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OK, so maybe we can be a *little* frightened.

Bad Astronomy
By Phil Plait
Jun 18, 2009 4:00 PMJun 27, 2023 7:41 PM


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If you can't take a bloody nose, go home and crawl under your bed. It's not safe out here. It's wondrous, with treasures to satiate desires both subtle and gross... but it's not for the timid. -Q

In my book, Death from the Skies!, I don't spend much time discussing magnetars. Although terrifying -- able to generate truly mind-numbing outbursts which I'll describe in a moment -- they are simply too rare and too far away to be much of a threat. Yeah, well, I might've been wrong. A little wrong, I mean; there's no reason to panic. Life on Earth won't be snuffed out by some rogue magnetar blasting away our atmosphere or anything like that. But one of my main premises for feeling completely safe has been eroded a bit, and to be fair I should talk about it.

Magnetars are neutron stars, superdense balls of tightly packed neutrons left over from the collapsed core of a massive star that's gone supernova. Neutron stars have about the mass of the Sun but are only a few kilometers across, making them fantastically dense and giving them surface gravities that can be billions of times the force you feel standing on the Earth. They also can possess magnetic fields literally trillions of times stronger than the Earth's. And in some cases, young neutron stars can be even more powerful: their field strength might be a quadrillion (1,000,000,000,000,000) times the Earth's! These beasts, called magnetars, probably lose that field strength rapidly, decaying in only a few thousand years. That makes them rare on a galactic scale. Still, several are known to exist. And they can have a nasty, nasty temper. See, the magnetic field is coupled to the crust of the neutron star. The crust is extremely rigid and under vast pressure from the gravity of the star. If the crust cracks -- a starquake, if you will -- the energy released makes the strongest earthquake ever recorded on our planet look like a friendly pat on the back. I once calculated the strength of such a starquake, and it would register as magnitude 32 on the Richter scale. This ultraviolent blast shakes the magnetic field of the star, which in turn reacts by slamming around subatomic particles... the bottom line is that such an event can trigger a phenomenal release of X-ray energy from the star. And by "phenomenal" I mean "pants-wetting terrifying". In December 2004, the magnetar SGR 1806-20 underwent such a starquake. In one-tenth of a second the subsequent blast released something like 2 times 10^46 ergs of energy -- equal to about 50 trillion times the Sun's output during that same period. Holy crap. This star sits about 50,000 light years from the Earth: literally halfway across the Milky Way galaxy from us. Yet, even from that forbidding distance, this titanic event was able to physically affect the Earth. It compressed our magnetic field and partially ionized our atmosphere, causing it to puff up measurably. Mind you, it was 500 quadrillion kilometers (300 quadrillion miles) from us at the time. So you can see why these things are a bit unnerving. But really, this one is so far away! Sure, it can hurt us, but at that distance really all it can do is what it did; we don't expect it can have a bigger event, so we're safe enough. Moreover, these objects are so bright in X-rays that we think we've found all the really big bruisers in the Galaxy. If one were closer to us, there's no way to hide it. We'd see it. Yeah, about that... Astronomers have announced they found a new magnetar, named SGR 0501+4516, and it's only 15,000 light years away. It turns out to be dark most of the time, emitting very little energy, which is how it escaped detection. But it had an outburst last year that lasted four months, allowing scientists time detect it and to get a good long look at it. This event was far less violent than the one from SGR 1806 in 2004, but still nothing to sneeze at. Is it capable of an SGR 1806-like event? Probably not -- that was an extraordinary event -- and I certainly hope not! At 1/3 the distance, the effects on Earth would be nine times as strong. That could damage satellites and possibly even cause some effects on Earth itself -- probably nothing that would be too big a deal, but still. Yikes. The thing is, in Death from the Skies!, I said we're safe from these things because they're far away, and it's not possible to hide any closer to us. Yet here is this one, three times closer than SGR 1806. It makes me wonder if there are any closer still. If one were, say, 5000 light years away and had a blast like the one in 2004, the effects would be 100 times larger! There could be serious satellite damage, and possibly even blackouts on Earth due to electric currents induced in our power grid. Let me be clear: I seriously doubt there's anything that close to us. This new one at 15,000 light years is something of a fluke, and it's entirely possible it's not capable of the same kind of explosive event as its more distant cousin. The odds of one being even closer are pretty small, so I'm not too concerned about it. If I were, believe me, I'd let you know! The point here is that we have to be careful when we talk in absolutes, and it's always good to question assumptions. If there's one thing we know for sure about the Universe, it's that it's capable of some pretty good surprises, and not all of them need be the happy fun kind. We're almost certainly safe from this particular threat... but maybe a little kick in the complacency isn't always such a bad thing. Image credit: NASA.

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