Twenty years ago today a tiny neutron star reached across the Milky Way to land a blow on Earth, ringing our planet like a bell. Despite being half the galaxy away, an explosion on that dead star’s surface was able to physically compress our planet’s magnetic field, overload some satellites and even partially ionize Earth’s upper atmosphere. Yet this all came from an object no bigger than about two dozen kilometers across.
Sometimes terrifying things come in small packages.
The culprit was SGR 1806-20, a magnetar located some 50,000 light-years away in the constellation Sagittarius. A magnetar is a special kind of neutron star that is already at the top end of what the extreme universe can produce. Forged in the fires of a supernova, a neutron star forms when a massive star’s core collapses even as the rest of the star explodes outward at a significant fraction of the speed of light. The core falls into itself, its density skyrocketing, until it becomes so compressed that its electrons are squeezed into neighboring protons (with an added antineutrino, for those subatomic particle bookkeepers keeping track), resulting in the formation of neutrons.
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The resulting neutron-packed object is almost beyond human comprehension. It holds more than the mass of the sun but is typically only about 20 kilometers across, making its density almost comically high: a single cubic centimeter of a neutron star, a portion roughly a fourth of the size of a standard six-sided die, would weigh 100 million metric tons. Imagine taking every single car in the U.S., smashing them together into a lump, then crushing that lump down into the size of a single sugar cube, and you’ll start to get the idea.
The surface gravity of a typical neutron star can be tens or even hundreds of billions of times that of Earth. Standing on the surface of a neutron star, a person would weigh billions or even trillions of tons. But they wouldn’t be able to stand; the immense gravity would flatten them into a vaporized smear of atoms less than a micron high.
Run-of-the-mill neutron stars are born with a strong magnetic field, billions of times stronger than Earth’s. Some can become even more powerful, however, with a field that can reach a staggering quadrillion times that of Earth. These are the magnetars, and they are among the most dangerous objects in the galaxy.
Just sitting out there in space, doing its thing, a magnetar is already an ultralethal astrophysical beast you wouldn’t want to tangle with. Yet sometimes these objects flare, a word that so undersells what actually happens that it’s laughable.
A neutron star’s intensely powerful magnetic field is embedded in the object’s crust, coupled to it like the hair growing out of your scalp. In some magnetars the crust can become unstable and eventually slip. Such a “starquake” is very similar to an earthquake, but remember that the crust is unimaginably dense and subject to fantastically high gravity. If the crust cracks and slips a single millimeter, the energy released is cosmically huge, creating temperatures high enough to vaporize hundreds of trillions of metric tons of matter on the surface. This shakes the magnetic field so violently that the field reshapes itself, the magnetic field lines snapping and recombining. When they do this, they release stored-up energy as well. The result is catastrophe on an epic scale.
Magnetars are relatively uncommon as far as neutron stars go and thus sparsely distributed throughout space. This usually means the effects of their flares are attenuated by great distances, so such outbursts are typically only detectable by specialized astronomical instruments. On rare occasions, however, a magnetar blasts out a superflare.
SGR 1806-20 suffered just such an event some 50,000 years ago. It was all over in the blink of an eye: in just a tenth of a second, the crust slipped, exploded and blasted the star’s magnetic field. The fireball had about 10 trillion times the sun’s total energy output over the same length of time. Much of that energy was in the form of super-high-energy gamma rays, though it also included x-rays and other forms of light as well.
To put this on an earthly scale—a nearly impossible task—the starquake was roughly the equivalent of a magnitude 32 earthquake, something like 32 sextillion times stronger than the most powerful earthquake ever recorded on our planet.
That energy rippled out across space for millennia, finally sweeping over Earth on December 27, 2004. The effects were felt immediately.
NASA had just launched the Swift satellite about a month before. Swift was designed to detect high-energy cosmic explosions from billions of light-years away, yet it was unprepared for SGR 1806-20’s outburst. The satellite’s gamma-ray detectors saturated with energy, even though Swift wasn’t even pointed in the direction of the blast; the energy penetrated the walls of the spacecraft and pummeled the detectors anyway.
The initial spike of energy endured for less than a second, but Swift’s exquisite instruments detected a long tail of energy that lasted for more than five minutes. The brightness of the superflare rose and dipped with a very well-determined period of 7.56 seconds, the rotation rate of the magnetar. As SGR 1806-20 spun, it swept the raging scar of the starquake’s location into and out of our view, creating the oscillations in brightness like a blinking Christmas light.
The energy was enough to physically impact our planet: it increased the ionization in the ionosphere—a layer of Earth’s atmosphere that reaches up to roughly 600 kilometers above the surface—and also measurably affected the magnetosphere. The overall effect was small, but bear in mind that the magnetar is 50,000 light-years from Earth, literally halfway across the Milky Way from us. Had it been much closer, the effects would have been much stronger, similar to a powerful solar flare that can fry the electronics of many satellites and create chaos down here on the surface.
The good news is that 50,000 light-years is a long way. There are some magnetars closer to us, yet none has been seen spitting such powerful superflares. SGR 1806-20 is still at the top of its class in terms of power.
So there’s probably no need to panic or even fret about a magnetar ruining our day. I remember that at the time of the superflare some pseudoscience crackpots speculated that it caused the massive Sumatra-Andaman earthquake and subsequent tsunami—an entirely terrestrial disaster that killed nearly a quarter of a million people. That earthquake, however, occurred more than a day before the magnetar blast hit us, when the explosion’s wave, traveling at the speed of light, was roughly 50 billion kilometers from Earth, still well outside the orbit of Neptune. The two events were unconnected.
But SGR 1806-20’s outburst does show how casually the universe wields such unimaginable forces. Stars explode, magnetars erupt, and other cosmic events cry havoc. The good news is that distance dwindles these colossal catastrophes so much that we didn’t even know they existed until relatively recently. Earth has been around for 4.6 billion years, and we’re still here.
So if you’re the sort to count your blessings as a new year starts, raise a toast to the sky and be happy space is so big—and thank science that we observe and try to understand it.
