# Explain why the core temperature of a star must reach approximately 10⁷ K before hydrogen fusion can occur.

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

> Stars generate energy through nuclear fusion reactions in their cores. Before fusion can begin, specific physical conditions must be met.

## Mark scheme (4 marks)

1. Protons are positively charged and so repel each other (Coulomb/electrostatic repulsion).
2. Protons must have sufficient kinetic energy to overcome this electrostatic potential energy barrier and come close enough for the strong nuclear force to act.
3. Kinetic energy of particles is proportional to (absolute) temperature, so a very high temperature is needed to provide this kinetic energy.
4. A temperature of ~10⁷ K is required so that a sufficient proportion of protons have enough energy to undergo quantum tunnelling through the barrier / to overcome the Coulomb barrier.

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