Cassiopeia A
Encyclopedia : C : CA : CAS : Cassiopeia A
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| Brightest radio source beyond our solar system |}
 Cassiopeia A (Cas A) is a supernova remnant in the constellation Cassiopeia and the brightest extra solar radio source in the sky, with a flux of 2720 janskies at 1 GHz. The supernova occurred approximately 10,000 light years away in the Milky Way. The expanding cloud of material left over from the supernova is now approximately 10 light years across, has an apparent magnitude of 6, and is faintly visible with the naked eye. It is believed to be approximately 300 years old but there are no historical records of any sightings of the progenitor supernova, probably due to interstellar dust absorbing optical wavelength radiation before it reached Earth. Possible explanations lean toward the idea that the source star was unusually massive and had previously ejected much of its outer layers. These outer layers would have cloaked the star and reabsorbed much of the light released as the inner star collapsed. Cas A is 3C461 in the Third Cambridge Catalogue of Radio Sources and G111.7-2.1 in the Green Catalog of Supernova Remnants. It is known that the expansion shell has a temperature of around 50 million degrees Fahrenheit (30 megakelvins), and is travelling at more than ten million miles per hour (4 Mm/s). Cas A is the strongest radio source in the sky beyond our solar system, and was among the first discrete sources to be found, in 1947. The optical component was first identified in 1950. In 1979, Shklovsky predicted that Cas A had a black hole (Shklovsky, 1979). In 1999, the Chandra X-Ray Observatory found a "hot point-like source" [link] close to the center of the nebula that is quite likely the neutron star or black hole predicted but not previously found. (Pavlov, et al, 2000) Calculations working back from the currently observed expansion point to an explosion around 1667, although astronomer William Ashworth and others have suggested that the Astronomer Royal John Flamsteed may have inadvertently observed the supernova on August 16, 1680, when he catalogued a star near its position. At any rate, no supernova in the Milky Way has been visible to the naked eye from Earth since. References
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