radial velocity

In astronomy, velocity of an object, such as a star or galaxy, along the line of sight, moving towards or away from an observer. The amount of Doppler shift (apparent change in wavelength) of the light reveals the object's velocity. If the object is approaching, the Doppler effect causes a blue shift in its light. That is, the wavelengths of light coming from the object appear to be shorter, tending toward the blue end of the spectrum. If the object is receding, there is a red shift, meaning the wavelengths appear to be longer, toward the red end of the spectrum.