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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?

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Choralone t1_j27c5yb wrote

Those are from density waves, which I believe have a different period than orbital.

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