Binary, Multiple & Variable Stars: Physical Mechanisms

Visual binaries are resolved as separate stars, spectroscopic binaries reveal periodic Doppler shifts, eclipsing binaries show recurring brightness dips, and astrometric binaries reveal small positional wobbles. Variable stars include intrinsic pulsators and eruptive stars as well as extrinsic systems whose brightness changes geometrically.

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.

Both stars orbit their common center of mass. Close binaries can exchange matter through Roche-lobe overflow, changing mass, spin, and evolutionary path. Pulsation instead arises from internal instability, with opacity changes in certain ionization zones able to drive repeated expansion and contraction.