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A rotation curve (or galaxy rotation curve) of a galaxy is a function over its radial dimension mapping to the orbital speed of the stars at that radius. Rotation curves are of interest in that they show a pattern other than that expected of Keplerian orbits based upon the mass observed in the galaxy's stars and clouds. This mismatch is a primary motivation for the concept of dark matter.
The determined rotation curve of the Milky Way is the basis for the concept of kinematic distance, determining the distance to Milky Way objects suggested by their radial velocity.