A galaxy that’s a factory for exploding stars loops high across the northern sky tonight. It’s to the left of the bowl of the Big Dipper as darkness falls. M82’s most recent supernova blasted into view in January. It was one of the brightest and closest in decades. M82 is only about 12 million light-years away — quite close as galaxies go. Like our own Milky Way, it’s a spiral galaxy — a flat disk with spiral arms that outline regions that have given birth to many new stars. Unlike the Milky Way, though, M82 cranks out new stars just about as fast as Hershey makes candy bars. A close encounter with another galaxy about a hundred million years ago compressed big clouds of gas and dust inside the galaxy. These clouds give birth to new stars at a prodigious rate. Many of these stars are big and bright, making M82 shine far brighter than the Milky Way. These hot new stars live short lives — in the millions of years versus billions of years for stars like the Sun. When such a star expires,
