A star begins its life as a cloud of gas and dust called a nebula. Describe the sequence of stages a star similar in mass to the Sun goes through from the moment nuclear fusion begins until it reaches its final stable state.

Eduqas A-Level Physics — 6.3 Stellar evolution and cosmology (A-Level only) · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

1. Nuclear fusion of hydrogen starts in the core, and the star enters the main sequence stage.
2. When hydrogen in the core is exhausted, the core contracts and the outer layers expand, making the star a red giant.
3. The red giant sheds its outer layers, creating a planetary nebula.
4. The remaining hot core becomes a white dwarf.
5. The white dwarf gradually cools over time, eventually becoming a black dwarf, the final stable state.

Examiner tips

  • Use the exact sequence of stages; each point counts as one mark.
  • Mention the core processes (hydrogen fusion, exhaustion) and the resulting structural changes.
  • Keep the answer concise and avoid extra detail beyond the required stages.

Common mistakes

  • Mixing up the order of the red giant and planetary nebula stages.
  • Using vague terms like "old star" instead of "white dwarf".
  • Including unnecessary information such as supernovae, which are not relevant for a Sun‑like star.

Mark scheme (5 marks)

  1. Nuclear fusion of hydrogen begins in the core, and the star enters the main sequence stage
  2. When hydrogen in the core is exhausted, the star expands to become a red giant
  3. The red giant sheds its outer layers, forming a planetary nebula
  4. The hot core that remains is a white dwarf
  5. The white dwarf gradually cools over time to become a black dwarf (final stable / cool state)

Key terms in this question

nuclear fusion

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