The Andromeda Galaxy, catalogued as M31, is the large spiral galaxy nearest our own, lying about two and a half million light-years away. It is the most distant object most people can see without a telescope, visible as a faint smudge on a dark night. Slightly larger than the Milky Way, it is approaching us at roughly a hundred and ten kilometers per second, and the two galaxies are expected to interact in about four to five billion years. Recent measurements of its sideways motion have made the outcome less certain than once assumed, with a clean merger no longer guaranteed.