Galaxies are grand yet fragile structures. When they pass close to each other or collide, their mutual gravity pulls them apart, twisting and distorting their shapes. Usually, two galaxies that collide will eventually merge to become one. Such mergers appear to be a standard feature in galaxy formation. During galaxy encounters, it is not the stars within the galaxies, but rather their gas clouds that collide.
Keywords: Astrophysics, Galaxies--Evolution, Gases, Gravitation