A star spends most of its life on the main sequence. Explain what is meant by the main sequence and describe the sequence of events that occurs when a star like the Sun eventually leaves the main sequence and reaches its final state.

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

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

Stars like the Sun are formed from clouds of gas and dust in space. After a long period of stability, they undergo dramatic changes before reaching their final state.

Model answer (5 marks)

The main sequence is the long, stable phase of a star’s life during which hydrogen is fused into helium in the core, producing enough outward pressure to balance gravity. When the core hydrogen is exhausted, the core contracts and heats up while the outer layers expand and cool, turning the star into a red giant. In the red‑giant stage the outer layers are lost to space, creating a planetary nebula, and the exposed core remains. This core becomes a hot, dense white dwarf, which then slowly radiates away its residual heat and will eventually cool into a black dwarf.

Examiner tips

  • Use the term "main sequence" and explain the balance of forces; mention hydrogen fusion into helium.
  • Describe the core hydrogen exhaustion, expansion to a red giant, ejection of layers, and formation of a white dwarf.
  • Show the final cooling to a black dwarf.
  • Keep each point concise to fit the 5‑mark limit.

Common mistakes

  • Confusing the main sequence with the entire life cycle; not mentioning the balance of pressure and gravity.
  • Omitting the planetary nebula stage or the white dwarf’s cooling phase.
  • Using vague terms like "old star" instead of specific stages such as red giant or white dwarf.

Mark scheme (5 marks)

  1. The main sequence is a stable period/stage in a star's life where nuclear fusion of hydrogen (into helium) provides an outward pressure that balances gravitational collapse
  2. When hydrogen in the core runs out, the star expands and cools to become a red giant
  3. The outer layers of the red giant are ejected/shed, forming a planetary nebula
  4. The remaining hot, dense core becomes a white dwarf
  5. The white dwarf gradually cools over time and will eventually become a black dwarf (a cold, dark, non-luminous object)

Key terms in this question

main sequence

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