SOMETHING with almost unimaginable power hit Earth in AD 775. Europe was in the grip of the dark ages, yet the skies were alight. “Fiery and fearful signs were seen in the heavens after sunset; and serpents appeared in Sussex, as if they were sprung out of the ground, to the astonishment of all,” recorded the 13th-century English chronicler Roger of Wendover.
We don’t just have his word for it. In the past year, new evidence has come to light confirming that something cataclysmic took place in the solar system that year. But what? There are no signs of a mass extinction or an environmental disaster which would normally accompany such an event. More mysterious is that no trace of it appears in the heavens today.
The only clues to what happened are found locked inside ancient tree rings. What they reveal is shocking. A supremely powerful blast of radiation struck our atmosphere out of the blue, changing its composition for millennia. While the medieval world emerged unscathed, we wouldn’t be so lucky today. Our technology-reliant society would be devastated by such an event: satellites would fry, power stations would melt, and we would be without communications and power for years. We might never bounce back.
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This makes identifying the source of the blast a priority. While various perpetrators have been proposed, we are now closing in on an answer. And the culprit, it seems, is alarmingly close to home.
With no technology to damage, the effects on the medieval world were slight. We would have missed them completely had it not been for Fusa Miyake of Nagoya University in Japan and her colleagues. They were searching for evidence of large, ancient radiation storms in tree rings from two long-lived Japanese cedar trees. In particular, they were looking for raised levels of carbon-14, a radioactive isotope that is created when energetic particles from space strike Earth’s atmosphere.
Archaeologists use carbon-14 to date organic artefacts because all living things absorb carbon. Trees are particularly good at recording any variations because they only grow for a few months of the year in many climates, so you can pinpoint the amount of carbon-14 in the air at a specific time.
What Miyake found was a startling spike in carbon-14 levels around the year AD 775. In other words, a radiation storm – and a big one at that. Nevertheless, a single detection is not definitive. To truly believe the discovery, a corroborating measurement was needed from somewhere else in the world.
“We saw and became interested in measuring the effect ourselves,” says , a physicist at the University of Oulu, Finland. His team turned to that once grew near the river Main. The measurement was clear. “We precisely confirmed the results,” he says. “Whatever it was in AD 775, it was a global phenomenon, and that points to an extraterrestrial source.” So what was it?
Miyake’s team calculated the energy needed to produce such enhanced levels of carbon-14. Her estimates for the energy were colossal – so large, in fact, that only exploding stars could provide the necessary deluge of particles. The problem is, no known supernova remnant is close enough to Earth to match the correct age. Nor are there any nearby dust clouds that could hide a remnant from our gaze.
“It wasn’t a supernova,” says Usoskin firmly. Instead, he and others cast a wary eye towards the sun. Reasoning that a solar flare would have produced aurorae on Earth, he went looking for evidence in the historical record.
Although there were no systematic observations of the night sky in the medieval world, people watched the sky for signs that could be interpreted religiously. Usoskin uncovered Roger of Wendover’s account of fiery and fearful signs and serpents in the Anglo-Saxon Chronicle. Fiery and fearful sounds a lot like aurorae. And although it is tempting to dismiss the reference to serpents as apocalyptic embroidery, Usoskin believes this points to the sinuous way that aurorae move across the sky. “Anyone who has seen aurorae knows that they look like serpents,” he says.
But there was no way to square the idea of a solar flare with Miyake’s energy estimate, which was at least 1000 times too large.
Nevertheless, another researcher was looking suspiciously at the sun. at the University of Kansas read Miyake’s paper and thought that something looked wrong. In calculating the energy of the solar flare needed to produce the radiation storm on Earth, Miyake had assumed the particles were flung from the sun equally in all directions. On the contrary, said Melott, particle eruptions from the sun are fairly well directed into space, like a geyser shooting from the ground.
Correcting this assumption dropped the necessary energy by one-hundredth. “At that energy output, the radiation storm is more likely to have its explanation in the sun,” says Melott.
Kamikaze comets
No one should think that this makes the cataclysm any less impressive. It is at least 20 times bigger than the biggest solar storm ever recorded, by English astronomer Richard Carrington in 1859. “We can absolutely say that what happened then was bigger than Carrington,” says Usoskin. It is also 100 times bigger than any flare in the last century, according to from Ben-Gurion University of the Negev in Israel.
As for whether the sun is capable of behaving like that, Eichler thinks it is, but “only with a bit of help.” He proposes that a comet collided with the sun and the resultant explosion provided so much energy that it drove a super solar flare. In his view, the power of the explosion came from the momentum of the comet. By the time the mountain of ice and rock struck the solar surface, it would have been travelling at more than 600 kilometres per second. “That’s per second,” emphasises Eichler, “not per hour.”
Video: Watch the largest sungrazing comet survive a roasting
Comets crash into the sun all the time. Known as sungrazers, some reach the surface, but most explode some way above it. They are so small, however, that the energy released by their destruction goes unnoticed. Eichler estimates that to spark a superflare, a comet the size of Hale-Bopp, which reappeared in the sky in 1997 and is estimated to be between 40 and 80 kilometres across, would be needed.
