Submitted by Sabre-Tooth-Monkey t3_zyesvt in askscience
alleyoopoop t1_j26ug0v wrote
Reply to comment by Aseyhe in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey
> Note that this implies the galaxy cannot be rotating rigidly. Objects closer to the center have shorter orbital periods.
Is that why many galaxies are spiral shaped?
Choralone t1_j27c5yb wrote
Those are from density waves, which I believe have a different period than orbital.
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