galaxy-mergers

The Milky Way did not grow peacefully. It reached its current size by absorbing smaller galaxies — tearing them apart with tidal forces, stripping their stars into long streams, and incorporating their material into the disk and halo. One major collision 10 billion years ago reshaped the Milky Way's entire inner halo. Another is happening right now. A third, larger one is coming in 4 billion years, and there is nothing that can stop it.

The Andromeda Galaxy is approaching the Milky Way at ~110 km/s and will collide in approximately 4.5 billion years. The two galaxies will merge over several billion years of passes, ultimately forming a giant elliptical galaxy — sometimes called 'Milkomeda.' Despite the collision of two galaxies each containing hundreds of billions of stars, individual stellar collisions are extremely rare. Earth will likely survive, though in a dramatically different night sky.