Kepler’s supernova, a type Ia explosion from 1604, lingers today with the x-ray glow of hot, leftover debris. By Daniel CleryThe white dwarf stars were zipping across the Milky Way at more than 1000 kilometers per second—thousands of times faster than a speeding bullet, so fast that they would eventually escape the gravitational clutches of the Galaxy. “They were not like anything we had seen before,” says Boris Gaensicke, an astronomer at the University of Warwick.Gaensicke and his colleagues suspected these burnt-out embers were fleeing scenes of violence: supernova explosions in which another white dwarf had detonated like an Earth-size hydrogen bomb.