Solid-Body Rotation speed rises with radius. 240 Myr Measuring Masses of Galaxies Star or Gas cloud is held in its orbit by the mass interior to the orbit. Don't move far from their birthplace. Spiral Arms are Density Waves in the Disk. Density Waves are a kind of orbital traffic jam Orbits crowd together in the arms, stars pile up and make the regions look brighter.
Gas clouds pile up, collide, fragment, and form new stars.
Density Waves Density waves pass through the disk like water waves pass over the ocean. Stars move through the spiral arms.