# Explain what happens to a star of approximately one solar mass after it exhausts the hydrogen fuel in its core, describing the changes in both its luminosity and radius.

> IB DP Physics Standard Level (2023 syllabus) — E.5 Fusion and stars · Explain · 4 marks

## Mark scheme (4 marks)

1. When hydrogen in the core is exhausted, fusion ceases in the core and the core contracts/gravitational collapse of the core occurs.
2. The released gravitational energy heats a shell of hydrogen surrounding the core, causing hydrogen shell fusion to begin.
3. The increased energy output causes the outer layers to expand greatly, so the radius increases significantly (star becomes a red giant).
4. The surface temperature decreases (surface cools) as the radius increases, so the star appears redder, but overall luminosity increases or remains comparable to main sequence luminosity.

## Key terms

- [luminosity](https://www.gradenine.co.uk/glossary/luminosity)

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-what-happens-to-a-star-96dead03) · Published by Druglandscape Ltd.