A supernova remnant. Image: NASA
When a star explodes into a supernova, it sends out cataclysmic shockwaves that incinerate anything within its blast radius. It’s no wonder that the terrifying notion of ending up in the fallout of these stellar detonations has become a common trope of science fiction (Outer Wilds fans: hello).
While humanity has so far avoided brushes with dangerously close supernovae, past Earthlings may not have shared our good fortune, reports a study published on Tuesday in the Proceedings of the National Academy of Sciences.
Led by Brian Fields, a physicist at University of Illinois, Urbana-Champaign, researchers propose that a mass extinction at the end of the Devonian period, some 359 million years ago, was triggered by the explosion (or explosions) of nearby stars.
The speculative supernova would have been beyond the “kill distance,” Fields’ team said, meaning that it did not total our planet. Still, a stellar explosion at a distance of about 65 light years would have bombarded the atmosphere with damaging radiation and would have brightened Earth’s skies, even in the daytime.
“Of course, it is irresistible to ask what it would look like if you were lucky/unlucky enough to see it,” Fields said in a call. “At its brightest, it would be putting out more light than the Full Moon.”
“The light of the Moon is spread over the face of the Moon, so it’s not a point in the sky,” he added. “But the supernova would be this very concentrated dot of bright light.”
This supernova, if it did exist, may have contributed to a complicated brew of pressures faced by life at the end of the Devonian, ultimately tipping the scales towards an extended mass extinction that killed more than 70 percent of animal species.
Biodiversity had been declining for millions of years before a series of major die-offs kicked off. Those events culminated in the Hangenberg Crisis, which is estimated to have wiped out nearly half of vertebrate families. A weakened ozone layer may have been the “kill mechanism” for the Hangenberg Crisis, according to a recent study in Science Advances. In the new study, Fields’ team outlines how radiation from supernovae, known as cosmic rays, could have been the major driver for this ozone loss.
Getting pelted by ultraviolet light, X-rays, and gamma rays could deplete the protective ozone layer, causing dangerous radiation to reach vulnerable ecosystems on its surface. Cosmic rays damage genetic material and boost the odds of developing diseases such as cancer, which is why it is so crucial to maintain an intact ozone layer even in the modern era.
Massive stars often hang out with their stellar siblings by spending their lives in binary systems with at least one other star. For this reason, the odds of multiple supernovae occurring within a short time span of a few million years are relatively high.
“Massive stars are incredibly social,” Fields explained. “What we’re suggesting is that since massive stars are born in clusters, if one hits you, it’s entirely possible or even likely that another one will hit you. So maybe multiple of these punctuated extinction events at the end of the Devonian could be due to supernovae.”
What’s more, traces of the hypothetical supernova(e) may be embedded into sediments and fossils laid down in this long-lost era, which could provide direct evidence of an explosion if they are ever detected in future studies.
Scientists have already detected the signature of a recent supernova in radioactive isotopes, which are variations of normal atoms, embedded in the seafloor. These particles rained down on Earth about two million years ago, after a star some 160 light years away blew up. Adrian Melott, co-author of the new study and professor emeritus at the University of Kansas, has already proposed that this supernova may have contributed to the extinction of the giant Megalodon species of shark.
Finding radioactive isotopes that date back to the Devonian, however, will be “a tall order,” Fields said. The iron isotopes that tipped off scientists to the recent supernova would not survive hundreds of millions of years. Only plutonium-244 and samarium-146 could hint at the presence of an explosion this old, but it will still be a challenge to detect it in Devonian sediments.
“I’m of the school that people can be surprisingly clever, so even if something sounds very hard, it’s better to suggest it and see if someone can be clever than not mention it,” Fields said. “That’s the spirit with which we’re writing this.”
If scientists were to pull off this feat, it would confirm that stars that died hundreds of millions of years ago may have taken some Earthlings to the grave with them, forever altering the course of life on our world. Detecting plutonium in these ancient samples would also reveal, for the first time, that stellar explosions are a source of this exotic element.
“That would be fun because you’re not only learning about extinctions, you’re learning about astrophysics,” Fields said. “You’re learning about element production in the cores of stars by looking at 360-million-year-old dead things.”
In case you’re worried about becoming a dead thing because stars close to Earth decide to blow up now, rest assured. We are far beyond the ominous “kill distance” of any dying stars.
“There are no threatening supernova candidates anywhere near us now.” Fields said. “That’s a super important point. I’m not trying to add to the misery of 2020.”
“If you care about your descendants, then you need to worry about it on million-year timescales,” he concluded. “But not today.”
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