Explain the changes in a star's internal structure and energy production that occur as it evolves from the main sequence stage to become a red giant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A star of mass similar to the Sun has exhausted the hydrogen fuel in its core after several billion years on the main sequence.
Model answer (4 marks)
When the core hydrogen is exhausted, fusion in the core stops and the core loses its radiation pressure, so it contracts under gravity.
The contraction of the core releases gravitational potential energy, which heats the surrounding hydrogen shell.
The heated shell ignites hydrogen fusion, producing a large increase in energy output.
The extra energy inflates the outer envelope; as it expands the surface temperature falls and the star becomes a red giant.
The contraction of the core releases gravitational potential energy, which heats the surrounding hydrogen shell.
The heated shell ignites hydrogen fusion, producing a large increase in energy output.
The extra energy inflates the outer envelope; as it expands the surface temperature falls and the star becomes a red giant.
Examiner tips
- Use the exact sequence: core stops → core contracts → shell ignition → envelope expands → red giant.
- Show cause–effect links and use terms like ‘radiation pressure’, ‘gravitational contraction’, ‘hydrogen shell burning’, ‘envelope expansion’.
- Keep each point brief and to the point.
Mark scheme (4 marks)
- When hydrogen in the core is exhausted, fusion in the core ceases, so the core loses its radiation pressure / outward pressure from fusion and begins to contract (gravitationally).
- The gravitational contraction of the core releases gravitational potential energy, heating the surrounding hydrogen shell, which then ignites hydrogen shell burning / fusion begins in a shell around the core.
- The increased energy output from shell burning causes the outer layers of the star to expand greatly / the envelope expands outward.
- As the outer layers expand they cool, causing the surface temperature to decrease and the star's colour to shift to red, forming a red giant.
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
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