= Collisionless stellar system
{title2=$T\ll t_{\rm rel}$}
A <collisionless stellar system> is one for which discrete gravitational encounters change its distribution negligibly over the <time> under study, usually $T\ll t_{\rm rel}$. <Stars> can <orbit> and respond collectively to a varying smooth <gravitational potential> even in this regime; collisionless does not mean force-free or stable against collective gravitational instabilities. Its smooth distribution follows the <Collisionless Boltzmann equation>.
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