable for binding. This positive cooperative binding is achieved through steric conformational changes of the hemoglobin protein complex as discussed above; i.e., when one subunit protein in hemoglobin becomes oxygenated, a conformational or structural change in the whole complex is initiated, causing the other subunits to gain an increased affinity for oxygen. As a consequence, the oxygen binding curve of hemoglobin is sigmoidal, or S-shaped, as opposed to the normal hyperbolic curve associated with noncooperative binding.
+
+The dynamic mechanism of the cooperativity in hemoglobin and its relation with the low-frequency resonance has been discussed.[65]
+
+Besides the oxygen ligand, which binds to hemoglobin in a cooperative manner, hemoglobin ligands also include competitive inhibitors such as carbon monoxide (CO) and allosteric ligands such as carbon dioxide (CO2) and nitric oxide (NO). The carbon dioxide is bound to amino groups of the globin proteins to form carbaminohemoglobin; this mechanism is thought to account for about 10% of carbon dioxide transport in mammals. Nitric oxide can also be transported by hemoglobin; it is bound to specific thiol groups in the globin protein to form an S-nitrosothiol, which dissociates into free nitric oxide and thiol again, as the hemoglobin releases oxygen from its heme site. This nitric oxide transport to peripheral tissues is hypothesized to assist oxygen transport in tissues, by releasing vasodilatory nitric oxide to tissues in which oxygen levels are low.[66]
+
+The binding of oxygen is affected by molecules such as carbon monoxide (for example, from tobacco smoking, exhaust gas, and incomplete combustion in furnaces). CO competes with oxygen at the heme binding site. Hemoglobin's binding affinity for CO is 250 times greater than its affinity for oxygen,[67][68] meaning that small amounts of CO dramatically reduce hemoglobin's ability to deliver oxygen to the target tissue.[69] Since carbon monoxide is a colorless, odorless and tasteless gas, and poses a potentially fatal threat, carbon monoxide detectors have become commercially available to warn of dangerous levels in residences. When hemoglobin combines with CO, it forms a very bright red compound called carboxyhemoglobin, which may cause the skin of CO poisoning victims to appear pink in death, instead of white or blue. When inspired air contains CO levels as low as 0.02%, headache and nausea occur; if the CO concentration is increased to 0.1%, unconsciousness will follow. In heavy smokers, up to 20% of the oxygen-active sites can be blocked by CO.
+
+In similar fashion, hemoglobin also has competitive binding affinity for cyanide (CN−), sulfur monoxide (SO), and sulfide (S2−), including hydrogen sulfide (H2S). All of these bind to iron in heme without changing its oxidation state, but they nevertheless inhibit oxygen-binding, causing grave toxicity.
+
+The iron atom in the heme group must initially be in the ferrous (Fe2+) oxidation state to support oxygen and other gases' binding and transport (it temporarily switches to ferric during the time oxygen is bound, as explained above). Initial oxidation to the ferric (Fe3+) state without oxygen converts hemoglobin into "hemiglobin" or methemoglobin, which cannot bind oxygen. Hemoglobin in normal red blood cells is protected by a reduction system to keep this from happening. Nitric oxide is capable of converting a small fraction of hemoglobin to methemoglobin in red blood cells. The latter reaction is a remnant activity of the more ancient nitric oxide dioxygenase function of globins.
+
+Carbon dioxide occupies a different binding site on the hemoglobin. Carbon dioxide is more readily dissolved in deoxygenated blood, facilitating its removal from the body after the oxygen has been released to tissues undergoing metabolism. This increased affinity for carbon dioxide by the venous blood is known as the Haldane effect. Through the enzyme carbonic anhydrase, carbon dioxide reacts with water to give carbonic acid, which decomposes into bicarbonate and protons:
+
+Hence, blood with high carbon dioxide levels is also lower in pH (more acidic). Hemoglobin can bind protons and carbon dioxide, which causes a conformational change in the protein and facilitates the release of oxygen. Protons bind at various places on the protein, while carbon dioxide binds at the α-amino group.[70] Carbon dioxide binds to hemoglobin and forms carbaminohemoglobin.[71] This decrease in hemoglobin's affinity for oxygen by the binding of carbon dioxide and acid is known as the Bohr effect. The Bohr effect favors the T state rather than the R state. (shifts the O2-saturation curve to the right). Conversely, when the carbon dioxide levels in the blood decrease (i.e., in the lung capillaries), carbon dioxide and protons are released from hemoglobin, increasing the oxygen affinity of the protein. A reduction in the total binding capacity of hemoglobin to oxygen (i.e. shifting the curve down, not just to the right) due to reduced pH is called the root effect. This is seen in bony fish.
