Cassiopeia A (Cas A) is a supernova remnant in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. It had a flux density of 2720±50 Jy at 1 GHz in 1980; its flux density at 1 GHz is decreasing at a rate of 0.97±0.04 percent per year.. This decrease means that at frequencies below 1 GHz Cas A is now less intense than Cyg A. The supernova occurred approximately 11,000 light-years (3.4 kpc) away in the Milky Way. The expanding cloud of material left over from the supernova is now approximately 10 light-years (3 pc) across. Despite its radio brilliance, however, it is extremely faint optically, and is only visible on long-exposure photographs.
It is believed that first light from the stellar explosion reached Earth approximately 300 years ago 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 (although it is possible that it was recorded as a sixth magnitude star 3 Cassiopeiae by John Flamsteed on August 16, 1680). 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.
The expansion shell has a temperature of around 50 million degrees Fahrenheit (30 megakelvins), and is expanding at 4000−6000 km/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.
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