Figure 10-15Spectral-Line Motion in Binary Star Systems The diagrams at the top indicate the positions and motions of the stars, labeled A and B, relative to Earth. Below each diagram is the spectrum we would observe for these two stars at each stage. The changes in colors (wavelengths) of the spectral lines are due to changes in the stars’ Doppler shifts, as seen from Earth. The graph displays the radial-velocity curves of the binary HD 171978. (The HD means that this is a star from the Henry Draper Catalogue of stars.) The entire binary is moving away from us at 12 km/s, which is why the pattern of radial-velocity curves is displaced upward from the zero-velocity line.