Galactic collisions have been simulated by computer (the calculations would have been virtually impossible in pre-computer days). These simulations show, step by step, how two galaxies collide with each other. As a relatively compact galaxy passes through a normal spiral one, the combined gravitational pull of their overlapping centers draws the spiral’s stars and gases inward. Then, as the interloper passes through and away, gravity lets up. Stars and dust are released to bounce back, resulting in a wave that carries galactic matter outward in a ring shape.
The computer also tells us that the ring galaxy will be unstable. Unlike the rings of the Saturn, the galactic material has no giant planet to orbit. In time, the ring galaxy may break into three to six separate galaxies. This is probably the origin of some galaxy groups we see today. A ring galaxy does not form overnight. Estimates put the time involved at about 100 million years. Surprisingly, such a galactic encounter will result in very few star-to-star collisions. Stars are too far apart in galaxies for crashes to be at all likely. And collisions between galaxies are so rare, there’s little chance we will ever see one in progress.
Interacting galaxy (Wikipedia)