Subgiant star is a class of stars that are brighter than normal main sequence (dwarf) stars, but not as bright as true giant stars. They are believed to be stars that are ceasing or already ceased fusing hydrogen in their cores and starting to swell up to become true giant stars. Subgiant stars have larger diameters and lower temperature than stars of similar mass in the main sequence. In Yerkes spectral classification their luminosity class is IV.