# A protostar forms from a nebula and eventually becomes a main sequence star. Explain why a main sequence star is described as being in a stable state.

> Pearson Edexcel International GCSE Physics (4PH1) — 8.2 Stellar evolution · Explain · 4 marks

> Astronomers classify stars according to the stage they have reached in their life cycle. A main sequence star, such as the Sun, can remain stable for billions of years before its hydrogen fuel runs out.

## Mark scheme (4 marks)

1. Nuclear fusion of hydrogen nuclei occurs in the star, producing an outward pressure (or force / radiation pressure)
2. Gravity pulls the star inwards (gravitational attraction acts inward / inward gravitational force)
3. The outward pressure due to fusion balances the inward gravitational force
4. Because the forces are balanced, the star does not expand or collapse / its size remains constant

## Key terms

- [main sequence star](https://www.gradenine.co.uk/glossary/main-sequence-star)
- [nebula](https://www.gradenine.co.uk/glossary/nebula)
- [protostar](https://www.gradenine.co.uk/glossary/protostar)

## Related

- [Revision notes for Pearson Edexcel International GCSE Physics (4PH1)](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/a-protostar-forms-from-a-nebula-1d064235) · Published by Druglandscape Ltd.