What Is A Binary Star?
QUICK ANSWER
A binary star is a system of two stars gravitationally bound and orbiting a common center of mass. Most stars in our galaxy are in binary or multiple systems rather than alone, like our Sun. Sirius is a famous binary, consisting of Sirius A and the white dwarf Sirius B.
Most stars in the universe aren't single like our Sun; they come in pairs or larger groups. A binary star system consists of two stars orbiting a common center of mass under their mutual gravitational attraction. Binary stars are fundamental to astronomy because they let us measure stellar masses directly. They also produce many of the most interesting phenomena in the universe, from supernovae to gravitational wave sources.
How common are binary stars?
Very common, possibly the majority. According to NASA, observations of stars like Sirius have revealed that most stars in our galaxy are in binary or multiple star systems. Our solar system, with its single Sun, is in the minority. Among the most massive stars, the binary fraction approaches 100%. Low-mass red dwarfs are more often single but still frequently come in pairs. Binary systems are clearly the norm in the universe.
What are the types of binary stars?
Several, classified by how we detect them. Visual binaries are wide enough to be resolved as two stars through a telescope, like Sirius A and B. Spectroscopic binaries are too close to resolve but are detected through Doppler shifts in their light as they orbit. Eclipsing binaries are systems whose orbital plane is edge-on from Earth, so the stars periodically pass in front of each other, causing detectable brightness dips. Astrometric binaries are detected by gravitational pull on their visible companion.
How are masses measured?
Through orbital mechanics. Binary stars let us measure stellar masses directly. From the orbital period and the separation between the two stars, astronomers can calculate the total mass of the system using Kepler's laws. With additional information about the individual orbits, the masses of each star can be determined. Most of what we know about stellar masses across the universe ultimately depends on binary star measurements.
What about more than two stars?
Triple, quadruple, and higher systems also exist. Many star systems contain more than two stars in stable orbits. Alpha Centauri, our nearest stellar neighbor, is a triple system with two main stars orbiting each other, plus the distant red dwarf Proxima Centauri. Castor in Gemini is a sextuple system (six stars in three pairs). Star formation often produces multiple stars together, though they don't always stay gravitationally bound.
A binary star is a system of two stars gravitationally bound and orbiting a common center of mass. Most stars in our galaxy are in binary or multiple systems, not single like the Sun. Sirius is a famous binary consisting of a main-sequence star and a white dwarf. Binary stars are essential for measuring stellar masses, since their orbits reveal the gravity (and therefore mass) of each star. Higher-order systems with three or more stars also exist.
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