* Fusion Requires Fuel: Hydrogen fusion is the process where hydrogen atoms combine to form helium, releasing tremendous energy. This process requires hydrogen as fuel.
* Core Depletion: When a star runs out of hydrogen in its core, fusion stops in that region. The core becomes inert.
* Shell Burning: However, the star doesn't simply die. The outward pressure from the core's previous fusion activity causes the star to expand. This expansion brings the outer layers of the star closer to the core, where the temperature and pressure are still high enough for hydrogen to fuse in a shell surrounding the inert core. This is called "shell burning."
So, after the core hydrogen is exhausted, hydrogen fusion continues in a shell around the core, but not within the core itself. This process ultimately leads to the star's evolution into a red giant or, for larger stars, a supernova.
