How Does the Sun Shine?

The SNO and Super-Kamiokande detectors have done a handy job of accounting for the neutrinos coming from the decay of boron-8 nuclei in the sun. But the flux from B-8 decays represents a mere 0.02% of the predicted flux of solar neutrinos, and one wants to study other types of nu production in order to get a better grip on nuclear physics in the sun's core. One would especially like to know more about neutrinos from Be-7, N-13 and O-15 decays (catalyzed by carbon-12), and from proton-proton reactions. (The p-p neutrinos, probably amounting to 90% of the sun's nu flux, have relatively low energies, below 0.5 MeV, whereas the nu's seen directly in terrestrial detectors typically have been in excess of 5 MeV.)

In the 1930's, nuclear pioneer Hans Bethe argued that energy produced in the nuclear reactions involving the heavier elements (the CNO cycle) were a more important energy-producing mechanism for the Sun than was the fusion of the lighter elements (the p-p cycle). Nowadays solar scientists believe the CNO reactions are predominant for stars a bit heavier than our sun but that in the sun itself the p-p cycle will be more important. A new paper by John Bahcall and Carlos Pena-Garay (Institute for Advanced Study) and Concha Gonzales-Garcia (Stony Brook) addresses this issue using recent data from solar neutrino and reactor experiments. Bahcall and his colleagues determine that the fraction of energy produced in the sun via CNO reactions is less than 7.3%. This is a tenfold improvement over the best previous estimation for the CNO contribution. (Bahcall et al., Physical Review Letters, 4 April 2003; contact John Bahcall, 609-734-8054; see neutrino website at www.sns.ias.edu/~jnb)