The Fusion Process and Stellar Stability:

* Hydrogen Fusion: Stars spend the majority of their lives fusing hydrogen into helium in their cores. This process releases enormous energy, creating outward pressure that counteracts the inward pull of gravity. This balance maintains the star's size.

* Core Hydrogen Depletion: When the core hydrogen is exhausted, fusion stops in the core. Without the outward pressure from fusion, gravity begins to dominate.

The Swelling Effect:

1. Core Contraction: The core collapses under its own gravity. This increases the temperature and density in the core.

2. Shell Burning: The increased temperature ignites a new shell of hydrogen fusion around the core. This shell fusion is more intense than the core fusion was, generating even more energy.

3. Expansion: The increased energy output from shell burning pushes the outer layers of the star outwards, causing it to expand significantly. The star becomes a red giant or a red supergiant, depending on its initial mass.

In Summary:

The depletion of core hydrogen leads to a decrease in outward pressure. This allows gravity to pull the core inward, leading to a rise in temperature and density. This triggers shell burning, which produces more energy, causing the star to expand into a red giant or red supergiant.

Important Note: The details of the expansion process vary depending on the star's initial mass. More massive stars experience more dramatic changes and may eventually explode as supernovae.