The term "Galaxy" can refer to several different concepts, depending on the context: 1. **Astronomy**: In astronomy, a galaxy is a massive system that consists of stars, stellar remnants, interstellar gas and dust, and dark matter, all bound together by gravity. Our own galaxy is called the Milky Way, which is just one of billions of galaxies in the observable universe.
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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]
Spirals in Galaxies by Jerry A. Sellwood and Karen L. Masters. Abstract and discussion of disk dynamics: spiral activity, stellar random motion and the dissipative gas component
