The Andromeda Galaxy is the nearest large spiral galaxy and is drifting toward the Milky Way.

The longer version

Andromeda lies close enough on cosmic scales that its gravity dominates over the general expansion in our neighborhood. Measurements of its motion show it moving toward the Milky Way, so the two large spirals are on a collision course. It is visible to the unaided eye as a faint smudge.

The encounter will unfold over an immense span of time, and because stars are separated by vast emptiness, direct collisions between them are unlikely. Gas clouds will collide and trigger bursts of star formation, and the merged remnant is often described as a single larger galaxy. Both galaxies already possess central black holes that will eventually merge.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.