12.14 Orbital motion reflected in the shifting spectral lines of a spectroscopic binary

When they are at positions 1(A) and 1(B), the two stars are moving across the line of sight. Because neither of them is moving toward or away from the Earth, neither of them has redshifted or blueshifted spectral lines. Their spectral lines are superimposed. When they are at positions 2, star A is moving away from the Earth, and its spectral lines are redshifted. At the same time, star B is moving toward the Earth and has blueshifted spectral lines. At positions 3, the two stars are again moving across the line of sight, and their spectral lines again are superimposed. At positions 4, star A is moving toward the Earth and has blueshifted spectral lines. Star B is moving away from the Earth and has redshifted spectral lines. Thus the lines of the two stars shift back and forth in the spectrum during each orbital period of the binary star system.