# Explain the changes in a star's internal structure and energy production that occur as it evolves from the main sequence stage to become a red giant.

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

> A star of mass similar to the Sun has exhausted the hydrogen fuel in its core after several billion years on the main sequence.

## Mark scheme (4 marks)

1. When hydrogen in the core is exhausted, fusion in the core ceases, so the core loses its radiation pressure / outward pressure from fusion and begins to contract (gravitationally).
2. The gravitational contraction of the core releases gravitational potential energy, heating the surrounding hydrogen shell, which then ignites hydrogen shell burning / fusion begins in a shell around the core.
3. The increased energy output from shell burning causes the outer layers of the star to expand greatly / the envelope expands outward.
4. As the outer layers expand they cool, causing the surface temperature to decrease and the star's colour to shift to red, forming a red giant.

## Key terms

- [main sequence](https://www.gradenine.co.uk/glossary/main-sequence)
- [red giant](https://www.gradenine.co.uk/glossary/red-giant)

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-changes-in-a-star-s-f052c3fb) · Published by Druglandscape Ltd.