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A black dwarf is a white dwarf that has cooled sufficiently that it no longer emits significant heat or light. Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe (13.8 billion years), no black dwarfs are expected to exist in the universe yet, and the temperature of the coolest white dwarfs is one observational limit on the age of the universe. A white dwarf is what remains of a main-sequence star of low or medium mass (below approximately 9 to 10 solar masses ()) after it has either expelled or fused all the elements for which it has sufficient temperature to fuse.〔§3, 〕 What is left is then a dense sphere of electron-degenerate matter that cools slowly by thermal radiation, eventually becoming a black dwarf. If black dwarfs were to exist, they would be extremely difficult to detect, because, by definition, they would emit very little radiation. They would, however, be detectable through their gravitational influence. Various white dwarfs cooled below 3900 K (M0 spectral class) have been found recently by astronomers using MDM Observatory's 2.4-meter telescope. They are estimated to be 11 to 12 billion years old.〔http://www.spacedaily.com/reports/12_Billion_Year_Old_White_Dwarf_Stars_Only_100_Light_Years_Away_999.html〕 Because the far-future evolution of stars depends on physical questions which are poorly understood, such as the nature of dark matter and the possibility and rate of proton decay, it is not known precisely how long it will take white dwarfs to cool to blackness., § IIIE, IVA. Barrow and Tipler estimate that it would take 1015 years for a white dwarf to cool to 5 K;〔Table 10.2, 〕 however, if weakly interacting massive particles exist, it is possible that interactions with these particles will keep some white dwarfs much warmer than this for approximately 1025 years.〔, § IIIE. If protons are not stable, white dwarfs will also be kept warm by energy released from proton decay. For a hypothetical proton lifetime of 1037 years, Adams and Laughlin calculate that proton decay will raise the effective surface temperature of an old one-solar-mass white dwarf to approximately 0.06 K. Although cold, this is thought to be hotter than the cosmic background radiation temperature 1037 years in the future.〔, §IVB. The name ''black dwarf'' has also been applied to substellar objects that do not have sufficient mass, less than approximately 0.08 , to maintain hydrogen-burning nuclear fusion. These objects are now generally called ''brown dwarfs'', a term coined in the 1970s.〔(brown dwarf ), entry in ''The Encyclopedia of Astrobiology, Astronomy, and Spaceflight'', David Darling, accessed online May 24, 2007.〕 Black dwarfs should not be confused with black holes or neutron stars. ==Surface and atmosphere== A black dwarf would have a mainly smooth surface due to the black dwarf's high gravity with very few irregularities (such as mountains). The surface would also be dry with no surface volatiles such as water. The atmosphere of the black dwarf would consist mainly of carbon, and would contain no clouds or weather system due to thinness of the atmosphere. 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「Black dwarf」の詳細全文を読む スポンサード リンク
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