The radial velocity of a star, denoted vr, is its velocity along the line of sight of an observer. It is calculated from the Doppler shift in the lines of stellar spectra, taking into account the nature of the star. By comparing the observed spectra of a star with reference spectra we can measure the wavelength shift of the reference lines and thus determine vr.

A star receding from us has a redshifted spectrum and a positive radial velocity, while a star coming towards us has a blueshifted spectrum and a negative radial velocity.