+
+It is necessary for hemoglobin to release the oxygen that it binds; if not, there is no point in binding it. The sigmoidal curve of hemoglobin makes it efficient in binding (taking up O2 in lungs), and efficient in unloading (unloading O2 in tissues).[72]
+
+In people acclimated to high altitudes, the concentration of 2,3-Bisphosphoglycerate (2,3-BPG) in the blood is increased, which allows these individuals to deliver a larger amount of oxygen to tissues under conditions of lower oxygen tension. This phenomenon, where molecule Y affects the binding of molecule X to a transport molecule Z, is called a heterotropic allosteric effect. Hemoglobin in organisms at high altitudes has also adapted such that it has less of an affinity for 2,3-BPG and so the protein will be shifted more towards its R state. In its R state, hemoglobin will bind oxygen more readily, thus allowing organisms to perform the necessary metabolic processes when oxygen is present at low partial pressures.[73]
+
+Animals other than humans use different molecules to bind to hemoglobin and change its O2 affinity under unfavorable conditions. Fish use both ATP and GTP. These bind to a phosphate "pocket" on the fish hemoglobin molecule, which stabilizes the tense state and therefore decreases oxygen affinity.[74] GTP reduces hemoglobin oxygen affinity much more than ATP, which is thought to be due to an extra hydrogen bond formed that further stabilizes the tense state.[75] Under hypoxic conditions, the concentration of both ATP and GTP is reduced in fish red blood cells to increase oxygen affinity.[76]
+
+A variant hemoglobin, called fetal hemoglobin (HbF, α2γ2), is found in the developing fetus, and binds oxygen with greater affinity than adult hemoglobin. This means that the oxygen binding curve for fetal hemoglobin is left-shifted (i.e., a higher percentage of hemoglobin has oxygen bound to it at lower oxygen tension), in comparison to that of adult hemoglobin. As a result, fetal blood in the placenta is able to take oxygen from maternal blood.
+
+Hemoglobin also carries nitric oxide (NO) in the globin part of the molecule. This improves oxygen delivery in the periphery and contributes to the control of respiration. NO binds reversibly to a specific cysteine residue in globin; the binding depends on the state (R or T) of the hemoglobin. The resulting S-nitrosylated hemoglobin influences various NO-related activities such as the control of vascular resistance, blood pressure and respiration. NO is not released in the cytoplasm of red blood cells but transported out of them by an anion exchanger called AE1.[77]
+
+Hemoglobin variants are a part of the normal embryonic and fetal development. They may also be pathologic mutant forms of hemoglobin in a population, caused by variations in genetics. Some well-known hemoglobin variants, such as sickle-cell anemia, are responsible for diseases and are considered hemoglobinopathies. Other variants cause no detectable pathology, and are thus considered non-pathological variants.[78][79]
+
+In the embryo:
+
+In the fetus:
+
+After birth:
+
+Variant forms that cause disease:
+
+When red blood cells reach the end of their life due to aging or defects, they are removed from the circulation by the phagocytic activity of macrophages in the spleen or the liver or hemolyze within the circulation. Free hemoglobin is then cleared from the circulation via the hemoglobin transporter CD163, which is exclusively expressed on monocytes or macrophages. Within these cells the hemoglobin molecule is broken up, and the iron gets recycled. This process also produces one molecule of carbon monoxide for every molecule of heme degraded.[80] Heme degradation is one of the few natural sources of carbon monoxide in the human body, and is responsible for the normal blood levels of carbon monoxide even in people breathing pure air. The other major final product of heme degradation is bilirubin. Increased levels of this chemical are detected in the blood if red blood cells are being destroyed more rapidly than usual. Improperly degraded hemoglobin protein or hemoglobin that has been released from the blood cells too rapidly can clog small blood vessels, especially the delicate blood filtering vessels of the kidneys, causing kidney damage.
