# Explain why a star similar in mass to the Sun remains stable during its main sequence stage, and describe what happens to the star once its hydrogen fuel runs out.

> Edexcel A-Level Physics (9PH0) — 7.1 Electric fields · Explain · 5 marks

> Stars spend most of their lifetime in a stable state before eventually changing as their fuel is used up.

## Mark scheme (5 marks)

1. During the main sequence, gravitational forces pulling the star inwards are balanced by the outward pressure from fusion reactions.
2. Fusion of hydrogen nuclei produces helium nuclei, releasing a large amount of energy that provides the outward pressure.
3. When hydrogen runs out, fusion can no longer maintain equilibrium, so the star is no longer stable and begins to change.
4. The star expands to form a red giant, as heavier nuclei are produced by fusion.
5. Once all fusion reactions end, the star contracts and cools to form a white dwarf.

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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