#density waves
A place where things crowd for a while and then move on, like a traffic jam that cars drive into and out of while the jam itself stays put or drifts slowly. A spiral galaxy's arms are density waves: no star stays in an arm, so the arms don't wind up as the inner stars outrun the outer ones. One way to draw one is to put each star on an oval and turn each oval a little further than the one inside it: the ovals crowd along spiral lines.
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