One billion years ago, a cluster of stars formed in our galaxy. Since then, that cluster has whipped four long circles around the edge of the Milky Way. In that time, the Milky Way's gravity has stretched that cluster out from a blob into a long stellar stream. Right now, the stars are passing relatively close to Earth, just about 330 light-years away. And scientists say that river of stars could help determine the mass of the entire Milky Way..