Evolution of Stars Off the Main Sequence (a) Hydrogen fusion occurs in the core of main-sequence stars. (b) When the core is converted into helium, fusion there ceases and then begins in a shell that surrounds 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 contrac
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