Galaxy (source code)
= Galaxy
{wiki}
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.<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]{ref}
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