A neutrino is an electrically neutral, almost massless, fermionic particle. In the particle physics Standard Model there are three kinds, or “flavors”, of neutrinos, which are associated with the electron and its heavier siblings, muon and tau particles, respectively. Because neutrinos only interact with other particles (and themselves) through the weak interaction, it is very difficult to detect them directly. This also means that matters that are opaque to photons can be completely transparent to neutrinos. For example, while it can take about 1 million years for a photon to diffuse out of the Sun, a neutrino escapes the Sun without any hindrance. In fact, the detection of solar neutrinos is the first direct proof that the Sun is powered by the nuclear fusion of hydrogen atoms into heliums.
If a neutrino is exactly massless as predicted by the Standard Model, it would always travel with the speed of light as a photon does. Although the physicists have not been able to pin down the masses of neutrinos, they determined that neutrinos could not be all massless by observation of “neutrino oscillations”. For example, although only electron-flavor neutrinos can be produced in the Sun, some of these electron-flavor neutrinos “oscillate” into neutrinos of other flavors on their way to the Earth.
Although it is rare, a nearby (core-collapse) supernova can provide another astronomical source of neutrinos. In such a supernova about a tenth of solar mass is converted into energy (about 1046 joules), and 99% of this energy is carried away by about 1058 neutrinos in 10 seconds. Detection of supernova neutrinos can provide a direct probe deep down to the surface of the nascent neutron star at the center of the supernova. Meanwhile, supernovae are important in the chemical evolution of the universe. They may have manufactured half of the elements heavier than iron in the universe. It is on the ashes of countless supernovae previously exploded inside the Milky Way that our solar system was formed. Because neutrinos dominate the energetics of the supernova and because they determine the neutron-to-proton ratio in the supernova ejecta, they are instrumental in supernova nucleosynthesis.