Here's a breakdown of what happens:
* Main Sequence: Stars spend the majority of their lives on the main sequence, fusing hydrogen into helium in their cores. This process generates energy, providing outward pressure that balances the inward pull of gravity.
* Hydrogen Depletion: Eventually, the hydrogen fuel in the core is depleted. The core contracts under its own gravity, causing it to heat up.
* Shell Burning: The increased core temperature allows hydrogen fusion to start in a shell surrounding the core. This shell burning produces even more energy, causing the outer layers of the star to expand dramatically.
* Cooling: As the star expands, its surface area increases significantly. This results in a decrease in surface temperature, making the star appear redder and cooler.
So, the contraction of the core triggers the expansion and cooling, ultimately transforming the star into a red giant.
