Describe the life cycle of a star that is similar in size to the Sun, starting from a nebula and ending at its final stage.

Edexcel GCSE Physics (1PH0) — 7.1 Astronomy (Phys only) · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

A nebula of gas and dust collapses under its own gravity to form a protostar.
The protostar contracts until core temperatures are high enough for hydrogen fusion, becoming a main‑sequence star where gravitational pressure balances the outward pressure from fusion.
When core hydrogen is exhausted the star expands into a red giant, fusing heavier elements.
After fusion ceases the outer layers are shed and the core cools and contracts to a white dwarf.

Examiner tips

  • Use the exact sequence: nebula → protostar → main‑sequence → red giant → white dwarf.
  • Show the balance of forces in the main‑sequence stage.
  • Mention fusion of heavier nuclei in the red giant.
  • Keep the answer concise – one sentence per stage.

Common mistakes

  • Confusing the protostar with the main‑sequence star.
  • Omitting the red giant phase.
  • Using the wrong final stage (e.g., neutron star) for a Sun‑like star.

Mark scheme (4 marks)

  1. A nebula (cloud of dust and gas) is pulled together by gravitational attraction to form a protostar
  2. The protostar becomes a main sequence star where gravitational forces and outward pressure (from fusion) are balanced
  3. When hydrogen runs out, the star expands to form a red giant (where heavier nuclei are produced by fusion)
  4. Once fusion reactions end, the star contracts and cools to form a white dwarf

Key terms in this question

nebula

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