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# Proton-Proton Chain

• Why the Sun is shining
• One of two known sets for nuclear fusion by which stars convert hydrogen to helium
• Only possible at temperatures and densities found at the centres of stars
• Even there, cannot overcome charge repulsion
• First step in the chain is very difficult
• Generally takes a long time

## Steps

1. ${}^1H + {}^1H \rarr {}^2H + e^+ + v_e$
• Two protons fuse
• Make deuterium, one positron, and one neutrino
2. ${}^1H + {}^2H \rarr {}^3He + γ$
• Deuterium nucleus fuses with a proton
• Makes helium-3, gamma ray
3. ${}^3He + {}^3He \rarr {}^4He + {}^1H + {}^1 H$
• Step 1 & 2 must happen twice
• Two helium-3 nuclei fuse to form helium-4
• Two excess protons are released

Physics