Describe the life cycle of a star that is similar in mass to the Sun, beginning from a nebula and ending as a white dwarf.

Cambridge International IGCSE Physics (0625) — 6.2 Stars 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 of dust and gas collapses under its own gravity to form a protostar.
When the core temperature rises to ~10^7 K, hydrogen fusion begins and the protostar becomes a stable main‑sequence star.
After the hydrogen in the core is exhausted, the core contracts and the outer layers expand, producing a red giant.
The outer layers are then ejected as a planetary nebula, leaving behind a hot core that cools to become a white dwarf.

Examiner tips

  • Use the word "nebula" and mention hydrogen gas. Show the sequence: nebula → protostar → main‑sequence star → red giant → planetary nebula → white dwarf. Include the key physical change (fusion starts, then hydrogen runs out, expansion, ejection).
  • Use concise, exam‑style sentences; avoid unnecessary words.

Mark scheme (4 marks)

  1. A nebula (cloud of dust and gas containing hydrogen) collapses under gravity to form a protostar.
  2. A stable star forms when the inward gravitational attraction is balanced by the outward gas pressure from nuclear fusion.
  3. When the star runs out of hydrogen fuel, it expands to form a red giant.
  4. The red giant contracts to form a planetary nebula, leaving a white dwarf at its centre.

Key terms in this question

nebula · white dwarf

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