By Sophia Dengo, CNN
History books may tell you that in the eighth century, the Moors invaded Spain and Mayan civilization was on the decline, but they don't say anything about the Earth being irradiated.
That event is not documented, but astronomers say a collision in space at that time could have resulted in the high levels of carbon-14 and beryllium-10 found in trees from the eighth century.
Astronomers Valeri Hambaryan and Ralph Neuhauser, based at the Astrophysics Institute of the University of Jena in Germany, published results in the journal Monthly Notices of the Royal Astronomical Society that suggest that two "compact stellar remnants" - which could be neutron stars, black holes or white dwarfs - collided and merged, resulting in a short-duration gamma-ray burst that hit Earth.
The astronomers started looking into this possibility on the heels of results announced in 2012.
Scientist Fusa Miyake suggested that high levels of carbon-14 and beryllium-10 found in tree rings from 775 A.D. could be evidence that Earth was hit by a flash of radiation around that time. The historical record rules out that the radiation could have come from a supernova or solar flare.
There is no human documentation of this happening in the eighth century. But Hambaryan and Neuhauser have explanations for why not: The collision of compact stellar remnants would have released a short burst of radiation, a couple of seconds in duration, without generating visible light. In addition, because life on Earth didn't appear to suffer adverse effects (for example, extinction!), the merging remnants could have been no closer than 3,000 light-years from Earth.
So what does this mean for us modern-day Earth dwellers? Could another similar event affect us?
Neuhauser tells CNN, "Highly energetic events take place in our galaxy and they could affect life on Earth, but we are protected against risks by the ozone layer."
He added: Electronics and orbiting satellites are more at risk from solar flares than from an event like the burst that may have occurred in the eighth century.