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..
Astronomers have seen these stars before, mixed in with lots of stars all around them. But until now, they didn’t realize the stars were part of a group. The river, which is 1,300 light-years long and 160 light-years wide, winds through the Milky Way’s vast, dense star field. But 3D-mapping data from Gaia, a European Space Agency spacecraft, showed that the stars in the stream moved together at roughly the same speed and in the same direction.
“Identifying nearby disk streams is like looking for the proverbial needle in a haystack. Astronomers have been looking at, and through, this new stream for a long time, as it covers most of the night sky, but only now realize it is there, and it is huge, and shockingly close to the sun,” João Alves, an astronomer at the University of Vienna and second author of the paper, said in a statement.