Hydrogen fusion: the proton-proton chain and the CNO cycle

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The two routes by which four protons become one helium-4 nucleus. The proton-proton chain begins with two protons fusing to deuterium via the weak interaction - an extraordinarily slow step, which is why the Sun burns for billions of years rather than detonating. The CNO cycle uses pre-existing carbon, nitrogen and oxygen as catalysts, consuming and regenerating them, and is far more temperature-sensitive, so it takes over in hotter cores. In the present-day Sun about 98.5 per cent of the energy comes from the pp chain and about 1.5 per cent from CNO; theoretical models put the crossover at roughly 1.3 solar masses. Either way about 0.7 per cent of the rest mass of the participating hydrogen is converted to energy, most of it as photons that take a very long time to random-walk out, and a small fraction as neutrinos that leave immediately and carry direct information about the core. Not a place.

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