Q: What causes the formation of spiral arms in galaxies? Do spiral arms form in other flattened groups of orbiting objects, like planetary rings and protostellar disks?
Robert Harrison
Albuquerque, New Mexico
A: Spiral arms in galaxies can form by a combination of processes. In a spiral galaxy, everything orbits at the same speed, meaning stars and gas near the center of the galaxy complete an orbit in less time than objects farther out. This effect is referred to as differential rotation. So,