+Iron is removed from heme and salvaged for later use, it is stored as hemosiderin or ferritin in tissues and transported in plasma by beta globulins as transferrins. When the porphyrin ring is broken up, the fragments are normally secreted as a yellow pigment called bilirubin, which is secreted into the intestines as bile. Intestines metabolise bilirubin into urobilinogen. Urobilinogen leaves the body in faeces, in a pigment called stercobilin. Globulin is metabolised into amino acids that are then released into circulation.
+
+Hemoglobin deficiency can be caused either by a decreased amount of hemoglobin molecules, as in anemia, or by decreased ability of each molecule to bind oxygen at the same partial pressure of oxygen. Hemoglobinopathies (genetic defects resulting in abnormal structure of the hemoglobin molecule)[81] may cause both. In any case, hemoglobin deficiency decreases blood oxygen-carrying capacity. Hemoglobin deficiency is, in general, strictly distinguished from hypoxemia, defined as decreased partial pressure of oxygen in blood,[82][83][84][85] although both are causes of hypoxia (insufficient oxygen supply to tissues).
+
+Other common causes of low hemoglobin include loss of blood, nutritional deficiency, bone marrow problems, chemotherapy, kidney failure, or abnormal hemoglobin (such as that of sickle-cell disease).
+
+The ability of each hemoglobin molecule to carry oxygen is normally modified by altered blood pH or CO2, causing an altered oxygen–hemoglobin dissociation curve. However, it can also be pathologically altered in, e.g., carbon monoxide poisoning.
+
+Decrease of hemoglobin, with or without an absolute decrease of red blood cells, leads to symptoms of anemia. Anemia has many different causes, although iron deficiency and its resultant iron deficiency anemia are the most common causes in the Western world. As absence of iron decreases heme synthesis, red blood cells in iron deficiency anemia are hypochromic (lacking the red hemoglobin pigment) and microcytic (smaller than normal). Other anemias are rarer. In hemolysis (accelerated breakdown of red blood cells), associated jaundice is caused by the hemoglobin metabolite bilirubin, and the circulating hemoglobin can cause kidney failure.
+
+Some mutations in the globin chain are associated with the hemoglobinopathies, such as sickle-cell disease and thalassemia. Other mutations, as discussed at the beginning of the article, are benign and are referred to merely as hemoglobin variants.
+
+There is a group of genetic disorders, known as the porphyrias that are characterized by errors in metabolic pathways of heme synthesis. King George III of the United Kingdom was probably the most famous porphyria sufferer.
+
+To a small extent, hemoglobin A slowly combines with glucose at the terminal valine (an alpha aminoacid) of each β chain. The resulting molecule is often referred to as Hb A1c, a glycosylated hemoglobin. The binding of glucose to amino acids in the hemoglobin takes place spontaneously (without the help of an enzyme) in many proteins, and is not known to serve a useful purpose. However, as the concentration of glucose in the blood increases, the percentage of Hb A that turns into Hb A1c increases. In diabetics whose glucose usually runs high, the percent Hb A1c also runs high. Because of the slow rate of Hb A combination with glucose, the Hb A1c percentage reflects a weighted average of blood glucose levels over the lifetime of red cells, which is approximately 120 days.[86] The levels of glycosylated hemoglobin are therefore measured in order to monitor the long-term control of the chronic disease of type 2 diabetes mellitus (T2DM). Poor control of T2DM results in high levels of glycosylated hemoglobin in the red blood cells. The normal reference range is approximately 4.0–5.9%. Though difficult to obtain, values less than 7% are recommended for people with T2DM. Levels greater than 9% are associated with poor control of the glycosylated hemoglobin, and levels greater than 12% are associated with very poor control. Diabetics who keep their glycosylated hemoglobin levels close to 7% have a much better chance of avoiding the complications that may accompany diabetes (than those whose levels are 8% or higher).[87] In addition, increased glycosylation of hemoglobin increases its affinity for oxygen, therefore preventing its release at the tissue and inducing a level of hypoxia in extreme cases.[88]
+
+Elevated levels of hemoglobin are associated with increased numbers or sizes of red blood cells, called polycythemia. This elevation may be caused by congenital heart disease, cor pulmonale, pulmonary fibrosis, too much erythropoietin, or polycythemia vera.[89] High hemoglobin levels may also be caused by exposure to high altitudes, smoking, dehydration (artificially by concentrating Hb), advanced lung disease and certain tumors.[48]
+
+A recent study done in Pondicherry, India, shows its importance in coronary artery disease.[90]
+
+Hemoglobin concentration measurement is among the most commonly performed blood tests, usually as part of a complete blood count. For example, it is typically tested before or after blood donation. Results are reported in g/L, g/dL or mol/L. 1 g/dL equals about 0.6206 mmol/L, although the latter units are not used as often due to uncertainty regarding the polymeric state of the molecule.[91] This conversion factor, using the single globin unit molecular weight of 16,000 Da, is more common for hemoglobin concentration in blood. For MCHC (mean corpuscular hemoglobin concentration) the conversion factor 0.155, which uses the tetramer weight of 64,500 Da, is more common.[92] Normal levels are:
+
+Normal values of hemoglobin in the 1st and 3rd trimesters of pregnant women must be at least 11 g/dL and at least 10.5 g/dL during the 2nd trimester.[95]
+
+Dehydration or hyperhydration can greatly influence measured hemoglobin levels. Albumin can indicate hydration status.
