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.
New to topics? Read the docs here!