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 of spectral type G and luminosity class V. 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 V star is in the process of converting hydrogen to helium in its core by means of nuclear fusion. Our Sun is the best known (and most visible) example of a G V 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 V 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.