How does the Sun shine?

John Bahcall
1989 Nature  
Assuming that MSW neutrino oscillations occur and ignoring all solar physics except for the constraint that nuclear fusion produces the solar luminosity, we show that new solar neutrino experiments are required to rule out empirically the hypothesis that the sun shines via the CNO cycle. In 1939, Bethe [1] showed that the energy required to enable the sun to shine for several billion years could be obtained by two alternative sets of reactions, which have come to be known as the pp chain and
more » ... CNO cycle. Although Bethe's original calculations favored the CNO reactions for the solar energy source, detailed solar models developed in the late 1950s and early 1960s indicated that the pp chain was dominant in the sun. Many authors (e.g., [2] [3] [4] [5] ) have summarized the specific reactions that occur, according to current understanding, in the pp chain and the CNO cycle. Do solar neutrino experiments confirm the theoretical calculations that indicate that the sun shines primarily by the pp fusion chain? The predominant opinion (to which we also subscribed before doing the calculations described in this paper) seems to be [6] that the pp neutrinos have been observed in the gallium solar neutrino experiments, GALLEX [7] and SAGE [8], establishing experimentally the predominance of the pp chain. The reasons for this view include the approximate agreement between the total observed rate in the gallium experiments, 74 ± 8 SNU and the total rate, 73 SNU, predicted in the standard solar model (SSM) [9] to arise from only the pp (and pep) neutrinos. The observed rate is about half the predicted standard rate from all neutrino sources. In addition, the CNO
doi:10.1038/340265a0 fatcat:5btv6gmibra3dka42eiq3b7tuy