Cassiopeia A (Cas A) is a supernova remnant in the constellation Cassiopeia around 11,000 light-years away and is approximately 10 light-years in diameter. Today it is the most powerful extrasolar radio source in the sky.

It is the remnant of a supernova explosion that could have been observed on Earth around 1680 if it had not occurred behind clouds of gas and dust.The supernova may have appeared as a sixth-magnitude star, which astronomer John Flamsteed cataloged as star 3 Cassiopeiae on August 16, 1680, but has since been untraceable.Cassiopeia A was discovered in 1947 and optical identification was achieved in 1950. Cassiopeia A was the youngest known supernova remnant in our galaxy until the discovery of supernova remnant G1.9+0.3.

Using a light echo, it was possible to subsequently observe the historical supernova spectrally. A light echo is caused by the scattering of dust particles from interstellar matter; Because the dust lies outside the Earth-supernova line of sight, the path of the light is longer, and the explosion flash can still be studied today. The MIPS instrument on the Hubble Space Telescope was used to examine the infrared spectrum of the supernova, which therefore belongs to type IIb. Accordingly, Cas A is the supernova remnant of a core collapse supernova of a former red supergiant that lost its hydrogen-rich atmosphere due to stellar wind before the explosion.

During observations with the Chandra X-ray telescope, a point-shaped X-ray source was found near the center of Cas A = 3C 461 = SNR G111.7-2.1. Since no variability of the source could be found in either the optical or X-ray range, it is very unlikely that it is a cataclysmic variable in the foreground or an active galactic nucleus in the background of Cas A. The X-ray spectrum is best described as that of a neutron star with a polar spot at a temperature of approximately 2.8 million Kelvin.

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