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Solar neutrino problem, long-standing astrophysics problem in which the amount of observed neutrinos originating from the Sun was much less than expected. In the Sun, the process of energy generation results from the enormous pressure and density at its centre, which makes it possible for nuclei to overcome electrostatic repulsion. (Nuclei are positive and thus repel each other.) Once in some billions of years, a given proton (1H, in which the superscript represents the mass...