【How-to】What is the helium fusion reaction and why does it require much higher temperatures than hydrogen fusion

Low-Mass Stars fuse hydrogen into helium, the proton-proton cycle. … When the core hydrogen is used up and no more nuclear fusion occurs, the star’s outer stellar layers expand and the core shrinks. At this point, the star becomes a Subgiant Star . The star’s outer layers continue to expand and the star brightens.