What is a defining characteristic of spectroscopic binaries?

Spectroscopic binaries are characterized by the presence of two stars that cannot be resolved as separate entities through a telescope. Instead, their presence is inferred through the analysis of their spectral lines. A defining feature of these systems is that they exhibit periodic shifts in the spectral lines of the observed light. This phenomenon is a result of the Doppler effect, which occurs when one of the stars moves towards us and the other moves away from us as they orbit a common center of mass.

The shift in spectral lines allows astronomers to detect the motion of the stars and determine their velocities, leading to the conclusion that they are in a binary system. This is fundamentally different from visual binaries, where both stars can be directly observed. The periodic nature of the shifts can indicate the orbital period of the binary system, providing valuable information about the stars involved.