Core-helium fusing stars have similar luminosities but a range of surface temperatures. They form a horizontal branch in the HR diagram. When the core of a main sequence star has converted all of its hydrogen into helium, it contracts and gets hotter. Hydrogen starts to fuse in a shell around the core. The envelope expands, and the star becomes a big, cool, luminous, red giant.
Red giants have extremely dense cores and large, diffuse envelopes.
When the core of the giant becomes sufficiently hot and dense, helium begins to fuse to carbon and oxygen. On the main sequence, more massive stars use up their fuel supply more quickly, so they have shorter main sequence lifetimes. As star clusters age, they lose their most massive, bluest stars. The color of the main sequence turnoff indicates a cluster's age. Giants with core-helium fusion form a clump in the HR diagrams of open clusters or a horizontal branch in the HR diagrams of globular clusters.