According to an Old English manuscript chronicling the history of the Anglo-Saxons, a mysterious "red crucifix" appeared in the "heavens" over Britain one evening in A.D. 774. Now astronomers say it may have been the supernova explosion that sprinkled unexplained traces of carbon-14 in tree rings that year, halfway around the world in Japan. Jonathon Allen, an undergraduate student at the University of California, Santa Cruz, made the connection this week after listening to a Nature podcast. He heard a team of Japanese scientists discussing new research in which they measured an odd spike in carbon-14 levels in tree rings...