Cassiopeia A (3C 461)
Cas A lies about 10,000 light-years away in Cassiopeia (R.A. 23h 23.4m, Dec. +58° 48.9') and coincides with a shell-type remnant of a Type II supernova whose light reached Earth in about 1667. Optically, it is a faint nebula, 10 light-years across, with an expansion velocity of about 800 km/s and a mass of a few solar masses. It is also an extended source of soft X-rays.
Related category• NEBULAE AND STAR CLUSTERS
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