  One of the most dramatic possible consequences of stellar rotation is its
influence on stellar death, particularly of massive stars. If the angular
momentum of the iron core when it collapses is such as to produce a neutron
star with a period of 5 ms or less, rotation will have important consequences
for the supernova explosion mechanism. Still shorter periods, corresponding to
a neutron star rotating at break up, are required for the progenitors of
gamma-ray bursts (GRBs). Current stellar models, while providing an excess of
angular momentum to pulsars, still fall short of what is needed to make GRBs.
The possibility of slowing young neutron stars in ordinary supernovae by a
combination of neutrino-powered winds and the propeller mechanism is discussed.
The fall back of slowly moving ejecta during the first day of the supernova may
be critical. GRBs, on the other hand, probably require stellar mergers for
their production and perhaps less efficient mass loss and magnetic torques than
estimated thus far.
