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A G-type main-sequence star (Spectral type: G-V), often (and imprecisely) called a yellow dwarf, or G dwarf star, is a main-sequence star (luminosity class V) of spectral type G. Such a star has about 0.8 to 1.2 solar masses and surface temperature of between 5,300 and 6,000 K.〔(Empirical bolometric corrections for the main-sequence ), G. M. H. J. Habets and J. R. W. Heintze, ''Astronomy and Astrophysics Supplement'' 46 (November 1981), pp. 193–237.〕, Tables VII, VIII. Like other main-sequence stars, a G-type main-sequence star is in the process of converting the element hydrogen to helium in its core by means of nuclear fusion.〔(Stellar Evolution: Main Sequence to Giant ), class notes, Astronomy 101, Valparaiso University, accessed on line June 19, 2007.〕 The Sun, the star to which the Earth is gravitationally bound in the Solar System, is by far the best known example of a G-type main-sequence star— being by far the closest star to earth with an Apparent magnitude dwarfing all other celestial objects. Each second, the sun fuses approximately 600 million tons of hydrogen to helium, converting about 4 million tons of matter to energy.〔(Why Does The Sun Shine? ), lecture, Barbara Ryden, Astronomy 162, Ohio State University, accessed on line June 19, 2007.〕〔(Sun ), entry at ARICNS, accessed June 19, 2007.〕 Besides the Sun, other well-known examples of G-type main-sequence stars include Alpha Centauri A, Tau Ceti, and 51 Pegasi.〔(Alpha Centauri A ), SIMBAD query result. Accessed on line December 4, 2007.〕〔(Tau Ceti ), SIMBAD query result. Accessed on line December 4, 2007.〕〔(51 Pegasi ), SIMBAD query result. Accessed on line December 4, 2007.〕 The term ''yellow dwarf'' is a misnomer, because G-type stars actually range in color from white, for more luminous types like the Sun, to only very slightly yellow for the less massive and luminous G-type main-sequence stars.〔(What Color Are the Stars? ), Mitchell N. Charity's webpage, accessed November 25, 2007〕 The Sun is in fact white, but appears yellow through Earth's atmosphere due to atmospheric Rayleigh scattering. In addition, although the term "dwarf" is used to contrast yellow main-sequence stars from giant stars, yellow dwarfs like the Sun outshine 90% of the stars in the Milky Way (which are largely much dimmer orange dwarfs, red dwarfs, and white dwarfs, the last being a stellar remnant). A G-type main-sequence star will fuse hydrogen for approximately 10 billion years, until it is exhausted at the center of the star. When this happens, the star expands to many times its previous size and becomes a red giant, such as Aldebaran (or Alpha Tauri).〔SIMBAD, (entry ) for Aldebaran, accessed on line June 19, 2007.〕 Eventually the red giant sheds its outer layers of gas, which become a planetary nebula, while the core rapidly cools and contracts into a compact, dense white dwarf.〔 ==Spectral standard stars== The revised Yerkes Atlas system (Johnson & Morgan 1953)〔(Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas ) H.L. Johnson & W.W. Morgan, 1953, Astrophysical Journal, 117, 313〕 listed 11 G-type dwarf spectral standard stars; however, not all of these have survived to this day as standards. The "anchor points" of the MK spectral classification system among the G-type main-sequence dwarf stars, i.e. those standard stars that have remained unchanged over years, are beta CVn (G0V), the Sun (G2V), Kappa1 Ceti (G5V), 61 Ursae Majoris (G8V).〔(MK ANCHOR POINTS ), Robert F. Garrison〕 Other primary MK standard stars include HD 115043 (G1V) and 16 Cygni B (G3V).〔(The Perkins Catalog of Revised MK Types for the Cooler Stars ), P.C. Keenan & R.C McNeil, "Astrophysical Journal Supplement Series" 71 (October 1989), pp. 245–266.〕 The choices of G4 and G6 dwarf standards have changed slightly over the years among expert classifiers, but often-used examples include 70 Virginis (G4V) and 82 Eridani (G6V). There are not yet any generally agreed upon G7V and G9V standards. 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「G-type main-sequence star」の詳細全文を読む スポンサード リンク
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