  The Crab Nebula is likely to be expanding into freely expanding supernova
ejecta, although the energy in the ejecta may be less than is typical for a
Type II supernova. Pulsar nebulae much younger than the Crab have not been
found and could have different properties. The search for such nebulae through
ultraviolet/optical line emission in core collapse supernovae, or through their
X-ray emission (which could show strong absorption) is warranted. Neutron stars
have now been found in many young supernova remnants. There is not a clear link
between neutron star and remnant type, although there is an indication that
normal pulsars avoid the O-rich remnants. In the later phases of a supernova
remnant, the pulsar wind nebula is crushed by the reverse shock front. Recent
simulations show that this process is unstable, which can lead to mixing of the
thermal and relativistic gases, and that the pulsar nebula is easily displaced
from the pulsar, which can explain the position of the Vela pulsar relative to
the Vela X radio nebula.
