Twenty years ago today, a small neutron star crossed the Milky Way to strike Earth, ringing our planet like a bell. Although half a galaxy away, an explosion on the surface of that dead star physically compressed our planet’s magnetic field, supercharged some satellites, and even partially ionized Earth’s upper atmosphere. However, all this was coming from an object no larger than two dozen kilometers across.
Sometimes scary things come in small packages.
The culprit was SGR 1806-20, a magnetar located 50,000 light-years away in the constellation Sagittarius. A magnetar is a special type of neutron star that is already at the top end of what the extreme universe can produce. Forged in the flames of a supernova, a neutron star is created when the core of a massive star collapses, even while the rest of the star has exploded at a fraction of the speed of light. The nucleus collapses in on itself, its density increasing, until it is so compressed that its electrons squeeze onto neighboring protons (with an additional antineutrino, for those accounting for subatomic particles that follow the trail), creating neutrons.
About supporting science journalism
If you like this article, please consider supporting our award-winning journalism subscribing. By purchasing a subscription, you’re helping to ensure a future of impactful stories about the discoveries and ideas that shape our world.
As a result, the neutron-laden object is almost beyond human comprehension. It holds more than the mass of the Sun, but is typically about 20 kilometers across, and its density is almost comically high: just one cubic centimeter of a neutron star, about a quarter the size of a standard six-sided die. would weigh 100 million metric tons. Imagine taking every car in the US, smashing it into a lump, then crushing that lump down to 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 hundreds of millions of times that of Earth. A person standing on the surface of a neutron star would weigh trillions or trillions of tons. But they would not be able to stand; immense gravity would flatten them into vaporized atoms less than a micron in size.
Neutron stars are born with a strong magnetic field, billions of times stronger than Earth’s. Some may be even stronger, however, with a field that can be a staggering four trillion times that of Earth. These are magnetars, and are among the most dangerous objects in the galaxy.
Sitting in space, doing its thing, a magnetar is an ultra-deadly astrophysical beast you don’t want to mess with anymore. However, sometimes these objects shine, a word that so underlines what’s actually happening, it’s ridiculous.
A neutron star’s strong magnetic field is embedded in the surface of the object, like hair growing on your skin. In some magnetars the crust can be unstable and eventually slip. This “starquake” is very similar to an earthquake, but remember that the earth’s crust is impossibly dense and has incredibly high gravity. If the Earth’s crust cracks and slips a single millimeter, the energy released is cosmically large, creating temperatures high enough to vaporize hundreds of trillions of metric tons of matter at the surface. This shakes the magnetic field so violently that the field reshapes itself, breaking and recombining the magnetic field lines. When they do this, they also release their stored energy. The result is disaster on an epic scale.
Magnetars are relatively rare as neutron stars go, so they are sparsely distributed throughout space. This means that the effects of their flares are usually attenuated with great distances, so such bursts are usually only detected by specialized astronomical instruments. Sometimes, however, a magnetar explodes a greatflare up
SGR 1806-20 experienced such an event about 50,000 years ago. It was all over in the blink of an eye: in a tenth of a second, the crust slipped, exploded, and blew up the star’s magnetic field. Fireballs hit him 10 trillion times the total energy output of the sun in the same period of time. Much of this energy was in the form of high-energy gamma rays, although it also included x-rays and other types of light.
To put this on a terrestrial scale—an almost impossible task—the starquake was roughly equivalent to a magnitude 32 earthquake, something like 32 sextillion. times stronger than the strongest earthquake ever recorded on our planet.
This energy spread through space for millennia, finally passing through Earth on December 27, 2004. The effects were felt immediately.
NASA had just launched the Swift satellite about a month earlier. Swift was designed to detect high-energy cosmic explosions billions of light-years away, but it was not prepared for the explosion of SGR 1806-20. The satellite’s gamma-ray detectors were saturated with energy, even though Swift was not even pointed in the direction of the blast; the energy penetrated the walls of the spacecraft and struck the detectors anyway.
The initial energy spike lasted less than a second, but Swift’s brilliant instruments detected a long tail of energy that lasted more than five minutes. The brightness of the superflare rose and dipped with a well-defined period of 7.56 seconds, the rotation speed of the magnetar. As SGR 1806-20 rotated, the frenzied scar of the starquake’s location moved in and out of our view, creating oscillating brightness like twinkling Christmas lights.
It was enough energy to physically affect our planet: increased ionization in the ionosphere – a layer of the earth’s atmosphere that reaches about 600 kilometers from the surface – and also measured the effect on the magnetosphere. The overall effect was small, but keep in mind that the magnetar is 50,000 light years from Earth, literally half way through the Milky Way. Had it been much closer, the effects would have been much stronger, similar to a powerful solar flare that could fry the electronics of many satellites and wreak havoc on Earth.
The good news is that 50,000 light years is a long way. There are some magnetars closer to us, but none have been seen doing such powerful superflares. The SGR 1806-20 is still at the top of its class in terms of power.
So there’s probably no need to panic or worry about a magnetar ruining our day. I remember at the time of the superflare some pseudoscience crackpots speculated that it had caused the massive Sumatra-Andaman earthquake and subsequent tsunami—a total land disaster that killed a quarter of a million people. That earthquake, however, happened over a day before we were hit by the magnetar blast, the blast wave, traveling at the speed of light, when it was about 50 billion kilometers from Earth, still outside Neptune’s orbit. The two events were unrelated.
But the outburst of SGR 1806-20 shows how unexpected forces the universe wields. Stars explode, magnetars explode, and other cosmic events wreak havoc. The good news is that distance dwarfs these colossal disasters so much that we didn’t even know they existed until recently. The earth has been around for 4.6 billion years, and we’re still here.
So if you’re one to count your blessings as a new year begins, raise a toast to the sky and be happy space is so big, and thanks to science we try to observe and understand it.
