= Stellar orbit
A <stellar orbit> is a <star>'s trajectory in the gravitational field of its surroundings. A close <binary star> can often be approximated by a <Kepler orbit>, while a <star> moving through a <galaxy> responds to an extended distribution of stars, gas and <dark matter>. Its path need not close into an ellipse.
Observed positions, <proper motion> and line-of-sight <velocity> constrain different components of the motion. Reconstructing an orbit also requires a distance and a model of the gravitational potential. Comparing many <stellar orbits> can therefore constrain an unseen mass distribution, but one projected track need not uniquely determine the full three-dimensional trajectory or potential.
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