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Supernova remnants, like this one in the constellation of Cassiopeia, named Cas
A, are the final debris from the supernova explosions that mark the end-point
of the lives of massive stars. The collision between the supernova shell,
expanding outwards at a speed of around 10,000 km per second, and interstellar
gas heats the gas to very high temperature. As a result in this ROSAT image we
can see the 10 million-degree gas in the supernova shell glowing brightly in
the X-ray band. The original supernova explosion that produced Cas A took place
around 400 years ago but was not seen by astronomers according to historical
records. The probable explanation for this is that the supernova explosion
occurred in a relatively obscured and distant part of our Milky Way,
making it much less dramatic than other supernovae in recorded history such
as those associated with the Crab Nebula, or SN1006.