As the article explains, Earth is constantly hit by cosmic rays, or radiation from space, but our atmosphere absorbs it and converts it into Carbon-14. This isn’t radiation like the threat from the Fukushima reactors, but it is more like miscellaneous particles flying through space from all kinds of sources. Earth usually gets hit by a steady amount of it every day, and so we have a steady amount of Carbon-14 daily, yearly, etc. More or less.
Carbon-14, or “radiocarbon”, is not the same as the standard Carbon-12 (tanso 炭素 in Japanese by the way), so it only exists in very small amounts on earth, and apparently trapped in trees. It also exists in your body by the way, and thus people are “naturally” radioactive. ;p
Anyhow, in the years 774 and 775, scientists in Japan have noticed that there was a 1% increase in Carbon-14 in tress from that time, and it’s not clear why. The 1% increase represents a big change, but it’s not clear what the source is.
Since Carbon-14 is produced in the atmosphere from cosmic rays, something hit the Earth with a lot of energy from 774-775, but what?
Researchers don’t believe it’s the Sun because solar-flares usually aren’t strong enough, and the timing doesn’t seem right.
So, they thought it might come from an exploding star. In space, most stars die in a typical way: when they start to run out of fuel, they swell into a red-giant star, then eventually collapse into a tiny white dwarf. Our sun will do this in 5 billion years.
But if a star is really heavy, at least 8 times more massive than our sun, it will have a different death called a supernova. The star’s own gravity is so strong, it will collapse, then cause an extremely powerful explosion of energy. Supernovae are among the brightest things in space. So, the energy from a supernova would definitely hit our atmosphere and create more carbon-14.
But supernova are rare and easy to track, because they leave behind colorful clouds called nebula, and scientists can estimate when nebula were created:
There are no nearby supernova that happened around 774-775 that could explain the mystery.
So, maybe there are other possibilities. Maybe Earth was hit by a distant gamma-ray burst. But some believe that if a gamma-ray burst hit Earth, all life would immediately die. Also, the evidence points to a long-sustained increase across 2 years, not a quick burst.
Maybe it’s a supernova that’s untracked? It’s possible, but supernova are rare and really obvious.
Maybe it was energy from a much more distant object that finally hit Earth? It’s possible, but hard to prove.
Maybe something unusual happened to the Sun? That’s also possible, but hard to prove.
Anyhow, until there’s more evidence, it’s a scientific mystery of what happened to the Earth in 774 and 775.