On 16 August 1680 Flamsteed catalogued a star, 3 Cassiopeiae, that later astronomers were unable to corroborate. Three hundred years later, the American astronomical historian William Ashworth suggested that what Flamsteed may have seen was the most recent supernova in the galaxy's history, an event which would leave as its remnant the strongest radio source outside of the Solar System, known in the third Cambridge (3C) catalogue as 3C 461 and commonly called Cassiopeia A by astronomers. Because the position of "3 Cassiopeiae" does not precisely match that of Cassiopeia A, and because the expansion wave associated with the explosion has been worked backward to the year 1667 and not 1680, some historians feel that all Flamsteed may have done was incorrectly note the position of a star already known.