Here's why:
* Hydrogen Fusion: Main sequence stars primarily fuse hydrogen into helium in their cores. This process generates the outward pressure that balances the inward pull of gravity, keeping the star stable.
* Fuel Depletion: As hydrogen fuel in the core is depleted, the core starts to contract under gravity. This contraction increases the core's temperature and pressure.
* Shell Fusion: The increased temperature and pressure trigger hydrogen fusion in a shell surrounding the core, causing the star to expand dramatically. This expansion cools the outer layers, giving the star a reddish appearance, hence the name "red giant".
* Evolutionary Path: The star's further evolution depends on its mass. Less massive stars will eventually shed their outer layers to become white dwarfs, while more massive stars will undergo further nuclear fusion stages, leading to a supernova explosion or a black hole.
