Adrian Melott, an astrophysicist from the University of Kansas and lead author of the new paper, has been studying supernovae for much of his career. But just a few years ago, researchers found iron-60d deposits around the globe hinting that a particular group of supernovae had exploded from a shared distance, during a shared timeframe. We can use the deposits to determine that the rays from these supernovae began hitting Earth as long as 8 million years ago, and that energy peaked as recently at 2.6 million years ago. For Melott and his colleagues, this timeframe distance matches up with activity originating about 123 light-years away, in a region called the Local Bubble—which possesses hot gas that looks like it was blown by a series of supernovae. The original stars that blew up were probably nine times the mass of the sun.