When a sun-like star runs out of hydrogen fuel in its core, it expands into a red giant.

Here's why:

* Hydrogen Fusion: Stars like our Sun generate energy through nuclear fusion, where hydrogen atoms fuse to form helium. This process creates outward pressure that balances the inward force of gravity.

* Hydrogen Depletion: When the hydrogen in the core is exhausted, fusion stops in that region. The core contracts under its own gravity, causing it to heat up.

* Shell Burning: The star begins fusing hydrogen in a shell surrounding the core. This process releases energy, causing the outer layers of the star to expand dramatically.

* Red Giant Stage: The star becomes much larger and cooler, giving it a reddish appearance – hence the name "red giant."

This is a significant stage in the life of a star, and it ultimately leads to the formation of a white dwarf.