Dynamics in Disk Galaxies A detailed understanding of the internal working of galaxies must incorporate their dynamics - just how the gravitational potential and internal motions balance to give galaxies their forms. This is all from Newton's laws at work, but distributed masses can give results that violate intuitions honed on the inverse-square law. All of this is treated well and exhaustively by Binney and Tremaine. The key is that observable stars and gas move in response to the totalpotential of the mass. Major surprises have come from recent observations both of spirals and ellipticals.DisksLocal density:Close to the disk plane, if we adopt a reference frame sharing the local rotational velocity, stellar motions consist mainly of small oscillations about the mean location (or guiding center) - shades of Ptolemaic astronomy! Radial, tangential, and vertical oscillations may occur, with the vertical frequency not in general matching the in-plane motion as it would for small perturbations to a Keplerian orbit (because galaxies have extended mass distributions). Small
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