Explain how a stable star maintains its size over a long period of time.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Nuclear fusion in the core generates an outward gas pressure that pushes the outer layers away.
Gravity pulls the material inward, creating an inward gravitational force.
The outward pressure and the inward gravity are in balance.
Because the forces are equal, the star neither expands nor contracts, so its size remains constant over a long time.
Gravity pulls the material inward, creating an inward gravitational force.
The outward pressure and the inward gravity are in balance.
Because the forces are equal, the star neither expands nor contracts, so its size remains constant over a long time.
Examiner tips
- Use the key terms ‘outward gas pressure’ and ‘inward gravitational force’.
- Show the balance of forces explicitly.
- Keep the answer concise and to the point.
Mark scheme (4 marks)
- Nuclear fusion produces an outward gas pressure (force)
- Gravity pulls the star inward / provides an inward (gravitational) force
- The inward gravitational force is balanced by the outward gas pressure
- Because the forces are balanced, the star neither expands nor contracts, maintaining a constant size
Key terms in this question
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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