= Galaxy formation
<Galaxy formation> is the assembly of the matter and <stellar populations> that make a <galaxy>. In a standard <dark matter> framework, <dark matter halos> provide <gravitational potential> wells in which <gas> can collect. <Radiative cooling> lets <gas> lose thermal support and contract, while <star formation> converts some dense <gas> into <stars>.
The process is regulated rather than a one-way collapse. Stellar explosions, <radiation> and accreting central <black holes> return <energy> and <momentum> to the <gas>; <galactic winds> can remove some of it. Mergers and <angular momentum> transport change the geometry. A formation model must therefore explain both how material assembles and why only part of the available <gas> becomes a long-lived <stellar population>.
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