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Brown dwarfs are substellar objects not massive enough to sustain hydrogen-1 fusion reactions in their cores, unlike main-sequence stars. They occupy the mass range between the heaviest gas giants and the lightest stars, with an upper limit around 75 to 80 Jupiter masses (). Brown dwarfs heavier than about are thought to fuse deuterium and those above , fuse lithium as well. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth. The defining differences between a very-low-mass brown dwarf and a gas giant () are debated.〔(【引用サイトリンク】title=A. J. Burgasser - Brown dwarfs: Failed stars, super Jupiters (2008) )〕 One school of thought is based on formation; the other, on the physics of the interior.〔 Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced fusion at some point in their history. Stars are categorized by spectral class, with brown dwarfs being designated as types M, L, T, and Y.〔 Despite their name, brown dwarfs are of different colors.〔 Many brown dwarfs would likely appear magenta to the human eye,〔 or possibly orange/red. Brown dwarfs are not very luminous at visible wavelengths. Some planets are known to orbit brown dwarfs: 2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144b At a distance of about 6.5 light years, the nearest known brown dwarf is Luhman 16, a binary system of brown dwarfs discovered in 2013. DENIS-P J082303.1-491201 b is listed as the most-massive known exoplanet (as of March 2014) in NASA's exoplanet archive, despite having a mass () more than twice the 13-Jupiter-mass cutoff between planets and brown dwarfs.〔()〕 ==History== The objects now called "brown dwarfs" were theorized to exist in the 1960s.〔 They were originally called black dwarfs, a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion. However: a) the term black dwarf was already in use to refer to a cold white dwarf; b) red dwarfs fuse hydrogen, and c) these objects may be luminous at visible wavelengths early in their lives. Because of this, alternate names for these objects were proposed, including planetar and substar. But in 1975, Jill Tarter suggested the term "brown dwarf", using brown as an approximate color.〔〔Planet Quest: The Epic Discovery of Alien Solar Systems, Ken Croswell, Oxford University Press, 1999, ISBN 9780192880833, pages 118–119〕 This has been the term used in astronomy ever since. The term black dwarf still refers to a white dwarf that has cooled to the point that it no longer emits significant light. However, the time required for even the lowest-mass white dwarf to cool to this temperature is calculated to be longer than the current age of the universe; hence such objects are thought not to exist yet. Early theories concerning the nature of the lowest-mass stars and the hydrogen-burning limit suggested that a Population I object with a mass less than 0.07 solar masses () or a Population II object less than would never go through normal stellar evolution and would become a completely degenerate star (Kumar 1963).〔 The first self-consistent calculation of the hydrogen-burning minimum mass confirmed a value between 0.08 and 0.07 solar masses for population I objects (Hayashi and Nakano 1963).〔 The discovery of deuterium-burning down to 0.012 solar masses and the impact of dust formation in the cool outer atmospheres of brown dwarfs in the late 1980s brought these theories into question. However, such objects were hard to find because they emit almost no visible light. Their strongest emissions are in the infrared (IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs. Since then, numerous searches by various methods have sought to find these objects. These methods included multi-color imaging surveys around field stars, imaging surveys for faint companions to main-sequence dwarfs and white dwarfs, surveys of young star clusters, and radial velocity monitoring for close companions. For many years, efforts to discover brown dwarfs were fruitless. In 1988, however, University of California, Los Angeles professors Eric Becklin and Ben Zuckerman identified a faint companion to a star known as GD 165 in an infrared search of white dwarfs. The spectrum of the companion GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf. It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All Sky Survey (2MASS) when Davy Kirkpatrick, of the California Institute of Technology, and others discovered many objects with similar colors and spectral features. Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs". Although the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD 165B would be classified as a brown dwarf or simply a very-low-mass star, because observationally it is very difficult to distinguish between the two. Soon after the discovery of GD 165B, other brown-dwarf candidates were reported. Most failed to live up to their candidacy, however, because the absence of lithium showed them to be stellar objects. True stars burn their lithium within a little over 100 Myr, whereas brown dwarfs (which can, confusingly, have temperatures and luminosities similar to true stars) will not. In other words, the detection of lithium in the atmosphere of a candidate object ensures that, as long as it is older than the relatively young age of 100 Myr, it is a brown dwarf. In 1995, the study of brown dwarfs changed substantially with the discovery of two incontrovertible substellar objects (Teide 1 and Gliese 229B), which were identified by the presence of the 670.8 nm lithium line. The more notable of these objects was the latter, which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in the atmospheres of giant planets and that of Saturn's moon Titan. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs, known as "T dwarfs", for which Gliese 229B is the prototype. The first confirmed brown dwarf was discovered by Spanish astrophysicists Rafael Rebolo (head of team), Maria Rosa Zapatero Osorio, and Eduardo Martín in 1994.〔(【引用サイトリンク】title=Instituto de Astrofísica de Canarias, IAC )〕 They called this object Teide 1 and it was found in the Pleiades open cluster. The discovery article was submitted to ''Nature'' in spring 1995, and published on September 14, 1995.〔 ''Nature'' highlighted "Brown dwarfs discovered, official" in the front page of that issue. Teide 1 was discovered in images collected by the IAC team on January 6, 1994 using the 80 cm telescope (IAC 80) at Teide Observatory and its spectrum was first recorded in December 1994 using the 4.2 m William Herschel Telescope at Roque de los Muchachos Observatory (La Palma). The distance, chemical composition, and age of Teide 1 could be established because of its membership in the young Pleiades star cluster. Using the most advanced stellar and substellar evolution models at that moment, the team estimated for Teide 1 a mass of , which is clearly below the stellar-mass limit. The object became a reference in subsequent young brown dwarf related works. In theory, a brown dwarf below is unable to burn lithium by thermonuclear fusion at any time during its evolution. This fact is one of the lithium test principles to examine the substellar nature in low-luminosity and low-surface-temperature astronomical bodies. High-quality spectral data acquired by the Keck 1 telescope in November 1995 showed that Teide 1 had kept the initial lithium amount of the original molecular cloud from which Pleiades stars formed, proving the lack of thermonuclear fusion in its core. These observations confirmed that Teide 1 is a brown dwarf, as well as the efficiency of the spectroscopic lithium test. For some time, Teide 1 was the smallest known object outside the Solar System that had been identified by direct observation. Since then, over 1,800 brown dwarfs have been identified,〔 even some very close to Earth like Epsilon Indi Ba and Bb, a pair of brown dwarfs gravitationally bound to a Sun-like star around 12 light-years from the Sun, and Luhman 16, a binary system of brown dwarfs about 6.5 light-years away. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「brown dwarf」の詳細全文を読む スポンサード リンク
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