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A galaxy is a gravitationally bound system of stars, stellar remnants and usually gas, interstellar dust and a substantial dark matter component. Its appearance reflects both its contents and their motion: a spiral galaxy has a rotating galactic disk, whereas an elliptical galaxy is largely supported by disordered stellar motion. Star formation makes luminous young populations from gas, while stellar feedback returns material and energy to the interstellar medium. A photograph therefore samples only part of the system: galaxy dynamics uses motions to constrain the underlying mass, including matter that emits little light. Spiral arms also redistribute angular momentum and changes the galactic disk over time.^[ref]

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.^[ref]

Jerry A. Sellwood studies the dynamics of galactic disks, including the origin and evolution of spiral density waves. His review with Karen L. Masters compares observations, dynamical theory and simulations of recurring spiral patterns.

The review emphasizes a physical feedback: transient spiral activity increases random stellar motion, which can make further spiral activity harder to sustain. A dissipative gas component can alter that evolution. This connects the visible shape of a spiral galaxy to the changing dynamical state of its stars and gas, rather than treating the arms as a permanently fixed decoration.^[ref]

Karen L. Masters studies the structure and evolution of galaxies. Her review with Jerry A. Sellwood connects observed spiral arms to the dynamics of galactic disks, comparing proposed mechanisms with observations and numerical numerical simulations.

A visible spiral pattern need not be a permanent set of the same stars. Recurrent patterns can rearrange a galactic disk and increase random stellar motion; continuing activity then depends in part on the dissipative gas component. Their review also used the number of spiral arms to discuss how much mass resides in the thin galactic disk relative to its surrounding halo.^[ref]

A spiral density wave is an organized density and gravitational-potential perturbation through which stars and gas can move; it is not generally a permanently labelled set of stars. Differential rotation winds a material arm rapidly. In a wave description, a perturbation has phase $m\varphi +\int kdr-\omega t$, with pattern speed ${\Omega }_p=\omega /m$ distinct from the local stellar angular speed.

For a thin fluid disk the local tightly wound dispersion relation isRotation and pressure resist self-gravity. A collisionless stellar disk requires its stellar response function instead of simply replacing $c_s$ by a dispersion; the familiar local axisymmetric stability parameter is $Q_{\star }={\sigma }_R\kappa /(3.36G\Sigma )$. This is distinct from the gas $Q_g=c_s\kappa /(\pi G\Sigma )$.

Waves exchange angular momentum at Lindblad resonances and near corotation radius. Swing amplification can strengthen a leading disturbance as shear turns it into a trailing one. Bars or companions can force a coherent spiral; recurrent self-excited disturbances also occur. Thus a single indefinitely steady wave is one model, not a description established for every spiral galaxy. Gas compression and shocks along arms can organize star formation, while gravitational torques redistribute angular momentum. Arm brightness, recent star formation and stellar mass density need not trace precisely the same pattern.^[ref]