Stellar dynamics
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Stellar dynamics is the branch of astrophysics which describes in a statistical way the collective motions of stars subject to their mutual gravity. The long range of gravity and the slow "relaxation" of stellar systems prevent the use of the methods of statistical physics. The motion of stars in a galaxy or in a globular cluster are principally determined by the average distribution of the other, distant stars, and little influenced by the nearest stars.
The "relaxation" of stars is the process deflecting the individual trajectories of stars from the one they would have if the distribution of matter was perfectly smooth. The "2-body relaxation" is induced by the individual star-star interactions, while the "violent relaxation" is induced by a large collective variation of the stellar system shape.