Describe the types of nuclear reactions involved in Main-Sequence and post-Main Sequence stars

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Describe the types of nuclear reactions involved in Main-Sequence and post-Main Sequence stars

  • Main sequence:
    • Proton-proton chain: convert 2 H atoms into deuterium which combines with a proton to form He isotopes. 2 of these combine to form He-4. Remainder is recycled, and energy is released as gamma rays and neutrinos. He accumulated at the centre to become next fuel store
    • CNO cycle: common in larger stars – uses C as a catalyst and requires hotter temperatures
      • C-12 + 1H à N-13 à C-13
      • C-13 + 1H à N-14
      • N-14 + 1H à O-15 à N-15
      • N-15 + 1H à He-4 + C-12
      • C-12 is recycled for next time
    • These two can occur simultaneously
  • Post-main:
    • Triple alpha fusion: fuses 3 He to form C, C fuses with He-4 to form O. Found in larger stars with mainly a C core. Core heated with contraction – fused all the way to iron (endothermic).