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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+1Hā†’2H+e++ve{}^1H + {}^1H \rarr {}^2H + e^+ + v_e
    • Two protons fuse
    • Make deuterium, one positron, and one neutrino
  2. 1H+2Hā†’3He+Ī³{}^1H + {}^2H \rarr {}^3He + Ī³
    • Deuterium nucleus fuses with a proton
    • Makes helium-3, gamma ray
  3. 3He+3Heā†’4He+1H+1H{}^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