When Cassiopeia A (3C 461) exploded as a supernova some 9,700 years ago, a gigantic, circular shell of gas raced out into space. Today, this still-expanding supernova remnant is too faint to see with amateur equipment, but its powerful energy is detectable at radio wavelengths. The radio output emission is created by high-speed electrons spiraling around magnetic field lines as the expanding cloud collides with thin gas between the stars.
Radio images of the cloud, the brightest radio source outside of the Solar System, show gas racing away from the spot where the star exploded. By calculating this speed and the distance traveled by the gas since the explosion, astronomers estímate that the light from the explosion reached Earth around 1680, creating a 5th magnitude star. No record exists of anyone noticing this short-lived supernova in Cassiopeia.
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