Explain what happens to a star of approximately one solar mass after it exhausts the hydrogen fuel in its core, describing the changes in both its luminosity and radius.

IB DP Physics Standard Level (2023 syllabus) — E.5 Fusion and stars · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

When the core hydrogen is exhausted, fusion stops in the core and the core contracts under gravity.
The gravitational energy released heats the surrounding hydrogen shell, so hydrogen shell fusion starts.
The extra energy output inflates the outer layers, so the radius increases dramatically and the star becomes a red giant.
The surface temperature falls as the radius grows, making the star appear redder, but its total luminosity rises or stays roughly the same as on the main sequence.

Examiner tips

  • Use the word ‘exhausts’ to show the core stops fusing hydrogen.
  • Mention core contraction and shell fusion explicitly.
  • Explain the radius increase and surface temperature drop as a cause of the red colour.
  • State that overall luminosity rises or remains comparable.”]

Mark scheme (4 marks)

  1. When hydrogen in the core is exhausted, fusion ceases in the core and the core contracts/gravitational collapse of the core occurs.
  2. The released gravitational energy heats a shell of hydrogen surrounding the core, causing hydrogen shell fusion to begin.
  3. The increased energy output causes the outer layers to expand greatly, so the radius increases significantly (star becomes a red giant).
  4. The surface temperature decreases (surface cools) as the radius increases, so the star appears redder, but overall luminosity increases or remains comparable to main sequence luminosity.

Key terms in this question

luminosity

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