Radial velocity

Gaia determined the radial velocity of its stellar candidates by looking at the way the stars emit light through the Doppler effect. That is, the frequency of light moving away from an observer decreases (redshift), while the frequency of light moving towards an observer will increase (blueshift).

This property can be accurately determined by studying high-resolution spectra of stars, by looking at the way the spectral lines move in the spectrum as the star moves. For example, the spectral lines appeared to move towards the red end of the spectrum as the star moved away from Gaia, while a star moving towards Gaia had its frequency of light compressed, shifting the spectral lines to the blue end of the spectrum.

Stellar radial velocities are needed to determine the velocity of stars with respect to the Sun, and are therefore essential to our understanding of the kinematics of our Galaxy.