# Explain the sequence of nuclear reactions that converts hydrogen into helium within a main sequence star, and state why energy is released in this process.

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

> Main sequence stars spend the majority of their lives fusing hydrogen nuclei in their cores. The proton–proton chain is the dominant fusion pathway in stars of similar mass to the Sun.

## Mark scheme (4 marks)

1. Protons (hydrogen nuclei) must overcome / tunnel through the electrostatic (Coulomb) repulsion barrier to fuse together.
2. Two protons combine to form a deuterium nucleus (releasing a positron and a neutrino), and then deuterium fuses with a further proton to form helium-3.
3. Two helium-3 nuclei fuse to form helium-4, releasing two protons; the overall result is four protons → one helium-4 nucleus.
4. Energy is released because the mass of the helium-4 nucleus is less than the combined mass of the four original protons; this mass difference (mass defect) is converted to energy via E = mc².

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