What Happens to Stars around the Size of the Sun?
STAGE 2: Red Giant Phase
When the star uses up its hydrogen fuel in the core, it can no longer hold up its outer layers. They fall inward. The center gets hotter. The hot temperatures cause the outer layers of the star to expand. As a result, the star gets bigger and the surface cools. The star is now a red giant.
While the outer atmosphere is expanding, the core inside collapses to about the size of the Earth.
The temperature again increases, reaching about 100 million degrees. Suddenly helium in the core begins to fuse to carbon and oxygen at a very rapid rate. Within a short time the helium is gone, and gravitational collapse continues.