Density Waves
Traffic jams are good examples of density
waves. Certain parts of the freeway may have a high density of cars, yet
individual cars do not stay with the pattern, but flow through it. They
move slowly when at high density, and move quickly when at low density.
The site of an accident might produce a stationary density wave (but again,
cars are always moving through it).
Thus, the spiral arms of a galaxy are just a
pattern that
may rotate slowly or not at all; individual stars will be passing
through it all the time.