+
+If the concentration is below normal, this is called anemia. Anemias are classified by the size of red blood cells, the cells that contain hemoglobin in vertebrates. The anemia is called "microcytic" if red cells are small, "macrocytic" if they are large, and "normocytic" otherwise.
+
+Hematocrit, the proportion of blood volume occupied by red blood cells, is typically about three times the hemoglobin concentration measured in g/dL. For example, if the hemoglobin is measured at 17 g/dL, that compares with a hematocrit of 51%.[96]
+
+Laboratory hemoglobin test methods require a blood sample (arterial, venous, or capillary) and analysis on hematology analyzer and CO-oximeter. Additionally, a new noninvasive hemoglobin (SpHb) test method called Pulse CO-Oximetry is also available with comparable accuracy to invasive methods.[97]
+
+Concentrations of oxy- and deoxyhemoglobin can be measured continuously, regionally and noninvasively using NIRS.[98][99][100][101][102] NIRS can be used both on the head and on muscles. This technique is often used for research in e.g. elite sports training, ergonomics, rehabilitation, patient monitoring, neonatal research, functional brain monitoring, brain computer interface, urology (bladder contraction), neurology (Neurovascular coupling) and more.
+
+Long-term control of blood sugar concentration can be measured by the concentration of Hb A1c. Measuring it directly would require many samples because blood sugar levels vary widely through the day. Hb A1c is the product of the irreversible reaction of hemoglobin A with glucose. A higher glucose concentration results in more Hb A1c. Because the reaction is slow, the Hb A1c proportion represents glucose level in blood averaged over the half-life of red blood cells, is typically 50–55 days. An Hb A1c proportion of 6.0% or less show good long-term glucose control, while values above 7.0% are elevated. This test is especially useful for diabetics.[103]
+
+The functional magnetic resonance imaging (fMRI) machine uses the signal from deoxyhemoglobin, which is sensitive to magnetic fields since it is paramagnetic. Combined measurement with NIRS shows good correlation with both the oxy- and deoxyhemoglobin signal compared to the BOLD signal.[104]
+
+Hemoglobin can be tracked noninvasively, to build an individual data set tracking the hemoconcentration and hemodilution effects of daily activities for better understanding of sports performance and training. Athletes are often concerned about endurance and intensity of exercise.  The sensor uses light-emitting diodes that emit red and infrared light through the tissue to a light detector, which then sends a signal to a processor to calculate the absorption of light by the hemoglobin protein.[105]
+This sensor is similar to a pulse oximeter, which consists of a small sensing device that clips to the finger.
+
+A variety of oxygen-transport and -binding proteins exist in organisms throughout the animal and plant kingdoms. Organisms including bacteria, protozoans, and fungi all have hemoglobin-like proteins whose known and predicted roles include the reversible binding of gaseous ligands. Since many of these proteins contain globins and the heme moiety (iron in a flat porphyrin support), they are often called hemoglobins, even if their overall tertiary structure is very different from that of vertebrate hemoglobin. In particular, the distinction of "myoglobin" and hemoglobin in lower animals is often impossible, because some of these organisms do not contain muscles. Or, they may have a recognizable separate circulatory system but not one that deals with oxygen transport (for example, many insects and other arthropods). In all these groups, heme/globin-containing molecules (even monomeric globin ones) that deal with gas-binding are referred to as oxyhemoglobins. In addition to dealing with transport and sensing of oxygen, they may also deal with NO, CO2, sulfide compounds, and even O2 scavenging in environments that must be anaerobic.[106] They may even deal with detoxification of chlorinated materials in a way analogous to heme-containing P450 enzymes and peroxidases.
