Twenty years ago today a magnetar's epic tantrum made our planet ring like a bell from tens of thousands of light-years away
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. On supporting science journalism If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today. 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…