Here's why:

* Main Sequence Stars: Stars like our Sun spend most of their lives fusing hydrogen into helium in their core. This process produces energy, causing the star to shine.

* Red Giant Phase: When a star exhausts the hydrogen fuel in its core, it starts to contract. This contraction heats the core, causing the outer layers to expand and cool, turning the star into a red giant.

* Helium Fusion: The intense heat in the core of a red giant is enough to initiate helium fusion. Helium nuclei fuse to form heavier elements like carbon and oxygen, releasing a new burst of energy.

So, while hydrogen is the main fuel for most of a star's life, it's helium that powers the red giant phase.