A star that has exhausted the hydrogen at its centre and is evolving into a giant. They are of luminosity class IV. The subgiants we see are usually less massive than the Sun, because more massive stars move very quickly through this stage into giants. Low-mass stars take many billions of years to evolve this far, so low-mass subgiants are very old. A subgiant branch on the Hertzsprung–Russell diagram, linking the main sequence to the giant branch, is therefore found only for old clusters such as globular clusters.