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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

A main sequence star is in a stable state because the outward pressure produced by nuclear fusion of hydrogen in its core balances the inward pull of gravity. The fusion reactions generate radiation pressure that pushes material outward, while gravity pulls the material inward. When these two forces are equal, the star neither expands nor contracts, so its size remains constant.

Examiner tips

  • Use the terms "outward pressure" and "inward gravitational force". Show the balance of forces. Mention that the size remains constant. Keep the answer concise and to the point.

Common mistakes

  • Confusing the direction of the forces. Forgetting to mention that the star’s size stays constant. Using vague language such as "stable because it is old".

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 in this question

main sequence star · nebula · protostar

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