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![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/cb/Cassiopeia_A_Spitzer_Crop.jpg/360px-Cassiopeia_A_Spitzer_Crop.jpg)
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. The supernova occurred approximately 11,000 light-years (3.4 kpc) away in the Milky Way.