Stars as Laboratories for Fundamental Physics - MPP Theory Group

38 Chapter 2
Fig. 2.8. The Crab nebula, remnant of the supernova of A.D. 1054. (Image
courtesy of the European Southern Observatory.)
At this point a shock wave forms at the edge of the core and moves
outward. The implosion can be said to be reflected and thus turned into
an explosion. In practice, it is difficult to account for the subsequent
evolution as the shock wave tends to dissipate its energy by dissociating
iron. Currently it is thought that a revival of the stalled shock is needed
and occurs by neutrinos depositing their energy in the “hot bubble”
below the shock. This region has a low density yet high temperature,
and thus a high entropy per baryon by common astrophysical standards.
Within about 0.3 s after collapse the shock has moved outward and
ejects the entire overburden of the mantle and envelope. This course
of events is the scenario of a type II supernova (SN) explosion.
What remains is an expanding nebula such as the Crab (Fig. 2.8)
which is the remnant of the SN of A.D. 1054, and a central neutron
star (radius about 10 km, mass about 1 M ⊙ ) which often appears in
the form of a pulsar, a pulsating source of radiation in some or all
electromagnetic wave bands. The pulsed emission is explained by a
complicated interplay between the fast rotation and strong magnetic
fields (up to 10 12 −10 13 G) of these objects.
Returning to the moment after collapse of the iron core, it is so dense
(nuclear density and above) and hot (temperature of several 10 MeV),