What phenomenon allows astronomers to detect spectroscopic binaries by analyzing their combined light spectrum?

Answer

Spectral lines alternately shifting due to the Doppler effect

As stars in a spectroscopic binary orbit, the Doppler effect causes spectral lines to alternately shift toward the red end (redshift) when moving away, and the blue end (blueshift) when moving toward the observer.

What phenomenon allows astronomers to detect spectroscopic binaries by analyzing their combined light spectrum?
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