Zapped! Part II 
Anyone looking up at just the right time on a night in the year 774 might have seen a bright new star in the sky. It didn’t last for long, but it might have had a long-lasting effect on Earth’s atmosphere, causing a spike in the amount of radioactive carbon-14.
Researchers in Japan discovered the uptick in carbon-14 in the tree rings of ancient Japanese cedars. They and others suggested the increase was caused by a sudden burst of gamma rays, either from the Sun or from an exploding star, called a supernova. Interactions between gamma rays and atoms in the upper atmosphere would set off a chain reaction resulting in the production of carbon-14.
But researchers in Germany disagreed. They ruled out the possibility of a large outburst from the Sun. And for a supernova to do the trick, it would have to be within a few hundred light-years of Earth, which would have been visible for weeks across the entire planet. But there’s no record of such an explosion, and no remains of a supernova at the right distance to account for the outburst.
Instead, the researchers suggested the gamma rays came from a gamma-ray burst that was triggered by the merger of two collapsed stellar cores.
The burst of gamma rays would have lasted just a couple of seconds, although the visible fireworks would have lasted a little longer. But that brief outburst would have pelted Earth with gamma rays - which left evidence of their creation in ancient tree rings.
Script by Damond Benningfield, Copyright 2013