Andromeda is a spiral galaxy approximately 2.5 million light-years from Earth and the nearest major galaxy to the Milky Way. Andromeda is approaching the Milky Way at about 110 kilometers per second (68 miles per second), and it is predicted that they will collide and merge in about 4 billion years to form a giant elliptical galaxy.

Scientists believe that Andromeda's current gravity and the gravitational forces between two satellite galaxies of Andromeda, M32 and M110, are the primary factors driving its motion toward the Milky Way.