Radiation in centuries-old tree rings reveals what occurred. If the event were to repeat itself today, it could wreak havoc on humankind’s technology. Phys.org writes:
A nearby short duration gamma-ray burst may be the cause of an intense blast of high-energy radiation that hit the Earth in the 8th century, according to new research led by astronomers Valeri Hambaryan and Ralph Neuhӓuser, based at the Astrophysics Institute of the University of Jena in Germany.
In 2012 scientist Fusa Miyake announced the detection of high levels of the isotope Carbon-14 and Beryllium-10 in tree rings formed in 775 CE, suggesting that a burst of radiation struck the Earth in the year 774 or 775.
Drs. Hambaryan and Neuhӓuser suggest that two compact stellar remnants, i.e. black holes, neutron stars or white dwarfs, collided and merged together. When this happens, some energy is released in the form of gamma rays. If they are right, then this would explain why no records exist of a supernova or auroral display.