G star Any member of a class of yellow-white stars, the spectra of which are characterized by relatively weak hydrogen lines, strong ionized calcium lines (FRAUNHOFER LINES H and K), and, as temperature declines, the beginning of strong neutral calcium and sodium lines (Fraunhofer g and D). MAIN-SEQUENCE dwarf stars range from 5300 K at G9 to 5900 K at G0, with zero-age masses from 0.9 solar mass to just over solar, zero-age luminosities from 0.5 to just above solar, and dwarf lifetimes from 10 billion years to greater than the age of the Galaxy. Giants and supergiants, which, because of lower densities, are about 800 K cooler than the dwarfs, are more difficult to distinguish spectroscopically than in other classes. However, the Wilson-Bappu effect (see KSTAR) can yield luminosities. G SUBDWARFS have weakened metal lines and lower metal abundances. The Sun is a G2 dwarf, allowing a good understanding of G stars.
G stars have deep convection zones, the Sun's occupying its outer third. Although they tend to be slow rotators (unless in binary systems where they are tidally spun up), G stars have active magnetic fields, which produce CHROMOSPHERES and hot CORONAE. The solar magnetic field is amplified within the Sun. Ropes of magnetic flux float upwards and produce SUNSPOTS where they exit and re-enter the Sun. Interacting and collapsing magnetic fields create bright solar FLARES, releasing the magnetically confined corona in CORONAL MASS EJECTIONS. Chromospheres produce EMISSION LINES. Stellar chromospheres can be recognized by emissions set within the calcium H and K absorptions; coronae can be confirmed through X-ray emission. About two-thirds of the G dwarfs exhibit Sun-like activity.
The highest luminosity (and longest period) Cepheids are class G supergiants. Numerous planets have been found orbiting G stars. Bright examples of G stars include the Sun (G2 V), Alpha Centauri-A (G2 V) and Capella (G0 III + G8 III).