* Helium Fusion: After a star exhausts its hydrogen fuel in its core, it begins to fuse helium into heavier elements like carbon. This process is called helium fusion and is what powers red giant stars.

* Expanding Outer Layers: The energy released during helium fusion causes the star's outer layers to expand dramatically, making it a red giant. This expansion also cools the star's surface, giving it its reddish hue.

Important Note: While red giants are the most prominent stars that use helium as fuel, other types of stars, like horizontal branch stars and asymptotic giant branch (AGB) stars, also fuse helium in their core during specific stages of their evolution.