There has been a long-standing discrepancy between the number of neutrinos expected from the sun and the number we actually detect. One possible interpretation for this was that our theoretical solar model was wrong. However, recent progress of helioseismology has shown that the real sun is very close to the latest solar models. On the other hand, very recent experiments of neutrino detection provided us evidence for neutrino oscillation. I discuss what we should do and what we can do in this situation for the neutrino physics from the astrophysical side.

Historical review: the solar neutrino problem

The energy source of sunshine (and shining of stars in general) is now thought to be nuclear fusion. To get direct evidence that nuclear reactions are really occurring in the sun is, however, a very challenging task. It takes ∼ 104 years for photons generated by nuclear fusion near the solar centre to reach the solar surface, because the photons interact so frequently with matter in the sun. Hence, the photons by which we can see the sun right now do not tell us the physical state of the present solar core. On the other hand, since neutrinos interact little with matter, unlike photons, and travel at the speed of light, the neutrinos generated by nuclear reactions in the sun reach the earth only eight minutes after they are generated.