G Stars Definition

Stars of spectral type G have surface temperatures between 5000 and 6050 K on the main sequence, and between 4600 and 5600 K among giants.

G Stars Description

The coolest G-type supergiants are about 4200 K. G dwarfs range in mass from 0.8 to 1.1 times that of the Sun, the giants and supergiants have generally evolved from more massive stars further up the main sequence. Almost all G stars are dwarfs with compositions similar to the Sun. A few are subdwarfs with lower metal abundances and a small fraction of the G giants have peculiar abundances that give them the name barium stars. The G stars are all slow rotators (unless in a short period binary system). Sun G2 V, Alpha Centauri G2 V, Capella G8 III.

As many as 512 or more stars of spectral type "G" (not including white dwarf stellar remnants) are currently believed to be located within 100 light-years or (or 30.7 parsecs) of Sol -- including Sol itself. Only around 64 are located within 50 light-years (ly), while some 448 are estimated to lie between 50 and 100 light-years -- a volume of space that is seven times as large as the inner sphere within 50 ly of Sol. A comparison of the density of G-type stars between the two volumes of space indicates that the outer spherical shell has around 100 percent of the spatial density of known G-type stars as the inner spherical volume, which suggests that astronomers have identified the great majority of the G-type stars that are actually located within 100 ly of Sol, assuming the same spatial distribution in the Solar neighborhood.

Of those five hundred some G-type stars, astronomers believe that five have evolved out of the main sequence into giant stars, while an additional 78 or so may be subgiants (more on nearby giants and subgiants). As many as 19 G-type stars have been identified as being located in Sol's immediate neighborhood (within 10 parsecs or 32.6 light-years). Due in part to their relative brightness and proximity (as well as abundance), many G-type stars can be seen with the naked eye in Earth's night sky. As of October 2005, astronomers have been able to detect the presence of planets around only 28 G-type stars (including Sol) -- or around 5.5 percent -- of those 511 stars located within 100 light-years of Earth.

Compared to hotter and brighter OBAF type stars, G type stars radiate more light towards the infrared end of the spectrum. For G-type stars, the spectral lines are characterized by the presence of many neutral and ionized metallic lines (iron, calcium II, and sodium) and weak hydrogen lines. Main-sequence G stars have surface temperatures of 5,250 to 5,950 K and around 66 to 150 percent of Sol's luminosity. G-type dwarf stars appear to have between 0.85 to 1.1 Solar-masses, which indicates in theory that these stars may spend from 15 to as few as seven billion years in the main sequence fusing core hydrogen (more from CSIRO Australia).

G-type giant stars, such as Capella, are more luminous than their main sequence counterparts. G-type supergiants can have more than nine solar-masses and a luminosity that can reach more than 10,000 times that of the Sun. At around 900 light-years from Sol, Mebsuta (Epsilon Geminorum) is a relatively close example of a rare G-type supergiant, with seven to nine Solar-masses and a luminosity of around 7,600 times that of the Sun. For supergiant stars, the surface temperature range for spectral type G is host to the "Yellow Evolutionary Void", where supergiant stars often swing between O or B (blue) and K or M (red) but do not stay for very long as a G star because this is an extremely unstable temperature range for supergiants. Rho Cassiopeiae is an example of an extremely rare yellowish hypergiant, that is much larger and brighter than Mebsuta.

G class main sequence stars are sun-sized. They fuse hydrogen slowly - their fuel will last for around ten billion years.
Their surface temperature is around 5,000° K. Their radiation output is centered on the yellow region of the spectrum.
The Sun is a G2V star. Alpha Centauri A is also a G2V star.

After they have exhausted their hydrogen fuel, they begin fusing helium. They then progress through a series of
Red Giant stages. Much of their outer layers are dispersed in a planetary nebula. Finally, all that remains is a hot,
extremely dense spent core - a White Dwarf.

G Stars Variablility/Peculiarity