[benjoffe]

It's believed that the spiral arms are quasi-static and that the stars move in and out of these regions of higher density.

See: https://en.wikipedia.org/wiki/Density_wave_theory

[mirimir]

Yes, that's what I vaguely recalled. But we don't have enough observations to test that hypothesis. I wonder, through, whether alternate-path gravitational lensing could provide enough snapshots of some other galaxy to be useful.