Subgiant

Hertzsprung–Russell diagram

Spectral type

Brown dwarfs

White dwarfs

Red dwarfs

Subdwarfs

Main sequence
("dwarfs")

Subgiants

Giants

Bright giants

Supergiants

Hypergiants

absolute
magni-
tude
(M_V)

A subgiant is a star that is slightly brighter than a normal main-sequence (dwarf) star of the same spectral class, but not as bright as true giant stars. Although certain subgiants appear to be simply unusually bright metal-rich hydrogen-fusing stars (in the same way subdwarfs are unusually dim metal-poor hydrogen-fusing stars), they are generally believed to be stars that are ceasing or have already ceased fusing hydrogen in their cores. In stars of roughly a solar mass (M_☉), this causes the core to contract, which increases the star's central temperature enough to move hydrogen fusion into a shell surrounding the core. This swells the star on the way to becoming a true giant. At the start of the subgiant phase (such as a star like Procyon A and Omicron² Cancri) the diameter and brightness have increased, but the star has yet to cool down or change color significantly. Later subgiants that are closer to becoming true giants have larger diameters and lower temperature than stars of similar mass in the main sequence. Overall luminosity changes little during the subgiant stage, as shown by a more or less horizontal evolution off the immediate main sequence; this feature is prominent in Hertzsprung–Russell diagrams of globular clusters, as the lateral extension after the turnoff point but before the "ascent" to red giant status.^[1]

Many subgiants are rich in metals, and commonly host orbiting planets.^[2] Partly for these reasons, and also because the subgiant phase can last up to a few billion years, subgiants are the only type of stars other than main-sequence stars believed capable of hosting life-bearing planets. In Yerkes spectral classification their luminosity class is IV.

Planets

Subgiants with planets include the B-type's Kappa Andromedae b.

References

Star Evolution Formation Pre-main-sequence star Main sequence Horizontal branch Asymptotic giant branch Dredge-up Instability strip Red clump PG1159 star Mira variable Planetary nebula Protoplanetary nebula Luminous red nova Luminous blue variable Wolf–Rayet star Stellar population Supernova Supernova impostor Hypernova Hertzsprung–Russell diagram Color–color diagram Protostars Molecular cloud H II region Bok globule Young stellar object Herbig–Haro object Hayashi track Henyey track Orion T Tauri FU Orionis Herbig Ae/Be Luminosity class Subdwarf Dwarf Blue Red White Yellow Black Brown Subgiant Giant Blue Red Bright giant Supergiant Blue Red Yellow Hypergiant Yellow Blue straggler Spectral classification O B A F G K M Be OB Subdwarf O Subdwarf B Late-type Peculiar Am Ap/Bp Oscillating Barium Carbon CH Extreme helium Lambda Boötis Lead Mercury-manganese S Shell Technetium Remnants White dwarf Black dwarf Helium planet Neutron star Pulsar Magnetar Stellar black hole Compact star Quark Exotic Stellar core: EF Eridani B Theoretical stars Dark-matter star Frozen star Q star Quasi-star Thorne–Żytkow object Iron star Nucleosynthesis Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Carbon burning Neon burning Oxygen burning Alpha process Silicon burning S-process R-process Fusor Nova Remnants Structure Core Convection zone Microturbulence Oscillations Radiation zone Atmosphere Photosphere Starspot Chromosphere Corona Stellar wind Bubble Asteroseismology Eddington luminosity Kelvin–Helmholtz mechanism Properties Designation Dynamics Effective temperature Kinematics Magnetic field Magnitude Absolute Mass Metallicity Rotation UBV color Variability Star systems Binary Contact Common envelope Multiple Accretion disk Star cluster Open cluster Globular cluster Super star cluster Planetary system Earth's Solar System Earth-centric observation of Pole star Circumpolar star Magnitude Apparent Extinction Photographic Color Radial velocity Proper motion Parallax Photometric-standard star Lists Star names Arabic Chinese Extremes Most massive Highest temperature Largest volume Smallest volume Brightest Historical Most luminous Nearest Nearest bright Stars with exoplanets Brown dwarfs White dwarfs Milky Way novae Notable supernovae Supernova candidates Supernova remnants Planetary nebulae Timeline of stellar astronomy Related articles Substellar object Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Supercluster Helioseismology Guest star Constellation Asterism Gravity Intergalactic star Infrared dark cloud Category:Stars · Star portal