My high-school astronomy class has just started chewing on supernovas, and in ticking off the recent bright ones I come up with a fairly short list. There's SN 1987A, of course, though its far-southern declination rendered it invisible to northern observers. Not so with the big bangs in 1572 and 1604, which were chronicled in Europe by Tycho Brahe and Johannes Kepler, respectively. And we have Chinese skywatchers to thank for records of "guest stars" in 1006 (arguably the brightest star in human history) and 1054.
But missing from my list is the supernova that led to the formation of Cas A, a dramatic remnant about 11,000 light-years away that's the brightest radio source in the sky. The problem is that there's no solid evidence that anyone witnessed the star's demise. By running the observed expansion rate in reverse, astronomers estimate that the explosion's burst of light should have reached Earth about 1667. Surely this event should have been the talk of the scientific world back then, but astronomers left no such "huzzahs" in their observing logs.
One clue comes from John Flamsteed, Britain's first Astronomer Royal. On August 16, 1680, he logged a 6th-magnitude star in Cassiopeia that's not there now. In 1980, William B. Ashworth Jr. posited that 3 Cassiopeiae, as Flamsteed had labeled it, was in fact an inadvertent sighting of the supernova. But Flamsteed didn't call particular attention to it, and his carefully recorded coordinates for 3 Cas don't really match where the supernova would have appeared.
Now for something really different! This week, at the annual meeting of the Royal Astronomical Society, British astronomer Martin Lunn and American historian Lila Rakoczy tossed out a radical new idea. They argue that the supernova was widely seen on May 29, 1630 — the birth date of Great Britain's future king Charles II. As Lunn notes in an RAS press release, "The number and variety of sources that refer to the new star strongly suggest that an astronomical event really did take place."
So how is it that royal historians noted the appearance of a star bright enough to be seen in broad daylight but astronomers of that era (not to mention astrologers) did not? All of Cassiopeia is circumpolar from England's latitude, so it surely should have been obvious in the nighttime sky. Moreover, could the dating of the Cas A supernova really be off by more than 10%?
I suspect we haven't heard the last of this story!