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supernova remnant(SNR) The expanding shell of gas from a supernova explosion, consisting of the supernova ejecta and ‘swept-up’ interstellar gas. Young (< 1000 year old) supernova remnants are generally optically faint but are fairly strong radio and X-ray sources; the Crab nebula is exceptionally bright because it is energized by a central pulsar. Older supernova remnants appear as rings of bright filaments, again with associated radio and X-ray emission. Supernova remnants that have been observed at sufficiently high resolution can be loosely classified into two types. In shell SNRs, which constitute about 90% of all SNRs (including Tycho and Kepler), most of the observed radiation comes from a filamentary, often spherical shell; they seem to have no central power source and their luminosity is exclusively derived from the interaction of the supernova shell with the external medium. Plerions (or filled or filled-center SNRs) are now widely thought to be powered by a central pulsar – the Crab nebula is the archetypal example – and the observed radiation originates from the whole of the remnant. Compression by an expanding supernova remnant can trigger star formation in interstellar clouds. See also emission nebula.
supernova remnant[¦sü·pər′nō·və ′rem·nənt]
A nebula consisting of an expanding shell of gas that has been ejected by a supernova. Abbreviated SNR.