Explain why a main sequence star remains in a stable equilibrium state for an extended period of its lifetime.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A main sequence star converts hydrogen into helium in its core via nuclear fusion. This process continues for millions to billions of years depending on the star's mass.
Model answer (4 marks)
A main‑sequence star is in hydrostatic equilibrium: the inward gravitational force is balanced by the outward pressure from the energy released in nuclear fusion. The fusion reactions produce radiation and thermal pressure that push against gravity. If the core contracts, the temperature rises, increasing the fusion rate and the outward pressure, which restores equilibrium; if the core expands, the temperature falls, the fusion rate drops, and the pressure decreases, again restoring balance. This self‑regulating mechanism keeps the star stable for a long time.
Examiner tips
- Use the terms ‘hydrostatic equilibrium’, ‘gravitational force’, ‘outward pressure’, and ‘self‑regulating’.
- Show the cause–effect chain: contraction → ↑T → ↑fusion → ↑pressure → expansion.
- Mention that the balance prevents net expansion or contraction.
Common mistakes
- Confusing radiation pressure with gas pressure or omitting the self‑regulating aspect.
- Using vague phrases like ‘the star stays the same’ without explaining the forces involved.
Mark scheme (4 marks)
- Gravitational force/pressure acts inward, tending to compress the star
- Radiation pressure (and thermal/gas pressure) from fusion acts outward, opposing gravitational collapse
- These two opposing forces/pressures are balanced, preventing net expansion or contraction — this is hydrostatic equilibrium
- The equilibrium is self-regulating: if the core contracts, temperature rises, fusion rate increases, raising outward pressure and restoring balance (or vice versa)
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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