Core collapse can fail to produce an ordinary successful ejecting supernova, leaving a black hole and retaining much of the processed core. Some envelope material can still escape, depending on the event; failure is not equivalent to a universal zero-yield prescription.
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A "failed supernova" refers to an event in which a massive star, instead of exploding as a supernova, collapses directly into a black hole or a neutron star without ejecting its outer layers. This can occur in stars that have insufficient energy or mass in their outer layers to produce the explosive conditions characteristic of a supernova.