The observation of neutrinos emitted by celestial objects. Neutrinos pass through large quantities of matter without significant absorption. For example, neutrinos produced by the nuclear processes in the Sun’s core have only one chance in 10¹⁰ of being absorbed in escaping from the Sun and so reach the Earth in huge numbers (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 ³⁷Cl, 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.