# Explain why a main sequence star remains in a stable equilibrium state for an extended period of its lifetime.

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

> 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.

## Mark scheme (4 marks)

1. Gravitational force/pressure acts inward, tending to compress the star
2. Radiation pressure (and thermal/gas pressure) from fusion acts outward, opposing gravitational collapse
3. These two opposing forces/pressures are balanced, preventing net expansion or contraction — this is hydrostatic equilibrium
4. The equilibrium is self-regulating: if the core contracts, temperature rises, fusion rate increases, raising outward pressure and restoring balance (or vice versa)

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