Cassiopeia A

Supernova remnant in the constellation Cassiopeia

Cassiopeia A (Cas A;) is a supernova remnant (SNR) in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies below 1 GHz. The supernova occurred approximately 11,000 light-years (3.4 kpc) away within the Milky Way; given the width of the Orion Arm, it lies in the next-nearest arm outwards, the Perseus Arm, about 30 degrees from the Galactic anticenter. The expanding cloud of material left over from the supernova now appears approximately 10 light-years (3 pc) across from Earth's perspective.

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