An artist's impression of a star going supernova and shooting out a gamma ray burst. Via
The tree-based key, in this case, is carbon. Most of the carbon in nature is the common and stable isotope carbon-12 that has six protons and six neutrons in its nucleus. But there’s another, less common but still present carbon isotope in nature: carbon-14, a radioactive atom with eight protons and eight neutrons that decays into nitrogen.
Carbon-14 is created when cosmic rays – those high energy particles that hit the Earth from space – interact with the nitrogen in our atmosphere. It’s a regular process that keeps the levels of carbon-14 on Earth at a more or less stable level. But in the Japanese trees, researchers noted a significant spike in carbon-14. By reading the trees’ rings, they determined the spike came from an event 1,200 years ago, in the year 774 or 775.
Initial research into what this event could have been quickly ruled out the explosion of a nearby massive star – a supernova. These are bright. Some astronomer would have seen the light and noted it, but no record exists that matches the date of the carbon-14 spike found in the trees. Solar flares are another way the Earth can be hit with radiation, but these generally aren’t very powerful. A solar flare big enough to produce the levels of carbon-14 researchers found in the trees would have to be more than 20 times larger than the average flare. It would have been bright, and someone definitely would have seen it and taken a note.
Gamma ray bursts are massive events, as suggested by this render. Via
But there’s another possible culprit: a gamma-ray burst.
Gamma ray bursts are a byproduct of a black holes’ birth, and there are a few ways we get black holes. One, and the most common way, is for a massive star to explode and collapse on itself at the end of its life. The forces involved send out beams of energy that we see as high-energy gamma rays.
Black holes can also form when two neutron stars merge. Neutron stars are failed black holes, stars that died but didn’t have the energy needed to compress into a black hole so instead became an incredibly small and dense ball of neutrons. So say there are two stars orbiting each other – a binary system – and both turn into neutron stars. Its possible that these orbiting dense balls orbiting each other would eventually fall into one another. When two things this heavy merge, it’s violent, and one of the byproducts is a short-lived twin beam of energy.
This is what scientists think happened based on their reading of the Japanese trees.
What two neutron stars might look like as they smash together. Via
Neutron star collisions and the resulting gamma ray bursts aren’t rare; we see them all the time. But we see them in distant galaxies, places too far for the radiation to hit the Earth and produce the spike of carbon-14 scientist found in Japan. So if this is the explanation behind the 774/775 radiation burst, the merging neutron stars must have been much closer to home. The event would have to have been somewhere between 3,000 and 12,000 light-years from the Sun. Any closer and the Earth would have been fried, and further and we wouldn’t be able to see any after effects.
It all works in theory, but finding the proof is another matter altogether. The short lived gamma ray burst from neutron stars merging is extremely quick, less than two seconds. Its unlikely anyone would have seen the event to record it, but double checking astronomical records is one way to go. Astronomers could also look for the likely remnant of the burst, a 1,200-year-old black hole or a neutron star 3,000 to 12,000 light-years away that lacks the characteristic gas and dust seen after a star goes supernova.
Finding the proof that merging neutron stars caused the gamma ray burst that hit the Earth with enough radiation to account for the exact rise in carbon-14 scientists found in tress in Japan will be like finding a needle in a universe-sized haystack. I guess that’s what grad students are for.