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Cassiopeia AAn intense radio source 2.8 kiloparsecs distant in the constellation Cassiopeia. It has the largest flux density at low frequencies of all the discrete radio sources (apart from the Sun), the value being about 8200 jansky at 178 megahertz, and a spectral index of –0.77. The flux drops by about 1% per year. It is a supernova remnant – probably of a supernova explosion (unrecorded) of a massive (˜50 MO) star in the late 17th century – and has a ringlike structure about four arc minutes in diameter. The radio radiation is by synchrotron emission and the radio shell is expanding at 2300 km s–1. It is also an extended source of soft X-rays.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
Cassiopeia A[‚kas·ē·ə′pē·ə ′ā]
One of the strongest discrete radio sources, located in the constellation Cassiopeia, associated with patches of filamentary nebulosity which are probably remnants of a supernova.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.