© Visual Unlimited/naturepi.comThe clues are in these rings: The rings capture microcosmic traces of particles in our skies - and tell us an explosion occurred
But there is no record of something happening in our skies in that period - except perhaps for one tiny, obscure account by a 13th-century historian.
The problem - and this is where we need to call in Mr Holmes of Baker Street - is that there should be a record.
If this was a supernova - a star exploding deep in space - we should either be able to spot the remains with modern telescopes, or find visual accounts in the written accounts of Chinese and European historians.
To get the technical details out of the way first: Trees capture particles from the atmosphere during photosynthesis, and one particle that gets buried within the annual growth rings is carbon-14.
Carbon-14 forms when cosmic rays - generally caused by massive solar flares, or by supernovae - interact with nitrogen and oxygen in our atmosphere.
In the two cedar trees - and doubtless many other tree records from the period - there was a giant increase of 1.2 per cent of carbon-14.
In comparison, the annual variation of the captured isotope is just 0.05 per cent, making this more than a 20-fold increase.
In recorded history, at least two supernovae have exploded in the skies visible from Earth, their light travelling across light-years to hit the eyes of humans.
In 1006 and 1056, two stars went nuclear - at least, the light from their deaths arrived on Earth in those years.
© EPAThe Crab Nebula exploded in 1054, and was witnessed by Chinese observers, making this one of the largest and earliest cosmic events in recorded history
Yet even such giant events, which impacted on those who saw them enough that the records survive to this day, were not powerful to result in much of a variation in the carbon-14 levels.
So the 774AD explosion must have been on a scale much bigger.
But if a supernova had exploded of a force even just equal to the other two witnessed supernovae, we should be able to witness gas remnants - the corpse of the star - in space. But there is nothing in the skies to suggest this.
The only contemporaneous record is from a 13th-century English chronicler, called Roger of Wendover, who, according to New Scientist, is quoted as saying: 'In the Year of our Lord 776, fiery and fearful signs were seen in the heavens after sunset; and serpents appeared in Sussex, as if they were sprung out of the ground, to the astonishment of all.'
This lends itself to just one other possibility, that of a solar flare. But if that was the case, it would be the biggest solar flare ever recorded by our sun.
© AP...But a giant solar flare would have caused huge Northern Lights, accounts of which would have survived to this day
Researcher Igor Moskalenko, an astrophysicist at Stanford University, California, who has followed the case but is not involved in the original study, says: 'I cannot imagine a single flare which would be so bright.'
Instead, he offers his own hypothesis: 'It may be a series of weaker flares over the period of one to three years.'
Other tree rings have also implied something big happened in in the mid-770s, this time in the UK.
Researchers from Queen's University Belfast, UK, also found the carbon-14 increase - but they have yet to publish their work.
Daniel Baker, a space physicist at the University of Colorado's Laboratory for Atmospheric and Space Physics in Boulder, Colorado also told New Scientist: 'The work looks pretty solid - Some very energetic event occurred in about 775.'
"The Crab Nebula exploded in 1054, and was witnessed by Chinese observers, making this one of the largest and earliest cosmic events in recorded history"
No, a nebula doesn't explode. In 1054, a star exploded violently -- went supernova -- and the Crab Nebula consists of the expanding remnants of that stellar event. (I just can't help it, folks!)
The Chinese didn't witness the Crab Nebula in 1054, because it didn't yet exist.
Yeah, I know -- "Picky, picky, picky..."