What Is a Binary Star? Most Stars Are Not Alone
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The sun having no companion is somewhat unusual, since a large share of stars orbit one or more partners. Those systems are not merely common, they are the only direct way to measure a star's mass, which makes them the calibration for essentially everything else known about stars.
How the mass measurement works
Mass is the most important property of a star, determining its brightness, temperature, lifespan and eventual fate, and it cannot be measured for an isolated star because nothing responds to its gravity in a way that can be observed. In a binary system the two stars orbit each other, and the orbit is governed by their combined mass, the size of the orbit and the period, related by a form of Kepler's third law. Measuring the period and the physical size of the orbit therefore yields the total mass directly, and measuring how far each star moves relative to their common centre of mass yields the ratio between them, which separates the total into two individual masses. That is the only direct route available, and every relationship between mass and other stellar properties, including the mass-luminosity relation that underpins stellar astrophysics, was calibrated using binaries. Systems where the orbit is seen edge-on are the most valuable, since eclipses give the sizes of the stars as well.
The kinds that can be observed
Binaries are classified by how they are detected, which reflects geometry and distance rather than any physical difference:
- •Visual binaries, where both stars are resolved separately and the orbit can be watched over years or centuries
- •Spectroscopic binaries, unresolved but revealed by periodic shifts in the wavelengths of spectral lines as the stars move towards and away from us
- •Eclipsing binaries, where the orbital plane is aligned so that each star passes in front of the other, producing a characteristic dip pattern in the combined brightness
- •Astrometric binaries, where only one star is seen and its position wobbles because of an unseen companion, which is how several faint companions were found
- •Optical doubles, which are not binaries at all but chance alignments of stars at different distances, and which must be distinguished by measuring motion
- •Higher multiples, since triples, quadruples and larger hierarchical systems are common, generally arranged as pairs orbiting pairs for stability
When they interact
Close binaries do not simply orbit, since material can pass between the stars and that transforms both. Each star has a region within which its gravity dominates, and if one expands to fill that region as it ages, gas flows onto the companion, which changes both masses and both futures. That mass transfer produces a range of phenomena including novae, where hydrogen accumulating on a white dwarf ignites in a surface explosion that recurs, and type one supernovae, where a white dwarf accumulating mass approaches a critical limit and detonates entirely. Those supernovae matter beyond the systems producing them, since their consistent peak brightness makes them a distance indicator across the universe and was the tool that revealed the accelerating expansion. Interacting binaries also produce X-ray sources where material falls onto a neutron star or black hole, which is how most stellar-mass black holes were identified, and they explain several otherwise puzzling stars whose properties do not fit any single-star evolutionary track.
How common and why
The fraction of stars in multiple systems depends strongly on mass, which is itself informative. Massive stars are overwhelmingly in multiples, with most having at least one companion and many having several, while low-mass red dwarfs, which are by far the most numerous stars, are mostly single. Sun-like stars sit in between, with roughly half in multiple systems by most estimates. That pattern reflects how stars form, since a collapsing cloud core carries angular momentum that is easier to shed by fragmenting into two objects than by any other route, and more massive cores fragment more readily. Dynamical interactions in young clusters then disrupt some pairs and create others. The consequences for planets are actively studied, since planets are known in binary systems both orbiting one star and orbiting both, and the stability of such orbits constrains where they can exist, which matters because if most stars are in multiples then most planetary systems are too.
The takeaway
Two stars orbiting each other is the only direct route to measuring stellar mass, since the period and the orbit size give the total and the relative motion splits it, which is what calibrated everything else known about stars. Eclipsing systems give sizes as well. Close pairs exchange material, producing novae and the supernovae used to measure the universe. Massive stars are nearly all in multiples and red dwarfs mostly are not.