The observation of neutrinos emitted by celestial objects. Neutrinos pass through large quantities of matter without significant absorption. For example, a neutrino produced by the nuclear processes in the Sun's core has only one chance in 10¹⁰ of being absorbed in escaping from the Sun. However, observations have detected only about one-third the number of neutrinos from the Sun predicted by theory (see solar neutrino unit).

Bursts of neutrinos are predicted to be produced in supernova explosions, and such a burst was detected from Supernova 1987A. Neutrinos can be detected in several ways. One uses the neutrino's interaction with the chlorine isotope ³⁷C1, which produces radioactive argon, ³⁷Ar. Detectors have also been built which utilize the conversion of gallium to germanium by a neutrino (⁷¹Ga to ⁷¹Ge). Neutrino telescopes detect the direction from which the neutrino comes, as well as its existence. These rely on the neutrino colliding with an electron inside a large tank of water. The electron is then detected via its Cerenkov radiation.