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A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. On an Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence. The term "subdwarf" was coined by Gerard Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate white dwarfs".〔Ken Croswell, ''The Alchemy of the Heavens'', (New York: Oxford UP, 1995), p. 87.〕 ==Cool subdwarfs== Like ordinary main-sequence stars, cool subdwarfs (of spectral types G to M) produce their energy from hydrogen fusion. The explanation of their underluminosity lies in their low metallicity: these stars are unenriched in elements heavier than helium. The lower metallicity decreases the opacity of their outer layers and decreases the radiation pressure, resulting in a smaller, hotter star for a given mass.〔James Kaler, ''Stars and their Spectra'', (Cambridge: Cambridge UP, 1989), p. 122.〕 This lower opacity also allows them to emit a higher percentage of ultraviolet light for the same spectral type relative to a Population I star, a feature known as the ultraviolet excess.〔Ken Croswell, ''The Alchemy of the Heavens'', (New York: Oxford UP, 1995), pp. 87–92.〕 Usually members of the Milky Way's halo, they frequently have high space velocities relative to the Sun. Subclasses of cool subdwarfs are as following:〔(Discovery of the Coolest Extreme Subdwarf ), Burgasser, Adam J. & Kirkpatrick, J. Davy, 2006.〕 * cool subdwarf: Example: SSSPM J1930-4311 (sdM7) * extreme subdwarf: Example: APMPM J0559-2903 (esdM7)〔(APMPM J0559-2903: The coolest extreme subdwarf known )〕 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「Subdwarf」の詳細全文を読む スポンサード リンク
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