The collisions (occurring in different galaxy pairs) are seen in different stages of the merger process show the sequence that occurs. In (a) and (b), the galaxies are still separated, but huge tidal forces of gravity are already pulling stars from the galaxies into enormous broad fans that stretch hundreds of thousands of light-years in space. In (c) and (d) the colliding galaxies have merged into single, larger galaxies. The violent past of these galaxies can be inferred from the tidal "debris" that still surrounds the newly formed galaxies. Credit: P. van Dokkum/Yale University and NOAO/AURA/NSF
