sciencehabit (1205606) writes "In 775 C.E., while Charlemagne was ruling his Frankish kingdom, something mysterious struck Earth. An analysis of the rings of two Japanese cedar trees reveals that from 774 to 775 C.E., the atmospheric level of radioactive carbon-14 jumped by 1.2%. This indicates that cosmic rays—high-speed, charged particles from space—bombarded our planet and converted some atmospheric nitrogen-14 into carbon-14. The scientists argue against two logical suspects: solar flares are too weak to do the job, and no supernova explosion was seen at the time, nor do any nearby supernova remnants date back to Charlemagne's time. So the cause remains a mystery, but whatever it was, something similar could presumably strike again."
Link to Original Source