M31 is the Andromeda galaxy, which, happens to be coming right at us or the Milky Way at it if you are not a Milky Way Centrist, and we will collide eventually. It will be disruptive, no doubt.
Do I have time to retire? :)
Whoops!
That's what I get for not looking it up. I'll buy a vowel and go with M32 (or was I thinking of M33?).