Here's why:
* Hydrogen Fusion Stops: Without hydrogen, the core can no longer sustain nuclear fusion, the process that provides the outward pressure that balances the inward force of gravity.
* Core Contracts: The core contracts under its own gravity, becoming denser and hotter.
* Shell Burning: This increased heat and pressure causes hydrogen fusion to begin in a shell around the core, where there is still some hydrogen.
* Star Expands: The energy from shell burning causes the outer layers of the star to expand significantly, making the star much larger and cooler. This is what gives the star its characteristic reddish color and earns it the name "red giant."
The exact fate of the star after the red giant phase depends on its initial mass. Smaller stars like our Sun will eventually become white dwarfs, while larger stars will go through further stages like the formation of a planetary nebula and ultimately end their lives as neutron stars or black holes.
