Stellar nucleosynthesis
Helium fusion requires higher temperature and pressure than hydrogen fusion. It can happen in all but the smallest stars. Helium fusion starts when the core runs out of hydrogen. When hydrogen fusion stops, the core stops making heat and light. Gravity collapses the core, making it smaller. This creates enough pressure and heat for helium fusion. The helium fusion makes the star much bigger. It becomes a red giant or red supergiant.