A Warning From the Dragon Stars

A fiery outburst warns of past and future disaster here on Earth

Photo credit: NASA. Shared under CC BY 2.0.

One hundred and ten light years from Earth lies the small and insignificant star EK Draconis. It is a yellow dwarf, rather like the Sun, and glows faintly, so faintly indeed, that it is invisible to the human eye. Only when a telescope is trained on it, somewhere in the direction of the great Dragon constellation, can its shimmering light be seen.

And yet, despite its apparent lack of importance, EK Draconis has long intrigued astronomers. In resembles, in many ways, our own star - but not as it is today, but as it was billions of years ago. That’s because EK Draconis is a young star, one less than a hundred million years old, but one that - in size and composition - matches the Sun.

Back in the 1990s, realising this resemblance, astronomers pointed the Hubble Space Telescope towards EK Draconis. They, and later measurements, found a fast spinning star, with spots coming and going on its surface – just as the Sun is sometimes marked by sun spots. Those brief surveys also showed it to be a violent star: Hubble saw traces of intense flares erupting.

That was an interesting discovery. Astronomers have long known that our own star occasionally throws out big flares. Some – like the events of 1859 or the near miss of 2012 – can be destructive to technological civilization, blasting satellites with radiation and overloading electrical grids. But just how big these flares can get has long been questioned.

Evidence from tree ring records hints that big flares have struck the Earth in the past. One, for example, probably came in 774 AD, another three, at least, have hit since the end of the last ice age. But because the tree ring record is not perfect, and perhaps could be fooled, astronomers are still not sure our star is capable of such eruptions.

Thus scientists have turned to the heavens, watching sun-like stars across the galaxy to see how often flares burst out – and how big they get. The results have been far from reassuring. Some stars, almost identical to our own, regularly throw out “superflares” – powerful eruptions dozens of times bigger than anything we’ve seen strike the Earth.

Of course, there might be something special about those stars. Some strange magnetic effect, maybe, that drives such anger. Perhaps these superflares are only a passing fancy – a brief moment of fury amid millions of years of calm. After all, we still do not fully understand how flares – super or not – come about.

This, then, brings us back to EK Draconis: a star that, by all appearances, is an perfect example of the young Sun. If it, too, throws out big flares, then it seems reasonable to believe our own star must once have done likewise. Is that, though, just youthful fury? Or do flares reoccur throughout the lifetime of a star, even in those more sedate and middle aged ones?

In the first months of 2020, between January and April, astronomers attempted to find out. They directed TESS – an orbiting observatory – to watch EK Draconis on nineteen different occasions. Their persistence paid off: on April 5th, as the world below was grappling with the emerging pandemic, TESS saw a sudden burst of energy from EK Draconis.

It was a flare: sixteen minutes long and twenty times bigger than the most powerful observed from the Sun. In the hours after, astronomers tracked a portion of the star breaking away from the surface – an event known as a Coronal Mass Ejection – and then watched it race away at hundreds of miles per second.

Such mass ejections are often linked to flares in our solar system. When one strikes the Earth, as happens several times per decade, its energetic particles interact with the upper atmosphere to create an intense aurora – dancing lights in the polar skies. They can, however, be deadly to satellites – and to astronauts lingering far from the Earth’s protection.

Indeed, a really big coronal mass ejection – as might come with an EK Draconis style flare – would be a catastrophe. Power grids around the planet would collapse, satellites would fail and astronauts would face grave danger. But such a flare could also strip the ozone layer; bathing the planet in deadly ultraviolet rays for decades.

Until last year’s observation, reported in a paper published last week, astronomers had no solid proof that mass ejections did accompany solar flares. Indeed, the evidence is still not enough to say the ejection really happened. TESS could not show if the eruption fell back to the star’s surface – though some surely did – or escaped into space.

Still, the discovery is enough to show that the Sun, too, probably had a violent youth. The early days of the solar system, long before life began, may have been marked by regular and intense superflares. Those must have stripped away primitive atmospheres and irradiated planetary surfaces. Only once the Sun calmed down – probably after a few hundred million years – could life have safely emerged.

In some ways, however, these observations are reassuring. Such superflares seem common on EK Draconis: it took TESS just nineteen days to see one. Our own Sun, at least, seems much calmer. In two centuries of observations we’ve never seen it do anything quite so violent.

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