Final answer:
When a star has used up its hydrogen, it enters the red giant phase, during which its core collapses and the outer layers expand. The stars' extended outer layers continue to fuse helium, generating a new flow of energy that causes the star to grow larger.
Step-by-step explanation:
When a star has expanded after using up its hydrogen, it enters a new phase of its life called the red giant phase. During this phase, the star's core collapses under its own weight, releasing heat that flows into a shell of helium just above the core. The helium, which had not been hot enough for fusion earlier, is now heated enough to begin fusion, generating a new flow of energy. This causes the outer layers of the star to expand, and the star reaches enormous proportions.