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SOMETHING with almost unimaginable power hit Earth in AD 775. Europe was in the grip of the dark ages, yet the skies were alight. 鈥淔iery 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,鈥� recorded the 13th-century English chronicler Roger of Wendover.

We don鈥檛 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鈥檛 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.

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鈥檚 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 鈥� 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.

鈥淲e saw and became interested in measuring the effect ourselves,鈥� says , a physicist at the University of Oulu, Finland. His team turned to that once grew near the river Main. The measurement was clear. 鈥淲e precisely confirmed the results,鈥� he says. 鈥淲hatever it was in AD 775, it was a global phenomenon, and that points to an extraterrestrial source.鈥� So what was it?

Miyake鈥檚 team calculated the energy needed to produce such enhanced levels of carbon-14. Her estimates for the energy were colossal 鈥� 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.

鈥淚t wasn鈥檛 a supernova,鈥� 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鈥檚 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. 鈥淎nyone who has seen aurorae knows that they look like serpents,鈥� he says.

But there was no way to square the idea of a solar flare with Miyake鈥檚 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鈥檚 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. 鈥淎t that energy output, the radiation storm is more likely to have its explanation in the sun,鈥� 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. 鈥淲e can absolutely say that what happened then was bigger than Carrington,鈥� 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 鈥渙nly with a bit of help.鈥� 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. 鈥淭hat鈥檚 per second,鈥� emphasises Eichler, 鈥渘ot per hour.鈥�

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鈥檚 closest approach took place on the sun鈥檚 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. 鈥淭he experts say that we can鈥檛 know that the explosion was triggered by Lovejoy but the timing was impeccable,鈥� he says.

Now he is biding his time. 鈥淚t 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,鈥� 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. 鈥淚 think it is highly unlikely that this comet will trigger anything,鈥� he says.

That鈥檚 because as sungrazers go, it鈥檚 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. 鈥淪ome break up when we don鈥檛 expect them to,鈥� he adds.

Eichler estimates it is only a matter of time before a large comet does strike. 鈥淭he odds of a comet hitting the sun are vastly larger than a comet striking Earth,鈥� he says, because the sun presents a bigger target.

Others think the sun is perfectly capable of sparking flares that big on its own. 鈥淵ou need a pretty big wallop to make carbon-14 in the atmosphere but the largest solar events can do it,鈥� 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鈥檚 field of view, .

Although that means just 0.2 per cent of sun-like stars are superflarers, Melott cautions against complacency. 鈥淭he really scary thing is that some of those flares are much greater than even the AD 775 event,鈥� 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鈥檛 just be our technology at risk. The flux of particles would destroy Earth鈥檚 ozone layer, allowing through the ultraviolet rays that cause sunburn and skin cancer. 鈥淚t would lead to a mass extinction level event,鈥� says Melott.

鈥淲ere a gigantic superflare to occur on the sun, it would destroy Earth鈥檚 ozone layer and lead to mass extinction鈥�

The good news is that the truly gigantic superflares came only from stars that displayed extraordinarily large 鈥渟tarspots鈥� 鈥� regions of intense magnetic fields and the source of solar flares 鈥� 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. 鈥淭here has been no greater event than AD 775 in the last 10,000 years,鈥� he says.

Knock out

While this offers some comfort, it doesn鈥檛 mean we can relax quite yet. That鈥檚 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鈥檚 atmosphere 鈥� essentially a torrent of energetic particles and magnetic fields 鈥� can be thrown into space.

鈥淪olar flares can spark a coronal mass ejection: a billion tonnes of the sun鈥檚 atmosphere can be thrown into space鈥�

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鈥檚 magnetic field induced electricity to flow in the world鈥檚 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-Qu茅bec power grid was knocked out in 1989 by a CME, it wasn鈥檛 the one with the highest energy that year. That occurred six months later. 鈥淭he whole thing ends up being very confusing,鈥� admits Melott. 鈥淲e are faced with trying to deduce new science from complicated data.鈥�

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鈥檚 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鈥檚 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 鈥淪tar burst鈥