Radial Velocity Curves

In the animation below the lower graph displays the radial velocity curves of a binary star system. The drawings at the top and middle indicate the positions of the stars (labeled A and B) and their spectra during each orbital period. Note how the doppler effect splits apart the absorption lines from stars A and B. The entire binary star system is moving away from Earth at 12 km/s, which is why the radial velocity curves are displaced upward from the zero-velocity line.