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 , 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.