What amazes me with M31 is that when you look at the picture, you can say to yourself that the galaxy we lie within, the Milky Way, resembles a great deal like M31. I always get to wonder about the trillions of objects that are hidden there that will, for now, remain unknown to us. In this picture, you see the whole galaxy with, at the bottom, a satellite dwarf galaxy that's hovering around it given the massive gravity it exerts. M31 galaxy contains roughly one trilllion stars, similar in ways to how many there are in our galaxy. One thing to remember is that our galaxy and M31 are only 2 out of hundreds of billions galaxies. Impressive isn't it!! Here's a Wiki link:
http://en.wikipedia.org/wiki/Andromeda_Galaxy