The Antennae Galaxies in Collision . ... Credit Copyright: B. Whitmore ( Space Telescope Science Institute ) et al. ... When two galaxies collide , however, the stars that compose them usually do not. This is because galaxies are mostly empty space and, however bright, stars only take up only a small amount of that space. During the slow, hundred million year collision , however, one galaxy can rip the other apart gravitationally, and dust and gas common to both galaxies does collide. ...