To Map a Thousand Million Stars

Our Milky Way galaxy is a huge place: It has something like 200 billion stars in it. When you go out on the darkest night imaginable and look up, it seems like the sky is filled with stars, but in reality you can only see a few thousand at most. That's a paltry figure compared to what's really out there. Over the years telescopes have mapped many more stars; the Hubble Space Telescope has a catalog that contains tens of millions of them. But even that is just scratching the surface. The European...