# Explain how a stable star maintains its size over a long period of time.

> Cambridge International IGCSE Physics (0625) — 6.2 Stars and the Universe · Explain · 4 marks

## Mark scheme (4 marks)

1. Nuclear fusion produces an outward gas pressure (force)
2. Gravity pulls the star inward / provides an inward (gravitational) force
3. The inward gravitational force is balanced by the outward gas pressure
4. Because the forces are balanced, the star neither expands nor contracts, maintaining a constant size

## Key terms

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

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-a-stable-star-maintains-34fe0b5b) · Published by Druglandscape Ltd.