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. The star then becomes a Red Giant Star . In about one billion years, the Sun will begin its Red Giant phase.
So how does the star expand and get brighter if it has ceased to fuse hydrogen in its collapsing core? Helium is left in the star’s core and gravity continues to shrink the core and the surrounding layers. These surrounding layers contain hydrogen; the surrounding shell of hydrogen begins to fuse, called Hydrogen Shell Burning . The now red giant star is now larger than out to the orbit of Mars.
Next, helium fusion begins, for which very high temperatures are required.
3 He→ 12 C
This occurs when three helium nuclei are fused into one carbon.
The star begins to dim and shrink in size. Now the cycle reverses ; hydrogen and helium are fused at a tremendous rate. This process takes a few million years. Now the star’s outer layers flow outward from the star, with the star’s core is mostly composed of carbon (from the fusion of the remaining helium into carbon).