= Stellar dynamics
{wiki}
Stellar dynamics treats <stars> as gravitating bodies whose collective motions reveal and reshape their environment. In a <collisionless stellar system>, stars mainly follow the smooth collective <gravitational potential>; in a sufficiently dense <star cluster>, accumulated gravitational encounters produce <two-body relaxation>. These encounters need not involve physical contact between stars. Observers combine <astrometry> with <radial velocities> to constrain <orbits> and infer unseen <mass>. The <stellar orbits at the Galactic centre> provide a particularly direct example: following a short-period star around <Sagittarius A*> measures the compact central mass, rather than estimating it from the light emitted by its surroundings.<Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits by Andrea M. Ghez and others>[Abstract: Keck astrometry and radial velocities constrain a complete stellar orbit and the central mass]{ref}
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