A cycle of nuclear reactions in which a nucleus loses energy by first absorbing an electron and then reemitting a beta particle (a high-speed electron) plus a neutrino-antineutrino pair. It is an important process in pre-supernova stars because, by cooling and decreasing the pressure in the star's core, it renders the core unable to support the weight of the overlying layers. These layers collapse onto the core and subsequently rebound and escape in the supernova explosion. "Urca" is not an acronym but the name of a casino in Rio de Janeiro at which George Gamow commented to the Brazilian astrophysicist and art critic Mario Schenberg (1914-1990) (who first pointed out the importance of neutrino emission in supernovae to Gamow): "the energy disappears in the nucleus of the supernova as quickly as the money disappeared at that roulette table." The process was dramatically confirmed by Supernova 1987A whose observation coincided with a burst of 11 neutrinos, detected by the the Super-Kamiokande in Japan, and a further eight registered independently by a detector in Ohio.
Related category STELLAR ASTROPHYSICS
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