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Van Maanen 2
Van Maanen 2 (van Maanen's Star) is a white dwarf. It is a dense, compact stellar remnant that is no longer generating energy, having about 68%〔 of the Sun's mass but only 1.1%〔 of the Sun's radius. Out of the white dwarfs known, it is the third closest to the Sun, after Sirius B and Procyon B, in that order, and the closest known solitary white dwarf.〔〔 Discovered in 1917 by Dutch–American astronomer Adriaan van Maanen,〔 Van Maanen 2 was the third white dwarf identified, after 40 Eridani B and Sirius B, and the first that was not a member of a multi-star system.〔
==Observation history==
While searching for a companion to the large-proper-motion star Lalande 1299, in 1917 Dutch–American astronomer Adriaan van Maanen discovered a star with an even larger proper motion located a few arcminutes to the northeast. He estimated the annual proper motion of the latter as 3 arcseconds. This star had been previously recorded on a plate taken November 11, 1896 for the ''Carte du Ciel'' Catalog of Toulouse, and it showed an apparent magnitude of 12.3. The initial spectral classification was type F0.〔
In 1918, American astronomer Frederick Seares obtained a refined visual magnitude of 12.34, but the distance to the star remained unknown.〔 Two years later, van Maanen published a parallax estimate of 0.246″, giving it an absolute magnitude of +14.8. This made it the faintest F-type star known at that time.〔 In 1923, Dutch-American astronomer Willem Luyten published a study of stars with large proper motions in which he identified what he called "van Maanen's star" as one of only three known white dwarfs, a term he coined.〔 These are stars that have an unusually low absolute magnitude for their spectral class, lying well below the main sequence on the Hertzsprung–Russell diagram of stellar temperature vs. luminosity.〔
The high mass density of white dwarfs was demonstrated in 1925 by American astronomer Walter Adams when he measured the gravitational redshift of Sirius B as 21 km/s.〔 In 1926, British astrophysicist Ralph Fowler used the new theory of quantum mechanics to show that these stars are supported by electron gas in a degenerate state.〔〔 British astrophysicist Leon Mestel demonstrated in 1952 that the energy emitted by a white dwarf is the surviving heat from a prior period of nuclear fusion. He showed that nuclear burning no longer occurs within a white dwarf, and calculated the internal temperature of van Maane 2 as . He gave a preliminary age estimate of 1011/''A'' years, where ''A'' is the mean atomic weight of the nuclei in the star.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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