The Andromeda Galaxy is the closest large spiral to our own Milky Way, and the only major Galaxy moving toward us.  Turns out its on a direct collision course, but we still have 3.5 Billion years to prepare, so its not exactly pressing news.  On the plus side, studying Andromeda allows us to infer properties of more distant galaxies, and it gives us a map of what our own Milky Way Galaxy may look like.  Astronomers using the Hubble Space Telescope have discovered a giant halo of gas around Andromeda, and the Milky Way may have a similar one. By...