What powers a star as it swells into a red giant after its core hydrogen runs out?
Hydrogen fusing in a shell around the contracting helium core.
- Hydrogen fusing in a shell around the core ✔
- Iron fusing in the core
- Chemical burning of the outer layers
Why: When core hydrogen is used up, the helium core shrinks and heats. That heat ignites hydrogen in a shell just outside the core. The extra energy pushes the outer layers out, so the star grows larger and brighter.
Huh, didn't know that: Most stars actually make more energy per second during shell fusion than they did on the main sequence.
When a star runs low on fuel it doesn't panic. It just comes out of its shell.
References
Answer: Hydrogen fusing in a shell around the core. When core hydrogen is used up, the helium core shrinks and heats. That heat ignites hydrogen in a shell just outside the core. The extra energy pushes the outer layers out, so the star grows larger and brighter.
- Astronomy 2e, 22.1 Evolution from the Main Sequence to Red Giants openstax.org/books/astronomy-2e/pages/22-1-evolution-from-the-main-sequence-to-red-giants
a shell of hydrogen nuclei just outside the core becomes hot enough for hydrogen fusion to begin.
Checked 2026-10-10.
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