Supernovae, the spectacular death rattles of the largest stars, are the astronomical gifts that keep giving.
The stellar explosions can add a temporary new jewel to the night (and sometimes day) sky, and they leave behind spectacular and intricate formations known as remnants. A certain class of supernovae helped astronomers realize that the universe’s expansion is speeding up, leading to the “discovery” of dark energy pervading our cosmos. And now, it seems, supernovae might ultimately be to blame for one of astronomy’s longest running mysteries: the origins of cosmic rays.
Scientists have known about these ridiculously energetic and high-velocity particles for nearly a hundred years. In daily life, cosmic rays may be familiar as the source of extra radiation airline passengers are exposed to. However scientists have been uncertain about where cosmic rays come from. The extreme conditions of temperature and speed that accompany supernovae and their remains made them a natural starting point for guesses. Now two separate Science papers finally provide evidence that cosmic rays do indeed come from supernovae remnants.
A page from the Anglo-Saxon Chronicle. The entry for
774 AD refers to a “red crucifix” appearing in the sky.
Earlier this month, researchers found that Japanese trees had preserved a centuries-old spike in the atmosphere’s proportion of carbon-14, apparently caused by a burst of cosmic rays that hit earth between 774 and 775 AD. But that left a perplexing mystery: the most likely source of that excess carbon-14 would be cosmic rays emitted by a supernova. But any supernova powerful enough to generate carbon-14 should have been visible to people alive then, and there was no known record of what should’ve been a pretty notable event.
Enter Jonathon Allen, a polymath undergraduate who majors in biochemistry and also has a deep interest in history. Allen dug around in a contemporary manuscript and found a reference to a “red crucifix” appearing in the sky in 774 AD. This celestial signal may have marked a supernova that birthed the cosmic rays and created the trees’ carbon-14 peak.