Astronomers from the US and France have mapped out the motion of a stars in the outskirts of the Andromeda galaxy (M31), and found that they're actually part of the galaxy's main disk. This means that this spiral galaxy is actually three times larger - 220,000 light years across - than astronomers had previously estimated. It was previously believed that these stars were part of a halo that surrounded Andromeda, but weren't actually part of its disk. This discovery hadn't been made until now because detecting the motion of these stars requires very precise observations.