Which kind of binary star is detected by periodic Doppler shifts in its spectral lines?

A binary star detected by periodic Doppler shifts in its spectral lines is a spectroscopic binary.

In a spectroscopic binary, the stars orbit their shared center of mass, but the system may be too close or distant to resolve as two separate points of light. As a star moves toward Earth, its spectral lines shift toward shorter, bluer wavelengths; as it moves away, they shift toward longer, redder wavelengths. Repeated measurements reveal the orbital motion through a periodic radial-velocity pattern.

Some systems show lines from both stars and are called double-lined spectroscopic binaries, or SB2 systems. Others show only one star’s lines moving back and forth and are called single-lined systems, or SB1 binaries. Spectroscopy can therefore reveal an unseen companion and provide estimates of orbital period, velocity, and constraints on stellar masses.

“Eclipsing binary” describes a system detected by periodic changes in brightness when one star passes in front of the other, while “visual binary” describes one whose components can be spatially resolved. These labels are not mutually exclusive: a binary can be both spectroscopic and eclipsing if it produces detectable line shifts and eclipses.