A red giant occurs when a star starts to fuse *1 pointHydrogen in the outer shellsHelium in the outer shellsCarbon in the coreIron in the core

A red giant occurs when a star starts to fuse Helium in the outer shells.

Here's the step by step process:

1. A star spends most of its life in the main sequence phase, where it fuses hydrogen into helium in its core.

2. Once the hydrogen in the core is exhausted, the core contracts and heats up, which causes the outer layers of the star to expand and cool, turning the star into a red giant.

3. In the core of the red giant, the temperature becomes high enough to start fusing helium into heavier elements like carbon and oxygen.

4. The helium fusion occurs in a shell around the core, which is why we say a red giant is a star that starts to fuse helium in the outer shells.