- Type of star which has left the main sequence, ceased burning hydrogen at its core, and swollen to many millions of kilometres in diameter. Red giants produce energy from burning helium in their cores and hydrogen in their outer layers. Stars enter the red giant phase when they exhaust the supplies of hydrogen in their cores. At this point in their history, the star changes fundamentally, as the core shrinks and heats up, allowing helium fusion to start there, and allowing the area around the core to become hot enough for hydrogen burning. At the same time, the amount of heat being produced increases so much that the star swells massively, its surface cools and it turns red, because it emits less heat per unit of its surface area than before.