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A massive dying star is not a peaceful event. It expels its outer material in a colossal explosion, irradiating space with high-energy gamma radiation for several years. However, empty space isn’t the only thing exposed to this radiation. New research suggests that Earth’s ancient trees may contain evidence of these celestial explosions.
“These are extreme events and their potential effects appear to match tree ring records,” explained geoscientist Robert Brakenridge of the University of Colorado Boulder.
Interestingly, it is quite difficult to tell how often stars in the Milky Way explode. Several different techniques have suggested that there should be one to three local supernovae per century, but the most recent recorded Milky Way supernova observation dates back more than 400 years.
There is evidence to suggest that supernovae can overrun us, like the rest of a star thought to have exploded some 120 years ago. So our human recordings are likely to be incomplete, although we would expect to be able to see supernova events quite clearly, if not with our eyes, then with telescopes, as brightness increases and dims over time.
Maybe we had to look a little closer to home to “see” these supernovae.
Brakenridge and his team found what may be fingerprints of nearby ancient supernovas in tree rings dating back 40,000 years. And over the past 15,000 years, the results indicate that there may have been four supernovae close enough to Earth to leave their signature on trees.
The clue is in the abundance of a radioactive isotope of carbon called carbon-14, or radiocarbon. Radiocarbon is found on Earth only in trace amounts compared to other naturally occurring carbon isotopes.
It was formed 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 trigger a nuclear reaction that produces radiocarbon. As cosmic rays constantly flow into space, the Earth receives a more or less constant supply of radiocarbon.
Some of these can be found, of course, in tree rings. And from time to time, a large radiocarbon spike appears in tree rings, fading over the course of several years. Since a rather significant known source of cosmic rays is solar activity, these peaks are usually interpreted as evidence of solar flares and storms.
But Brakenridge and his team think there is another explanation.
“There are really only two possibilities: a solar flare or a supernova,” he said. “I think the supernova hypothesis was shelved too quickly.”
To test the validity of the supernova hypothesis, he and his team hit records. First, they compiled a list of supernovae known over the past 40,000 years, traceable through the nebula’s supernova remnants they leave behind. Then, they compared this list to the record of radiocarbon spikes in tree rings from the same period.
Interestingly, they found that the eight closest supernovae to Earth all appear to correspond to a radiocarbon peak. And four, in particular, stood out.
The supernova Vela, which exploded about 12,300 years ago at a distance of 800 light years from Earth, corresponded to a 3% increase in radiocarbon. Supernova G114.3 + 00.3, which exploded about 7,700 years ago at a distance of about 2,300 light years, corresponded to a 2% increase.
Vela Jr., whose timing is difficult to estimate, may have occurred 2,800 years ago, corresponding to a radiocarbon peak of 1.4%. Finally, HB9, which exploded 5,400 years ago at a distance between 1,000 and 4,000 light years, corresponds to a radiocarbon peak of 0.9%.
As for the evidence, this is far from conclusive at this stage. Given the difficulty of dating, for example, the supernova Vela Jr., it cannot be said with certainty that the radiocarbon peak absolutely corresponds to the explosion.
But the results suggest that the hypothesis absolutely merits further investigation.
“What keeps me going,” Brakenridge said, “is when I look at the Earth record and say, ‘My God, the predicted and modeled effects seem to be there.’
And, if it holds up, tree rings could be an excellent tool for studying the history of our galaxy’s explosions. They could finally help pinpoint those ancient supernovae that turned out to be slippery. And, in turn, this could help us put together a census of nearby supernovae that sheds light on how often Milky Way stars become kaboom.
The research was published in International Journal of Astrobiology.
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