Worlds - I mean Galaxies - in collision

Galaxies are vast islands in space, stars, gas, and dust held together by their mutual gravity. Many sport grand shapes: elliptical, spirals, barred spirals, and so on. Some are just a mess, irregular in shape. Others are called peculiar: they appear to have some overall shape, but it's weird, distorted, like something bad happened. For the Antennae Galaxies, something very bad indeed happened: they collided. Two massive spiral galaxies slammed into each other, their mutual gravity ripping out long streamers of gas and stars from each other. In general, the stars will pass by each other without a physical collision. Stars are very small compared to the space between them, and it would be like two dust motes inside an otherwise empty football stadium colliding with each other by chance. But the gas clouds are big, and they do in fact collide. At the centers of the galaxies these clouds ...