We study all aspects of weak interactions during supernovae from the precollapse structure of a massive star, through the core bounce, and the cooling of the proto-neutron star. The precollapse core is supported by the electron degeneracy pressure, so the point at which the core becomes unstable to gravitational collapse is critically dependent upon the loss of electrons by electron capture. We calculate the competition between electron capture and beta decay and the effect of this competition on the developing supernova. As the core collapses and heats, thermally produced neutrino-antineutrino pairs become trapped. As they slowly escape from the core their interactions with heated material behind the shock supply the energy needed to drive a successful supernova explosion. Their emergent spectrum is a possible probe of conditions reached within the collapsing core. As the proto-neutron star cools, escaping neutrinos continue to interact with material on the surface of the neutron star producing a hot wind which is an ideal site for heavy-element nucleosynthesis. Also, the escaping electron and tau neutrinos could experience flavor mixing. If this occurs, it would substantially influence the nucleosynthesis yields. We study limits on the tau neutrino mass and mixing angle imposed by this possibility.
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