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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Bahcall–Wolf cusp ： ウィキペディア英語版 Bahcall–Wolf cusp Bahcall–Wolf cusp refers to a particular distribution of stars around a massive black hole at the center of a galaxy or globular cluster. If the nucleus containing the black hole is sufficiently old, exchange of orbital energy between stars drives their distribution toward a characteristic form, such that the density of stars, ''ρ'', varies with distance from the black hole, ''r'', as :$$ \rho(r) \propto r^ . So far, no clear example of a Bahcall–Wolf cusp has been found in any galaxy or star cluster. This may be due in part to the difficulty of resolving such a feature. ==Distribution of stars around a supermassive black hole== Supermassive black holes reside in galactic nuclei. The total mass of the stars in a nucleus is roughly equal to the mass of the supermassive black hole. In the case of the Milky Way, the mass of the supermassive black hole is about 4 million Solar masses, and the number of stars in the nucleus is about ten million.〔 〕 The stars move around the supermassive black hole in elliptical orbits, similar to the orbits that planets follow around the Sun. The orbital energy of a star is :$$ E = \boldsymbol^2/2 - GM/r where ''v'' is the star's velocity, ''r'' is its distance from the supermassive black hole, and ''M'' is the supermassive black hole's mass. A star's energy remains nearly constant for many orbital periods. But after roughly one relaxation time, most of the stars in the nucleus will have exchanged energy with other stars, causing their orbits to change. Bahcall and Wolf showed that once this has taken place, the distribution of orbital energies has the form :$$ N(E)\, dE = N_0 |E|^ dE, which corresponds to the density ''ρ''=''ρ''0 ''r'' −7/4. The figure shows how the density of stars evolves toward the Bahcall–Wolf form. The fully formed cusp〔The term "cusp" refers to the fact that a graph of density vs. radius has a cuspy appearance if plotted on linear axes, rather than the logarithmic axes used in the figure.〕 extends outward to a distance of roughly one-fifth the supermassive black hole's influence radius. It is believed that relaxation times in the nuclei of small, dense galaxies are short enough for Bahcall–Wolf cusps to form. 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「Bahcall–Wolf cusp」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.