Transcript: Why does the galaxy have spiral arms? Some analogies just don’t work. A garden sprinkler sends out spiral patterns of water, but in this case the water is moving radially rather than the circular orbits within the Milky Way. If you stir cream into your coffee it can give the illusion of a spiral pattern, but in the case of the Milky Way stars have made fifty orbits in the history of the Milky Way and so the spiral pattern would be hopelessly scrambled after that many windings. The current idea for the spiral pattern is the idea of density waves, a pattern of enhanced activity traced by young stars but not the movement of the young stars themselves. Increased general density leads to the tracers of star formation activity, and the spiral arms that result actually move more slowly than the constituent stars. The best analogy is a traffic pileup that might occur on a freeway behind a slowly moving vehicle, say, painting the road stripe. Cars approach at normal speed, they bunch up near the obstruction, and then they space out again as they reach their normal speed. The result is a moving traffic jam where a high density of cars is created, but the set of cars involved in the high density concentration is constantly changing.