This allows to identify systematic differences to spectroscopic values for stars of different type, and to calibrate wavelength displacements of complex spectra in terms of true stellar velocities. Astrometric signatures of radial velocity For any star, its component of radial motion may become visible as a secular change in its parallax and proper motion. Although the theoretical possibility of thus deducing radial velocities was realized long ago, only Hipparcos space astrometry reached an accuracy level permitting a meaningful search for differences to spectroscopic velocities.
Proper motion varies as the angle to the space-velocity vector changes.
Astrometric Radial Velocities. Moving clusters, whose stars are grouped around the same average velocity vector, offer a special possibility to determine astrometric radial velocities. As the cluster moves in the radial direction, it appears to contract or expand at a rate equal to the relative rate of change in distance. The moving-cluster method for determining astrometric radial velocities. Stars in open moving clusters share the same velocity vector.