Binary, Multiple & Variable Stars: Structures & Vocabulary

Many stars are members of binary or multiple systems, and many also vary in brightness. Orbital motion in binaries makes stellar masses measurable, while variability can reveal pulsation, eclipses, rotation, accretion, spots, or explosive events.

Two bound stars orbit their common center of mass according to gravity. If the orbit is favorably aligned, one can eclipse the other. In close systems, a star can fill its Roche lobe and transfer matter, changing both stellar evolution and orbital dynamics. Pulsating variables expand and contract because of instabilities in their outer layers.

Periodic radial-velocity shifts reveal orbital periods and velocity amplitudes, while eclipse light curves constrain radii and inclination. Combining orbital size with period yields masses. Repeated photometry separates pulsational, rotational, and eclipsing variability by characteristic patterns.

Binary categories describe how a system is detected—visual, spectroscopic, eclipsing, or astrometric—rather than mutually exclusive physical types. Variables likewise include intrinsic pulsators and eruptive stars as well as geometric variability from orbital eclipses.