A star begins its life as a nebula. Describe the sequence of stages in the life cycle of a star similar in mass to our Sun, starting from the nebula stage and ending with its final state.

WJEC GCSE Physics (Wales) — 2.1 Light, lenses and the universe · 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 is a cloud of gas and dust that is pulled together by gravity to form a protostar.
The protostar contracts until its core temperature is high enough for hydrogen nuclei to fuse to helium, becoming a main‑sequence star.
When core hydrogen is exhausted the core contracts and the outer layers expand, turning the star into a red giant.
The red giant sheds its outer layers, leaving a hot core that cools to become a white dwarf, the star’s final state.

Examiner tips

  • Use the exact sequence: nebula → protostar → main‑sequence → red giant → white dwarf.
  • Mention fusion of hydrogen to helium in the main‑sequence stage.
  • Show the star expands when hydrogen runs out.
  • State that the final remnant is a white dwarf.

Common mistakes

  • Confusing the protostar with the main‑sequence star; they are distinct stages.
  • Forgetting to mention that the red giant sheds its outer layers.
  • Using vague terms like “star dies” instead of “becomes a white dwarf.”

Mark scheme (4 marks)

  1. A nebula is a cloud of gas and dust that is pulled together by gravity to form a protostar
  2. The protostar becomes a main sequence star where hydrogen nuclei fuse to form helium (nuclear fusion), releasing energy
  3. When hydrogen runs out the star expands into a red giant
  4. The red giant sheds its outer layers to leave behind a white dwarf as the final state

Key terms in this question

nebula

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