Planetary nebula
The outer layers of the star become much bigger because of the heat of the core, and become much cooler. The star becomes a red giant. The core gets even smaller and hotter. When its reaches 100 million K, helium begins to fuse into carbon and oxygen. When this happens, the core stops shrinking. Helium burning soon forms a core of carbon and oxygen, with both a helium and a hydrogen shell surrounding it.