SGR 1806-20 is a "magnetar": a rapidly spinning neutron star that has both an incredible density, trillions of times greater than ordinary matter, and an incredibly strong magnetic field. Tens of thousands of years ago, a "starquake" disrupted the magnetar's surface. The result was an explosive release of energy, which sent a pulse of gamma rays racing across the cosmos at the speed of light.

Credit: Trent L. Schindler, National Science Foundation

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