The largest sungrazing comet actually observed was , which flirted with fiery death in 2011. Drawing to within 137,000 kilometres of the sun and estimated to be 0.5 kilometres across, it was sufficiently distant and large enough to survive the roasting. Yet Eichler thinks the shock wave it created in the solar atmosphere during its high-speed fly-by triggered a measurable eruption of solar particles.
Cosmic misfortune robbed him of a direct observation because Lovejoy’s closest approach took place on the sun’s far side, out of direct view from Earth. Simultaneously, an eruption of solar particles was seen expanding into space behind the sun. For Eichler, this was tantalising and frustrating in equal measure. “The experts say that we can’t know that the explosion was triggered by Lovejoy but the timing was impeccable,” he says.
Now he is biding his time. “It is possible that at some time in the near future a sungrazing comet will produce an energetic particle event and that will teach us a great deal,” he writes in a it ().
Indeed, astronomers already know of one inbound comet that will skim the sun later this year. Comet ISON will draw to within 1 million kilometres of the fiery surface on 14 December. Nevertheless, at the University of Glasgow, UK, who studies the survival rates of sungrazing comets, thinks we are probably safe. “I think it is highly unlikely that this comet will trigger anything,” he says.
That’s because as sungrazers go, it’s not approaching that closely. Brown estimates that a comet would have to hit the sun to pose any risk of sparking a superflare, but he does hedge his bets a little because comets are so unpredictable. “Some break up when we don’t expect them to,” he adds.
Eichler estimates it is only a matter of time before a large comet does strike. “The odds of a comet hitting the sun are vastly larger than a comet striking Earth,” he says, because the sun presents a bigger target.
Others think the sun is perfectly capable of sparking flares that big on its own. “You need a pretty big wallop to make carbon-14 in the atmosphere but the largest solar events can do it,” says Melott. Although we have never seen the sun do it, we have seen this behaviour in other stars.
In a paper published last year, Hiroyuki Maehara and colleagues at Kyoto University in Japan analysed 120 days of observations by the telescope and found that out of 83,000 sun-like stars in the telescope’s field of view, .
Although that means just 0.2 per cent of sun-like stars are superflarers, Melott cautions against complacency. “The really scary thing is that some of those flares are much greater than even the AD 775 event,” he says. Some blasted 1000 times the estimated energy of the medieval flare into space. At those magnitudes, were one to occur on the sun, it wouldn’t just be our technology at risk. The flux of particles would destroy Earth’s ozone layer, allowing through the ultraviolet rays that cause sunburn and skin cancer. “It would lead to a mass extinction level event,” says Melott.
“Were a gigantic superflare to occur on the sun, it would destroy Earth’s ozone layer and lead to mass extinction”
The good news is that the truly gigantic superflares came only from stars that displayed extraordinarily large “starspots” – regions of intense magnetic fields and the source of solar flares – much larger than those seen on the sun.
Nevertheless, researchers are scouring tree-ring data for more large events. Miyake has found a second in the year AD 992. Although large by previously known standards, it was only about half the size of the flare that hit in AD 775. Usoskin too has been analysing his data. “There has been no greater event than AD 775 in the last 10,000 years,” he says.
Knock out
While this offers some comfort, it doesn’t mean we can relax quite yet. That’s because the flares can spark something much more dangerous and hard to predict: a coronal mass ejection (CME), in which a billion tonnes of the sun’s atmosphere – essentially a torrent of energetic particles and magnetic fields – can be thrown into space.
“Solar flares can spark a coronal mass ejection: a billion tonnes of the sun’s atmosphere can be thrown into space”
The trouble is that no two CMEs are the same. Some have high energy but weak magnetic fields, which cause little damage to infrastructure. Others have strong magnetic fields but weak energy. These are the ones we should worry about, but it is hard to spot them in the historical record because it is the energetic particles alone that cause the carbon-14 spikes that researchers look for.
This was demonstrated by the 1859 Carrington event, when the battering of the Earth’s magnetic field induced electricity to flow in the world’s telegraph lines, stunning operators unconscious and causing telegraph offices to burst into flames. Yet there is no sign of it in the carbon-14 records.
Conversely, a CME with a huge number of high-energy particles struck in 1956, yet it caused little disruption to communications. And when the Hydro-Québec power grid was knocked out in 1989 by a CME, it wasn’t the one with the highest energy that year. That occurred six months later. “The whole thing ends up being very confusing,” admits Melott. “We are faced with trying to deduce new science from complicated data.”
The more measurements we can study, the better. Usoskin, for example, has turned from tree rings to the lunar rocks brought back by the Apollo missions. Exposed on the moon’s surface, the rocks act like sponges soaking up all the energetic particles that the sun has been spitting out during the course of the moon’s 4.6-billion-year lifetime.
They should allow us to know the size of the greatest solar events that have ever exploded from the sun, and not just the ones during the last two millennia. Perhaps the next time we see serpents in the sky, we will truly appreciate how dangerous living next to a star can be.
This article appeared in print under the headline “Star burst”
