object might be a supernova remnant, and the race for a full description was on.
More evidence that Cas A was a supernova remnant came with the x-ray images taken from the Einstein Observatory, an Earth-orbiting satellite launched by NASA in 1978. Measuring the speed of the nebulaΓÇÖs expansion, astronomers deduced that the light from the explosion probably reached Earth in the mid-1600s. Seen from Earth, the exploding star would have been brighter than any object in the sky other than the Sun and full Moon. Indeed, it should have been visible even in daylight. And the supernovaΓÇÖs position in the sky would have made it visible throughout the Northern Hemisphere. Yet despite
