In spectroscopic binary stars, we see the feature of the spectrum (spectral lines)
moving back and forth in wavelength. This change is due to the Doppler Effect, and
indicates that the sources of the radiation (the stars) are moving too, The idea of a
spectroscopic binary is shown in Figures 21.4 and 21.5 on p483 of the textbook.
Figures with spectroscopic binaries.
Spectroscopic binaries also tend to be close-packed binaries with periods of a few days,
and in some cases less than a day. Some of these are the so-called contact binaries.