The fields related to neutrino particles and astrophysics are rich, diverse and developing rapidly. So it is impossible to try to summarize all of the activities in the field in a short note. That said, current questions attracting a large amount of experimental and theoretical effort include the following: What are the masses of the various neutrinos? How do they affect Big Bang cosmology? Do neutrinos oscillate? Or can neutrinos of one type change into another type as they travel through matter and space? Are neutrinos fundamentally distinct from their anti-particles? How do stars collapse and form supernovae? What is the role of the neutrino in cosmology?
One long-standing issue of particular interest is the so-called solar neutrino problem. This name refers to the fact that several terrestrial experiments, spanning the past three decades, have consistently observed fewer solar neutrinos than would be necessary to produce the energy emitted from the sun. One possible solution is that neutrinos oscillate--that is, the electron neutrinos created in the sun change into muon- or tau-neutrinos as they travel to the earth. Because it is much more difficult to measure low-energy muon- or tau-neutrinos, this sort of conversion would explain why we have not observed the correct number of neutrinos on Earth.