Here's why:
* Hydrogen Fusion: Stars like our Sun primarily fuse hydrogen into helium in their cores, releasing energy in the process. This outward pressure balances the inward pull of gravity, keeping the star stable.
* Hydrogen Depletion: Once the hydrogen fuel in the core is exhausted, fusion stops. This means the outward pressure weakens, allowing gravity to take over.
* Core Contraction: The core begins to contract and heat up, compressing the remaining hydrogen in a shell surrounding the core.
* Shell Burning: The increased temperature ignites hydrogen fusion in the shell, causing the star to expand dramatically.
* Red Giant Formation: As the star expands, its surface cools down, giving it a reddish hue. This is why it becomes a red giant.
It's important to note that the evolution of a star depends on its initial mass. Large stars have a more complex life cycle, potentially leading to supernova explosions and the formation of neutron stars or black holes.
