A star of similar mass to the Sun is currently a stable main sequence star. Explain what happens to this star from the point at which it runs out of hydrogen fuel, ending with the formation of a white dwarf.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
When the core hydrogen is exhausted the core can no longer sustain the high temperatures required for fusion, so the outward pressure falls and the core collapses under its own gravity.
The loss of pressure causes the outer layers to expand enormously, and the star becomes a red giant.
In the red‑giant phase the core temperature rises until helium fusion can start, but eventually the core runs out of fuel again.
With no fusion to support it, the core collapses further, cools, and settles as a dense white dwarf.
The loss of pressure causes the outer layers to expand enormously, and the star becomes a red giant.
In the red‑giant phase the core temperature rises until helium fusion can start, but eventually the core runs out of fuel again.
With no fusion to support it, the core collapses further, cools, and settles as a dense white dwarf.
Examiner tips
- Show the sequence: core collapse → red giant → core exhaustion → white dwarf.
- Use the terms ‘outward pressure’, ‘inward gravitational force’, ‘expand’, ‘contract’, ‘cool’ as they appear in the mark scheme.
- Keep the answer concise – 4 points only.
Common mistakes
- Mixing up the order of expansion and contraction; the star expands first, then contracts to a white dwarf.
- Using the wrong terminology such as ‘collapse of the whole star’ instead of ‘core collapse’.
Mark scheme (4 marks)
- When hydrogen is used up, nuclear fusion (producing outward pressure) stops / decreases, so the star collapses under its own gravitational attraction / inward gravitational force exceeds outward pressure
- The star expands massively to become a red giant
- Fusion reactions in the red giant eventually come to an end / hydrogen/fuel is exhausted in the red giant
- The star then contracts / collapses under gravity and cools to form a white dwarf
Key terms in this question
main sequence star · white dwarf
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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