Spectroscopy makes it possible to study binary systems in which the two stars are close together

• Some binaries can be detected and analyzed, even though the system may be so distant or the two stars so close together that the two star images cannot be resolved

• A spectrum binary appears to be a single star but has a spectrum with the absorption lines for two distinctly different spectral types

• A spectroscopic binary has spectral lines that shift back and forth in wavelength

• This is caused by the Doppler effect, as the orbits of the stars carry them first toward then away from the Earth