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The Question
(Submitted February 15, 1998)
My understanding of a supernova is that the explosion itself is due primarily
to a shock wave caused by rebound of the neutron star, but I do not know how
to connect this picture to a supernova that leads to a black hole.
Does the collapse pause at the baryon degeneracy level long enough to rebound,
regardless of whether continued collapse occurs?
The Answer
Yes, in the formation of a neutron star the infall is so great that it
is compressed to up to 50 % greater than its normal density. In a neutron
star, the nuclear force is strong enough to cause a rebound from this
compression, which gives an outward push to the remaining outer layers of
the stellar interior.
However, the rebound is only part of the mechanism by which the supernova
generates its energy. The binding energy of a neutron star is much less than
that of a non-collapsed stellar core. The tremendous amount of energy generated
by the neutron star formation drives the supernova. The same is true with
the formation of a black hole, save that the binding energy of the black
hole is even less than a neutron star and hence the explosion would be
somewhat more energetic. So the formation to a black hole still includes
an explosion.
Another way of thinking of the same issue: if the rebound was the only
source of energy driving the supernova, the surface layers of the star would
only bounce back up to their original radius prior to the stellar collapse
if no energy was generated by the formation of the neutron star, rather like
dropping a superball onto the floor and letting it bounce back. The rebound
effect is rather like having the floor jump up at the superball, so it
bounces somewhat higher. But in this case, the binding energy released
in the explosion is even greater: the superball is thrown off into
outer space instead of just bouncing higher.
Jesse Allen and Jim Lochner
for "Ask an Astrophysicist"
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