Nena

NeNa, short for the NeNa cycle, is a set of proton-capture and beta-decay reactions that occur during hydrogen burning in stars. It operates in hot hydrogen-burning environments and interconverts neon and sodium isotopes, producing 23Na and cycling material back to 20Ne. The principal sequence begins with 20Ne capturing a proton to form 21Na. Depending on temperature, 21Na may beta-decay to 21Ne or capture another proton to 22Mg, which decays to 22Na. 22Ne can capture a proton to 23Na, and 23Na can capture a proton and release an alpha particle to return to 20Ne via 23Na(p,α)20Ne. Net effect is the production of sodium and the alteration of neon isotopes.

Occurrences: The NeNa cycle operates in the hydrogen-burning shells of massive stars, during hot-bottom burning in asymptotic giant branch stars, and in novae on white dwarfs, at temperatures roughly 20–60 million kelvin. Its efficiency increases with temperature and density, and with the rate of the key proton-capture channels.

Significance: The cycle contributes to sodium production in stars and influences isotope ratios of neon and sodium in stellar envelopes and ejecta. It is related to observed abundance patterns such as sodium enrichment in some giant stars and globular cluster Na-O anticorrelation, though details depend on stellar evolution and reaction-rate uncertainties.