A star spends most of its life as a main sequence star. Describe the life cycle of a star that has a similar mass to the Sun, from the main sequence stage until it reaches its final state.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A main‑sequence star is in hydrostatic equilibrium, with gravity balanced by the outward pressure of nuclear fusion.
When the core hydrogen is exhausted the core contracts, heating up while the outer layers expand and cool, producing a red giant.
In the red‑giant phase the core temperature rises enough for helium fusion (the triple‑alpha process) to begin, creating carbon and oxygen.
Once the core is exhausted the outer layers are expelled, forming a planetary nebula.
The exposed core remains as a white dwarf, which slowly cools over time.
When the core hydrogen is exhausted the core contracts, heating up while the outer layers expand and cool, producing a red giant.
In the red‑giant phase the core temperature rises enough for helium fusion (the triple‑alpha process) to begin, creating carbon and oxygen.
Once the core is exhausted the outer layers are expelled, forming a planetary nebula.
The exposed core remains as a white dwarf, which slowly cools over time.
Examiner tips
- Use the exact terms: hydrostatic equilibrium, red giant, helium fusion, planetary nebula, white dwarf.
- Show the sequence in order and link each stage to the previous one.
- Include the final cooling of the white dwarf to earn the last mark.
Common mistakes
- Confusing the red giant with the main sequence stage; forgetting to mention the planetary nebula.
- Using vague phrases like "the star dies" instead of the specific stages.
- Omitting the cooling of the white dwarf or the helium fusion step.
Mark scheme (5 marks)
- A main sequence star is in a state of balance/equilibrium between the inward force of gravity and the outward radiation/thermal pressure from nuclear fusion
- When hydrogen fuel runs out the core contracts/collapses and the outer layers expand, forming a red giant
- The red giant undergoes helium fusion / fuses heavier elements in its core
- The outer layers are ejected/shed to form a planetary nebula
- The remaining core becomes a white dwarf, which gradually cools over time
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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