Subgiant
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Subgiant star is a class of stars that are brighter than normal main sequence (dwarf) stars of the same spectral class, 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. 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) 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, a feature prominent in Hertzsprung-Russell diagrams of globular clusters.[1] In Yerkes spectral classification their luminosity class is IV.
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