A diffuse nebula consisting of the remains of the outer layers of a star that have been blown into space by a supernova explosion, together with swept-up interstellar matter. The expansion velocity can approach a few per cent of the speed of light. SNRs are strong sources of X-ray emission and radio waves (synchrotron radiation). SNRs can be classed as either shell remnants, such as Cassiopeia A, or the rare filled-centre remnants (plerions), such as the Crab Nebula. Plerions are believed to be energized by a central pulsar, the collapsed core of the exploded star.
http://www.spitzer.caltech.edu/images/2060-sig08-016-Vivid-View-of-Tycho-s-Supernova-Remnant http://www.spitzer.caltech.edu/images/1276-ssc2004-15a%20-NASA-s-Great-Observatories-Provide-a-Detailed-View-of-Kepler-s-Supernova-Remnant
The glowing remains of a star that has been destroyed in a supernova explosion. The brightest and most famous example is the Crab nebula.