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The shock in a supernova of Type II first moves slowly and then suddenly breaks out with a velocity of order 109 cm s-1. This breakout is due to the recombination of nucleons into α-particles. It is shown that this recombination provides enough energy to lift the matter against gravity. The net energy after escape from the star is about 4 x 1018 ergs g-1. Supernovae of Type II differ mainly in the binding energy of the mantle of the progenitor.
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Hans A. Bethe (1996) studied this question.
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