CNO cycle

A self-maintaining CNO chain starts at approximately 15×106 K, but its energy output rises much more rapidly with increasing temperatures[1] so that it becomes the dominant source of energy at approximately 17×106 K.[4] The Sun has a core temperature of around 15.7×106 K, and only 1.7% of 4He nuclei produced in the Sun are born in the CNO cycle.

The first reports of the experimental detection of the neutrinos produced by the CNO cycle in the Sun were published in 2020 by the BOREXINO collaboration.

[2][9][10] Under typical conditions found in stars, catalytic hydrogen burning by the CNO cycles is limited by proton captures.

Because of the long timescales involved, the cold CNO cycles convert hydrogen to helium slowly, allowing them to power stars in quiescent equilibrium for many years.

In 2006 it was experimentally measured down to stellar energies, revising the calculated age of globular clusters by around 1 billion years.

The total momentum received by the positron and the neutrino is not great enough to cause a significant recoil of the much heavier daughter nucleus[a] and hence, its contribution to kinetic energy of the products, for the precision of values given here, can be neglected.

The essential idea is that a radioactive species will capture a proton before it can beta decay, opening new nuclear burning pathways that are otherwise inaccessible.

When the cycle is run to equilibrium, the ratio of the carbon-12/carbon-13 nuclei is driven to 3.5, and nitrogen-14 becomes the most numerous nucleus, regardless of initial composition.