A subgiant is a type of star with a luminosity (and generally size) in between that of a main sequence star and that of a giant star. Subgiants fuse hydrogen in a shell near their outer layers, as they have depleted all of the hydrogens at their cores. On an H-R diagram, the subgiant branch is visible as a bent stripe slightly above the main sequence.
Low-mass stars do not develop into subgiants, but more massive stars spend varying amounts of time at this stage; generally, the more massive the star, the quicker it passes through the subgiant stage. After this, stars evolve into giants or supergiants.
|Stellar Luminosity Classes|
|-1 · 0 · I · II · III · IV · V · VI · VII|