neutrino

In physics, any of three uncharged elementary particles (and their antiparticles) of the lepton class, having a mass that is very small. The most familiar type, the antiparticle of the electron neutrino, is emitted in the beta decay of a nucleus. The other two are the muon and tau neutrinos.

Supernova 1987A was the first object outside the Solar System to be observed by neutrino emission. The Sun emits neutrinos, but in smaller numbers than theoretically expected. This shortage of solar neutrinos was one of the biggest mysteries in modern astrophysics until July 2001, when a team of US, Canadian, and British physicists discovered that the Sun emits electron neutrinos that transform into tau and muon neutrinos, which are harder to detect. When this is allowed for, the amount of neutrinos emitted by the Sun tallies with the amount predicted. The experiment also confirmed that neutrinos do have mass.