G-type main-sequence star
A G-type main-sequence star (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., Tables VII, VIII. Like other main-sequence stars, a G-type main-sequence star is in the process of converting hydrogen to helium in its core by means of nuclear fusion. The Sun is the best known (and most visible) example of a G-type main-sequence star. Each second, it fuses approximately 600 million tons of hydrogen to helium, converting about 4 million tons of matter to energy. Besides the Sun, other well-known examples of G-type main-sequence stars include Alpha Centauri A, Tau Ceti, and 51 Pegasi.
The term yellow dwarf is a misnomer, as G stars actually range in color from white, for more luminous types like the Sun, to only slightly yellow for the less luminous GV stars. Our own Sun is in fact white, but appears yellow through the Earth's atmosphere due to 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 Galaxy (which are largely orange dwarfs, red dwarfs, and white dwarfs, the latter being a post-main-sequence star).
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 (Alpha Tauri). Eventually the red giant sheds its outer layers of gas, which become a planetary nebula, while the core cools and contracts into a compact, dense white dwarf.
Spectral Standard Stars 
The revised Yerkes Atlas system (Johnson & Morgan 1953) 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 (G0 V), the Sun (G2 V), Kappa1 Ceti (G5 V), 61 Ursae Majoris (G8 V). Other primary MK standard stars include HD 115043 (G1 V) and 16 Cygni B (G3 V). The choices of G4 and G6 dwarf standards have changed slightly over the years among expert classifiers, but often-used examples include 70 Virginis (G4 V) and 82 Eridani (G6 V). There are not yet any generally agreed upon G7 V and G9 V standards.
See also 
- Brown dwarf
- K V star
- Red dwarf
- Solar twin
- Star count, survey of stars
- Stellar classification, Class G
- 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.
- Stellar Evolution: Main Sequence to Giant, class notes, Astronomy 101, Valparaiso University, accessed on line June 19, 2007.
- 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.
- 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.
- What Color Are the Stars?, Mitchell N. Charity's webpage, accessed November 25, 2007
- SIMBAD, entry for Aldebaran, accessed on line June 19, 2007.
- 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
- MK ANCHOR POINTS, Robert F. Garrison
- 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.