CNO cycle

There is more than one way for a star to turn hydrogen into helium. In stars hotter and more massive than the Sun, the dominant route is the CNO cycle, a chain of reactions in which carbon, nitrogen, and oxygen take turns swallowing hydrogen nuclei and then spitting out a finished helium nucleus. The C, N, and O are not used up — they act as catalysts, cycling around again and again, like a relay team passing a baton, while the net effect is still four hydrogen nuclei becoming one helium.

The cycle starts when a carbon nucleus captures a proton, and through a series of captures and small radioactive decays it works its way through nitrogen and oxygen, eventually shedding a helium nucleus and returning to carbon to begin again. The crucial feature is its ferocious sensitivity to temperature: the CNO rate climbs roughly as the fifteenth to twentieth power of temperature, so a small rise in core temperature multiplies the energy output dramatically. Because all those nuclei carry more charge than plain hydrogen, the cycle needs a hotter core to get going — above roughly 17 million kelvin it overtakes the proton-proton chain.

Which path a star uses shapes its whole structure. The Sun, just below the crossover temperature, makes only a small fraction of its energy by the CNO cycle and most by the proton-proton chain. But a star even modestly more massive runs mostly on CNO, and that fuel's extreme temperature sensitivity concentrates the energy generation into a tight central core — which forces that core to convect rather than radiate. The CNO cycle was worked out in the 1930s by Hans Bethe (and independently Carl von Weizsäcker), part of the work that first explained why stars shine.