supernova remnant

The expanding shell of material created by the ejection of the outer layers of a star that explodes as a supernova. Some supernova remnants are observable visually; others have been detected through their radio and X-ray emission. A shock wave precedes the ejected shell, colliding with and heating the interstellar gas. A reverse shock, moving inwards, is created, which heats the ejected material and the interstellar material, causing it to emit X-rays. Electrons accelerated by the shocks emit radio waves by the synchrotron radiation mechanism. The ejected material breaks up into clumps, so the radiation emitted from the shell often does not make up a uniform ring. A small proportion of supernova remnants, including the Crab Nebula, have a rather different appearance. In these, the synchrotron radiation coming from within the shell far outshines any from the shell itself. This type of supernova remnant has been termed a plerion. A continuing supply of electrons travelling at relativistic speeds is needed to account for the emission. In the Crab Nebula, the known pulsar can produce the electrons, but for plerions where no pulsar has been detected, it is assumed that we are observing at the wrong angle to pick up the pulses from the central pulsar. Some other well-known examples of supernova remnants are Cassiopeia A, Kepler's Star, Tycho's Star and the Cygnus Loop.