A plot of the radial velocity of an object against time, derived from the Doppler shift of spectral lines. With a spectroscopic binary, it may be possible to derive a curve for each component and thence obtain the relative orbit and the mass ratio. With a pulsating star, a radial-velocity curve provides details of the star’s expansion and contraction, and thus of the phase relationship between the variations in radius and the changes in brightness, surface temperature, and spectrum.