4 THE SUN

Obviously, the star for which the best and most detailed data exist is our sun. Thus, it is natural that our general understanding of stellar structure and evolution be checked in detail against solar observations.

We will see that the theoretical ideas that are combined in the standard solar model (Section 4.1) are generally in reasonable overall agreement with observation. In particular, the current neutrino data seem to confirm that the sun generates its energy from the fusion of protons to helium. We will discuss solar hydrogen burning as well as some of the nuclear reactions involved in Section 4.2. Nevertheless, even in the era of second-generation solar-neutrino detectors, the famous, or perhaps we should say notorious, solar-neutrino problem is still with us, i.e., the number of solar neutrinos observed by earth-bound detectors is significantly less than predicted by the standard solar model (Section 4.3). We will argue that the cross sections of those nuclear reactions that are involved in solar hydrogen burning are known well enough to exclude the nuclear physics input as a significant source of the solar neutrino problem (Section 4.4). Because of the work of Bahcall and collaborators (see Ref. 49 and references cited therein), the standard solar model is also unlikely to be responsible for this outstanding problem. At this time, the explanation of the solar-neutrino problem is expected by many astrophysicists to be in the physics beyond the standard model of the weak interaction. Resonant matter-induced neutrino oscillations are an especially...