Étoile binaire
A binary star is a system consisting of two gravitationally bound stars orbiting their common center of mass. Contrary to what intuition might suggest when viewed from Earth, binary stars are the rule rather than the exception: it is estimated that approximately 50 to 85% of the stars in the Milky Way are part of a multiple system containing two, three, or more stars.
The different types of binaries
Binaries are classified based on how they are detected. Visual binaries are separated enough to be resolved by a telescope—two distinct points are visible. Spectroscopic binaries are too close together to be resolved, but spectral analysis reveals their orbital motion through the Doppler effect. Eclipsing binaries are aligned with our line of sight: each star periodically passes in front of the other, producing measurable variations in brightness. Astrometric binaries are detected by the wobble of the visible star, which betrays the presence of an invisible companion.
True and False Pairs
Not all stars that appear close together in the sky are actually related. Optical binaries are merely coincidences of perspective—two stars that happen to be aligned but are at very different distances. Mizar and Alcor in the Big Dipper are sometimes cited as examples, although their actual status is complex: Mizar is actually a sextuple system, and Alcor is likely gravitationally bound.
Importance in Astronomy
Binary stars are crucial to stellar physics. It is by analyzing their orbital motion that we can measure stellar masses—one of the most fundamental quantities in astrophysics. Eclipsing binaries also allow us to accurately measure the radii and temperatures of stars. Close binary systems give rise to spectacular phenomena: mass transfers, novae, Type Ia supernovae, and X-ray sources.
A concrete example: Sirius
Sirius, the brightest star in the sky, is actually a binary system. Sirius A is an A-type white star, and Sirius B is a white dwarf discovered in 1862 and visually confirmed in 1915. It was the detection of Sirius B that led to the first identification of a white dwarf. The orbital period of the system is 50 years.
Did you know?
The Alpha Centauri system, the star closest to the Sun, is actually a triple system: Alpha Centauri A and B form a close binary system, and Proxima Centauri orbits them at a great distance. Proxima hosts at least one exoplanet in its habitable zone.
Frequently Asked Questions
Does the Sun have a binary companion?
No, the Sun is a solitary star. This characteristic—which is actually rare in the galaxy—may have contributed to the orbital stability of the solar system. An old hypothesis regarding a hypothetical companion named Nemesis has been refuted by modern infrared surveys.
Can a planet exist in a binary system?
Yes. Exoplanets are detected in binary systems either orbitingjust one of the two stars (S-type orbit), or around both stars (P-type circumbinary orbit). The Kepler-16 system contains a planet orbiting a binary star system, reminiscent of the famous double sunset on Tatooine in Star Wars.
How do binary stars form?
During star formation, a collapsing molecular cloud can fragment into several clumps, each of which forms a star. If these clumps are close enough together, they remain gravitationally bound—giving rise to a binary or multiple star system.