Blast from an unseen explosion.
Here, photographed in X-rays by the HEAO-2
satellite, supernova remnant Cassiopeia A is the
expanding blast from a star that exploded in
the late seventeenth century. Historical
records suggest that astronomers of the time
missed the event, although one observer may
have seen a faint star at the location of the
explosion. Cassiopeia A is one of the most
conspicuous objects found with X-ray telescopes,
although only very dim nondescript
wisps ofgas are seen on visible-light
photographs.
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