A few weeks ago, Jonathon Allen, a biochemistry major at the University of California, Santa Cruz, was listening to the Nature podcast when he heard about a team of researchers in Japan who had found an odd spike in carbon-14 levels in tree rings. The spike probably came from a burst of high-energy radiation striking the upper atmosphere, increasing the rate at which carbon-14 is formed (see ‘Mysterious radiation burst recorded in tree rings‘).
But there was a problem: the only known causes of such radiation are supernova explosions or gigantic solar flares, and the researchers knew of no such events in AD 774 or 775, the dates indicated by the tree rings.
Which gave Allen an idea. He pored over the pertinent entries in the Anglo-Saxon Chronicle, and in 774 found this:
Allen found a reference to a “red crucifix” that appeared in the heavens “after sunset”.
“It made me think it’s some sort of stellar event,” Allen says. Furthermore, he notes, the redness might indicate that the source was hidden behind a dust cloud dense enough to scatter all but a small amount of red light. Such a cloud might also prevent any remnants of the proposed supernova being seen by modern astronomers.
While Allen apparently wasn’t the first to spot this reference, give him credit for knowing what to do with the information once he got it.