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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
Protons in the core must first tunnel through the Coulomb barrier to fuse.
1. Two protons fuse to give a deuterium nucleus, emitting a positron and a neutrino.
2. The deuterium then fuses with another proton to produce a helium‑3 nucleus.
3. Two helium‑3 nuclei combine to form a helium‑4 nucleus, releasing two protons.
Overall four protons → one helium‑4 nucleus.
Energy is released because the mass of the helium‑4 nucleus is less than the sum of the four original protons; the mass defect is converted to energy via E=mc².
1. Two protons fuse to give a deuterium nucleus, emitting a positron and a neutrino.
2. The deuterium then fuses with another proton to produce a helium‑3 nucleus.
3. Two helium‑3 nuclei combine to form a helium‑4 nucleus, releasing two protons.
Overall four protons → one helium‑4 nucleus.
Energy is released because the mass of the helium‑4 nucleus is less than the sum of the four original protons; the mass defect is converted to energy via E=mc².
Examiner tips
- Use the exact sequence of reactions and include the positron/neutrino step; this shows full understanding. Show the mass defect explanation to earn the energy‑release mark. Keep the answer concise – 4 marks only.
- Use the word ‘tunnel’ to demonstrate knowledge of quantum tunnelling, a key point in the mark scheme.
Common mistakes
- Omitting the positron and neutrino emission in the first step. Confusing the order of the reactions (e.g., helium‑3 + helium‑3 → helium‑4 + 2p is correct but sometimes written backwards). Failing to mention the mass defect as the source of energy.
Mark scheme (4 marks)
- Protons (hydrogen nuclei) must overcome / tunnel through the electrostatic (Coulomb) repulsion barrier to fuse together.
- 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.
- Two helium-3 nuclei fuse to form helium-4, releasing two protons; the overall result is four protons → one helium-4 nucleus.
- 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.
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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