Subgiant star

(astronomy) A member of the family of stars whose luminosity is intermediate between giants and the main sequence in the Hertzsprung-Russell diagram; spectral classes G and K are most frequent.

An evolving star of luminosity class IV. Such a star is brighter than the main-sequence dwarfs and fainter than the true giants in its spectral class, lying between the two on the Hertzsprung-Russell diagram. The classic subgiants fall in a small region from class F to K (with effective temperatures ranging from 7000 to 4000 K or 12,000 to 7000°F). In class G they lie at absolute visual magnitude +3 with luminosities about five times the solar luminosity. Classic subgiants have masses around 1.3 times that of the Sun and violate the mass-luminosity relation as too bright for their masses. The concept is extended to the hot part of the Hertzsprung-Russell diagram from class F to O, in which the distinctions between subgiants and neighboring dwarfs and giants are much smaller, only a magnitude or less.

Main-sequence dwarfs run on the fusion of hydrogen to helium in their cores. The subgiant stage begins when the core hydrogen mass fraction drops to around 0.1, and then continues as the fraction goes to zero and the star evolves toward, but not onto, the red-giant branch with a contracting helium core. See also Dwarf star; Giant star; Hertzsprung-Russell diagram; Spectral type; Star; Stellar evolution.