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SYFY WIRE Bad Astronomy

Stellar Explosion Dwarfs Our Sun’s Best Shot

By Phil Plait

Regular readers of my blog know that the Sun can get pretty feisty. I’ve written about many a solar flare, magnetically induced explosions on the surface of the Sun so huge that the energy released vastly outguns our planet’s entire supply of nuclear weapons.

It turns out, though, that the Sun’s biggest effort is actually pretty weak stuff when compared with the epic tantrum the star DG Canes Venaticorum threw last April. When DG CVn gets ticked, it gets really ticked: It blew out a series of flares so huge that they were easily detectable here on Earth, 60 or so light-years away*. And the biggest of these flares was something like 10,000 times more powerful than anything the Sun has ever been seen to produce.

Ten thousand times. If the Sun were to produce a flare like that, and it were on the side of the Sun facing the Earth, it would wipe out our high-tech civilization without working up a sweat. I can only imagine what it would do to our planet physically.

And to make this even more incredible: DG CVn is an M star (actually, a binary pair of two very similar stars; it’s not known which one is flaring, so to keep it simple I’ll keep this generic), a dim bulb red dwarf with only one-third the Sun’s diameter, a third its mass, and usually shines with far less than 1 percent the Sun’s luminosity.

How can such a dinky star produce such terrifyingly powerful flares?

Here’s a video from NASA with a synopsis. I’ll give details after you watch it.

As Stephen Drake says in the video, DG CVn is young. Stars rotate much like planets do, but their rotation slows as they age. DG CVn is still in the flush of youth, rotating about once per day. This helps energize its magnetic fields, which are in part powered by the star’s spin.

Magnetic fields in a star get their start in what’s called the convective zone inside the star, where superhot gas (actually, ionized gas called plasma) rises toward the surface. Moving charged particles create magnetic fields, which is what happens in these columns of plasma. (I’m simplifying of course, but this is good enough to get the concept.)

In stars like the Sun, the convection zone is only a small fraction of the Sun’s radius. But small red dwarf stars are entirely convective, from their cores out to their surface. This, coupled with the star’s rotation whipping things up, allows magnetic fields to build up and become huge. When these cells of magnetized plasma reach the surface they can tangle up, and prevent the cooling gas from sinking again. The gas cools, darkens, and becomes what we call a sunspot.

On the Sun these spots are generally not much bigger than Jupiter, and most far smaller. On a red dwarf, however, starspots can be an appreciable fraction of the star’s surface! And those magnetic fields can be incredibly strong, which means they can store a lot of energy.

If those field lines get too tangled up (and that happens because they’re connected to the gas, which is moving around on the star’s surface) they can, in essence, snap. They release their stored energy, and that energy can be mind-numbingly vast.

That’s what happened on DG CVn in April. There must have been a huge region of magnetic field lines all higgledy-piggedly, and at some point things got too much for it. It snapped, exploding with energy—and what a blast it was. In just X-rays (a high energy form of light), the explosion outshone the rest of the star’s energy at all wavelengths … and it got 10 times brighter than normal in visible light, and 100 times brighter in ultraviolet. The X-ray energy released was staggering.

The flash was detected by NASA’s orbiting Swift telescope, designed to detect high-energy X- and gamma rays from space. It quickly swiveled to put DG CVn in its sights, and continued to study the star. Usually, M dwarf superflares die down in a day or so, but not this time: DG CVn continue to blast out ever-smaller flares over the next three weeks!

Incredible. I’m used to the Sun being something of a bruiser, and of course more massive stars doing all sorts of horrifying things like exploding as supernovae and <shudder> gamma-ray bursts. But it still amazes me that such low wattage stars like red dwarfs are capable of such fury. As they point out in the video, many red dwarfs have planets. It’s possible DG CVn has planets orbiting the two stars, but with flares like what we just saw, the chances of life there are slim …

But there’s time. The system is only 30 million years old. Any hypothetical Earth-sized planets orbiting them are likely still cooling from formation, and may not even have solid surfaces yet. As the stars age, they’ll calm, and flares will be less likely. In a few billion years, who knows?

A lot of us had pretty energetic youths, but calmed down later in life. As go we, so go the stars.

*Correction, Oct. 8, 2014: I initially had some trouble corroborating the numbers in the NASA video about DG CVn's distance, mass, and so on, so I went with the numbers I found from a stellar database. However, Francis Reddy pointed me to a paper that came out just this year showing that the distance to the star and the other numbers used in the NASA video are correct, so I updated the post to reflect that.

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