How to compute a star's space velocity

In the diagram, the true space velocity of a star is denoted by V. V can be thought of as the addition of the vectors Vt and Vr. Vr can be measured by looking at a star's spectra and measuring the doppler shift of the absorption lines. Vt is computed by multiplying the distance to the star to the small angle that the star appears to move on the sky as a result of its motion. This small angle is known as the star's proper motion.

So from the observed radial velocity (Vr), proper motion, and distance to the star we can calculate the true space velocity of the star relative to the Earth and Sun.