Computer Models of Colliding Galaxies

Galaxies come in a wide variety of sizes and shapes. Most have an ordered structure and can be classified as spirals or ellipticals. Spiral galaxies are flattened disks in which nearly all the stars orbit in the same direction about a common center; ellipticals are oval swarms of stars distributed in complicated three‐dimensional orbits. These are highly symmetric forms, suggesting a good degree of stability. Some galaxies, however, possess markedly irregular features. The nature of such “peculiar” galaxies and their relationship to more ordinary galaxies has long been a matter of controversy. Computer models, combined with increasingly powerful observations, offer a resolution: Many peculiar systems are ordinary galaxies in collision, and these collisions are transforming galaxies from one type to another.