3C461, also known as
Cassiopeia A (Cas A) is a supernova remnant in the Milky Way; it is
believed that the supernova explosion was recorded visually on 1680 by
John Flamsteed, the first British Astronomer Royal, who catalogued it
as star "3 Cassiopeiae", although, because the position of "3
Cassiopeiae" does not precisely match that of 3C461, 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. In that case, there would be no historical records of any
sightings of the 3C461's progenitor supernova, probably due to
interstellar dust absorbing optical wavelength radiation before it
reached Earth.
3C461 means that
it is the 461st object in the Third Cambridge Catalogue of Radio Sources,
it was one of the first objects discovered in early years of radio
astronomy. One of the brightest radio source was in the region of the
constellation Cassiopeia and so was named Cassiopeia A (then Cas A); indeed it is the brightest radio source in the sky beyond
our solar system. Despite so powerful
radio emission, its optical appearance is extremely faint; in fact,
whereas Cas A was among the first discrete radio sources to be
found, in 1947, the optical component was first identified in 1950.
