Binary stars and their masses

Two stars that look close in the sky may be an optical pair (actually far apart) or a physical binary – two stars bound by gravity, revolving about their common centre of mass. Physical binaries are found in three ways. Visual binaries – both stars are seen separately in a telescope (Sirius A and B, period ≈ 50 years; Alpha Centauri A and B, ≈ 80 years). Spectroscopic binaries – the components are too close to separate, but the Doppler effect makes the spectral lines shift periodically to the red and to the blue. Eclipsing binaries – the orbital plane is nearly along our line of sight, so the stars cover each other and the brightness dips periodically (Algol, β Persei, period ≈ 2.87 days). Binaries are important because they allow stellar masses and radii to be measured directly; about half of Sun-like stars are in multiple systems.