= Galaxy dynamics
{wiki}
Galaxy dynamics studies how <gravity> organizes the motions of <stars>, <gas> and <dark matter> in <galaxies>. <Doppler shifts> of <spectral lines> measure motion along the line of sight; <proper motions>, where measurable, supply transverse motion. A <rotation curve> constrains the radial gravitational force, while stellar random motions constrain support in a <galactic bulge> or <elliptical galaxy>. The interpretation depends on geometry and on whether the observed material follows nearly circular <orbits>. <Stellar bars> and <spiral arms> exert <torques>, exchange <angular momentum> with the surrounding <galactic disk> and can drive <gas> inward. Thus the dynamics both measures the <mass> distribution and explains how that distribution changes.<Spirals in Galaxies by Jerry A. Sellwood and Karen L. Masters>[Discussion of disk dynamics and secular changes driven by spiral structure]{ref}
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