Explain the life cycle of a star that is similar in size to the Sun, starting from a nebula and ending at its final state.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A nebula (cloud of dust and gas) is pulled together by gravitational attraction to form a protostar
The protostar becomes hot enough for hydrogen nuclei to undergo fusion, forming a main‑sequence star where gravitational forces and outward pressure are balanced
When hydrogen is used up, the star expands to form a red giant, where heavier nuclei are produced by fusion
Once all fusion reactions are complete, the star contracts under gravity
The star cools down to form a white dwarf as its final state
The protostar becomes hot enough for hydrogen nuclei to undergo fusion, forming a main‑sequence star where gravitational forces and outward pressure are balanced
When hydrogen is used up, the star expands to form a red giant, where heavier nuclei are produced by fusion
Once all fusion reactions are complete, the star contracts under gravity
The star cools down to form a white dwarf as its final state
Examiner tips
- Use the exact sequence of stages – nebula, protostar, main‑sequence, red giant, contraction, white dwarf
- Show the cause–effect link (gravity → heating → fusion → pressure balance → expansion → contraction)
- Keep the answer concise, 5 points only
- Use UK spelling and correct terminology
Common mistakes
- Mixing up the order of stages (e.g. putting red giant before main‑sequence)
- Using vague terms such as ‘big star’ instead of ‘red giant’
- Omitting the final state or misidentifying it as a neutron star or black hole
Mark scheme (5 marks)
- A nebula (cloud of dust and gas) is pulled together by gravitational attraction to form a protostar
- The protostar becomes hot enough for hydrogen nuclei to undergo fusion, forming a main sequence star where gravitational forces and outward pressure are balanced
- When hydrogen is used up, the star expands to form a red giant, where heavier nuclei are produced by fusion
- Once all fusion reactions are complete, the star contracts under gravity
- The star cools down to form a white dwarf as its final state
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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