- Cassiopeia A
Cassiopeia A is a supernova remnant (SNR) located within the stream of the Milky Way between Cassiopeia and Cepheus. It is the remains of a supernova that exploded at a distance of around 11,000 LY from us. There is no difinitive record of anyone seeing this exploding star. The rate of expansion of the SNR suggests that it could have been visible to observers on Earth around the year 1667. It has been suggested that the British astronomer John Flamsteed may have accidently observed it when he recorded a 6th-magnitude star near the SNR location in August of 1680. It has also been suggested that the supernova was not observed because it was shrouded by dust and gases previously ejected by the star before its final explosion. Like other SNRs, Cassiopeia A is a strong radio source, and has the designation 3C461 in the Third Cambridge Catalog of Radio Sources.
This image reveals Cassiopeia A as an incomplete shell with the primary components being a more prominent Northern Arc and a less prominent Southern Arc. This structure is similar to that of the Veil Nebula in Cygnus, which also reveals itself as a fragmented shell. The physical size of the Cassiopeia A SNR is around 10 LY across. In 1999, the Chandra X-Ray Observatory detected a "hot, point-like source" of X-Rays close to the center of the expanding SNR. It is possible that this is the neutron star that was left after the supernova explosion.
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