# 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 centre of stars
• Even there, cannot overcome charge repulsion
• First step in the chain is very difficult
• Generally takes a long time

## Steps

1. ${}^1H + {}^1H \rarr {}^2H + e^+ + v_e$
2. ${}^1H + {}^2H \rarr {}^3He + \gamma$
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