+
+The structure of hemoglobins varies across species. Hemoglobin occurs in all kingdoms of organisms, but not in all organisms. Primitive species such as bacteria, protozoa, algae, and plants often have single-globin hemoglobins. Many nematode worms, molluscs, and crustaceans contain very large multisubunit molecules, much larger than those in vertebrates. In particular, chimeric hemoglobins found in fungi and giant annelids may contain both globin and other types of proteins.[14]
+
+One of the most striking occurrences and uses of hemoglobin in organisms is in the giant tube worm (Riftia pachyptila, also called Vestimentifera), which can reach 2.4 meters length and populates ocean volcanic vents. Instead of a digestive tract, these worms contain a population of bacteria constituting half the organism's weight. The bacteria oxidize H2S from the vent with O2 from the water to produce energy to make food from H2O and CO2. The worms' upper end is a deep-red fan-like structure ("plume"), which extends into the water and absorbs H2S and O2 for the bacteria, and CO2 for use as synthetic raw material similar to photosynthetic plants. The structures are bright red due to their content of several extraordinarily complex hemoglobins that have up to 144 globin chains, each including associated heme structures. These hemoglobins are remarkable for being able to carry oxygen in the presence of sulfide, and even to carry sulfide, without being completely "poisoned" or inhibited by it as hemoglobins in most other species are.[107][108]
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+Some nonerythroid cells (i.e., cells other than the red blood cell line) contain hemoglobin. In the brain, these include the A9 dopaminergic neurons in the substantia nigra, astrocytes in the cerebral cortex and hippocampus, and in all mature oligodendrocytes.[12] It has been suggested that brain hemoglobin in these cells may enable the "storage of oxygen to provide a homeostatic mechanism in anoxic conditions, which is especially important for A9 DA neurons that have an elevated metabolism with a high requirement for energy production".[12] It has been noted further that "A9 dopaminergic neurons may be at particular risk since in addition to their high mitochondrial activity they are under intense oxidative stress caused by the production of hydrogen peroxide via autoxidation and/or monoamine oxidase (MAO)-mediated deamination of dopamine and the subsequent reaction of accessible ferrous iron to generate highly toxic hydroxyl radicals".[12] This may explain the risk of these cells for degeneration in Parkinson's disease.[12] The hemoglobin-derived iron in these cells is not the cause of the post-mortem darkness of these cells (origin of the Latin name, substantia nigra), but rather is due to neuromelanin.
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+Outside the brain, hemoglobin has non-oxygen-carrying functions as an antioxidant and a regulator of iron metabolism in macrophages,[109] alveolar cells,[110] and mesangial cells in the kidney.[111]
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+Historically, an association between the color of blood and rust occurs in the association of the planet Mars, with the Roman god of war, since the planet is an orange-red, which reminded the ancients of blood. Although the color of the planet is due to iron compounds in combination with oxygen in the Martian soil, it is a common misconception that the iron in hemoglobin and its oxides gives blood its red color. The color is actually due to the porphyrin moiety of hemoglobin to which the iron is bound, not the iron itself,[112] although the ligation and redox state of the iron can influence the pi to pi* or n to pi* electronic transitions of the porphyrin and hence its optical characteristics.
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+Artist Julian Voss-Andreae created a sculpture called Heart of Steel (Hemoglobin) in 2005, based on the protein's backbone. The sculpture was made from glass and weathering steel. The intentional rusting of the initially shiny work of art mirrors hemoglobin's fundamental chemical reaction of oxygen binding to iron.[113][114]
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+Montreal artist Nicolas Baier created Lustre (Hémoglobine), a sculpture in stainless steel that shows the structure of the hemoglobin molecule. It is displayed in the atrium of McGill University Health Centre's research centre in Montreal. The sculpture measures about 10 metres × 10 metres × 10 metres.[115][116]
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+Hemoglobin variants:
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+Hemoglobin protein subunits (genes):
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+Hemoglobin compounds:
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+Hardison, Ross C. (2012). "Evolution of Hemoglobin and Its Genes". Cold Spring Harbor Perspectives in Medicine. 2 (12): a011627. doi:10.1101/cshperspect.a011627. ISSN 2157-1422. PMC 3543078. PMID 23209182.
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+Related Questions:
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