Double, Multiple, and Binary Stars

More than half of all stars are part of multiple star systems.

Binary Stars

Two stars orbiting their common center of mass:

• Visual binaries - Both stars visible through telescopes.
• Spectroscopic binaries - Detected by periodic Doppler shifts in their spectral lines.
• Eclipsing binaries - One star periodically passes in front of the other, causing regular brightness variations.
• Astrometric binaries - One star's "wobble" reveals an unseen companion.

Multiple Star Systems

Three or more stars bound by gravity, usually in hierarchical arrangements:

• Triple systems (e.g., Alpha Centauri)
• Quadruple systems
• Open clusters with dozens to thousands of stars

Optical Doubles

Stars that appear close together in the sky but are actually at very different distances - not true binary systems.

Why Study Binary Stars?

Binary stars allow astronomers to measure stellar masses, radii, and luminosities directly. They're crucial for testing theories of stellar evolution.