What happens when two galaxies collide?

The answer

When galaxies collide, their stars almost never actually hit each other because the distances between stars are so vast relative to their sizes. Instead, gravitational interactions distort both galaxies into spectacular tidal tails and bridges of stars over hundreds of millions of years. Gas clouds do collide, triggering intense bursts of star formation. The Milky Way and Andromeda galaxy are approaching each other at 110 km/s and will merge in about 4.5 billion years, eventually forming a giant elliptical galaxy nicknamed 'Milkomeda.'