Subgiant star

Article By:

Kaler, James B. Department of Astronomy, College of Liberal Arts and Sciences, University of Illinois, Urbana-Champaign, Illinois.

Last reviewed:January 2021

DOI:https://doi.org/10.1036/1097-8542.663800

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 similar to or a bit greater than 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. The apparently brightest star usually considered a subgiant is Procyon, though it is more a transition between dwarf and subgiant.

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