Figure 11-20Evolution of Stars Off the Main Sequence (a) Hydrogen fusion occurs in the cores of main-sequence stars. (b) When the core is converted into helium, fusion there ceases while the same fusion begins in a shell just outside the core. The star expands into the giant phase. This newly formed helium sinks into the core, which heats up. (c) Eventually, the core reaches 10⁸ K, and core helium fusion begins. This activity causes the core to expand, slowing the hydrogen shell fusion and thereby forcing the outer layers of the star to contract.