Dying giant star, not a peaceful event. It explodes its outer material in a tremendous explosion, irradiating the space with high-energy gamma radiation for many years. The empty space is not just exposed to this radiation. New research suggests that ancient trees on Earth may have evidence of celestial explosions.
“These are extreme events and their potential effects seem to match the tree ring records,” explains Robert Breckenridge, a geologist at the University of Colorado Boulder.
Interestingly, it is difficult to say how often the stars in the galaxy explode. Some different techniques have suggested that there should be one to three local supernovae per century, but the most recent galaxy supernova observation on record was years ago.
There is suggested evidence that supernovae could pass us by, much like the remains of a star that exploded about 120 years ago. So it is likely that our human records are incomplete, yet we would expect to see supernova events quite clearly, if not with our eyes, then with telescopes, bright peaks and fades over time.
Maybe we needed to look a little closer to home to ‘see’ this supernova.
Brackenridge and his team found out what fingerprints might be in a tree ring from a nearby supernova that dates back 10,000 years. And over the last 15,000 years, their results suggest that, close to Earth, there may be four supernovae enough to leave their signature in the trees.
The key lies in the abundance of radioactive isotopes of carbon called carbon-of, or radiocarbon. Radiocarbons on Earth only contain trace amounts compared to naturally occurring carbon isotopes.
It forms in the upper atmosphere under the bombardment of cosmic rays from space. When cosmic rays enter the atmosphere, they interact with local nitrogen atoms to react with the molecules that produce radiocarbons. As global rays continue to flow through space, the Earth receives a more or less constant supply of radiocarbon.
Some of these can be found naturally, in tree rings. And every now and then, large radiocarbon spikes disappear over many years, turning into tree rings. Since solar activity is a significant known source of global rays, these spikes are generally interpreted as evidence of solar flares and storms.
But Breckenridge and his team feel there is another explanation.
“There are really only two possibilities: solar flares or supernovae,” he said. “I think the supernova hypothesis has been dismantled very quickly.”
To test the validity of the supernova hypothesis, he and his team hit the records. First, they made a list of supernovae known over the last 40,000 years, which can be traced back to the nebula supernova fossils they left behind. They then compared the list with a record of radiocarbon spikes in tree rings of the same period.
Interestingly, they discovered that the eight closest supernovae to Earth seem to be compatible with all radiocarbon spikes. And four, in particular, standing outside.
The vine, which exploded about 800 light-years from Earth about 12,300 years ago, is associated with a 3 percent increase in supernova radiocarbon. The G114.3 + 00.3 supernova, which exploded about 7,700 years ago at a distance of about 2,300 light-years, corresponds to an increase of 2 percent.
With a 1.8 percent radiocarbon spike, Vela Jr., which is difficult to measure over time, could have been 2,800 years ago. And finally, HB9, which spread 5,400 years ago, corresponds to a 0.9 percent radiocarbon spike, at a distance of 1,000 to 4,000 light-years.
As far as the evidence goes, this decision is far from over at this stage. Given the difficulty in dating, for example, the vine junior supernova, it cannot be said with certainty that the radiocarbon spike corresponds exactly to the explosion.
But the findings suggest that the hypothesis fully guarantees the investigation.
“It keeps me going,” Breckenridge said, “when I look at the terrestrial record and I say, ‘My God, the predictive and modeling effects are there.’
And, if it is maintained, the tree ring can be an excellent tool for studying the galaxy’s eruption history. They can finally help cut down those ancient supernovae that have proven to be slippery. And, in turn, it can help us calculate the nearest supernova population that will shed light on how often the stars of the galaxy come to Kabum.
Research has been published in International Journal Astrobiology.
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