A Binary Star is a pair of stars that orbit each other. Double Star means the same thing except that it also includes stars that are not orbiting and not close together but happen to be on the same line of sight from Earth, referred to as Apparent Binary stars.

Binary (or more) star systems are said to be common: the best number I've found is that roughly third of all star systems have two or more stars. They are extremely useful in the study of stellar physics, both to use the orbital dynamics for Stellar Parameter Determination, and for those close enough to interact further, giving additional situations to observe, to infer and test the physics of Stellar Structure.

A system can have three or more stars. The term Multiple Star System is sometimes reserved for this case, and the phrase Higher-Order Multiple Star System is sometimes used.

A common classification of binary stars is based on the method by which they were determined to be binary:

• Visual Binary - separate stars can be seen or photographed.
• Eclipsing Binary - a periodic variation in Luminosity suggesting a star passing in front of its companion.
• Spectrum Binary - a star whose spectrum looks like the combined spectrum of two stars.
• Astrometric Binary - measurement of the star's position in the sky over time suggests an orbit-like movement around an unseen companion.
• Spectroscopic Binary - measurement of the spectrum over time shows an apparent Doppler Shift signifying the type of radial motion that orbiting a companion would produce.

Another classification is based upon how close they are and how much they interact:

• Compact Binary - small orbits: one criteria is that they rotate in five days or less.
• Close Binary Star - similar meaning: close enough to consider their cross-influence and interaction.
• Detached Binary - sufficiently distant that the stars are (basically) spherical.
• Semidetached Binary - sufficiently close that one of the stars transfers mass to the other, but not touching.
• Contact Binary - touching; at minimum, both stars "stretched" by Gravity to a contact point.

A binary star's Mass Ratio (μ) is the ratio of the two masses, i.e., 1 for stars of equal mass. When the Total Mass can also be determined, e.g., from the orbital period and size, the mass of each star is evident.

Binary stars generally have similar composition (as shown by their spectra), as if they were formed together.

Given the large range of distances between the stars and the different sizes/Spectral Classes of the individual stars, binary stars show a wide variety and interactions between them produce characteristics unseen in non-binary stars. For example:

• Barium Star - a star showing unexpected barium lines, typically interpreted as a Red Giant that previously received mass from a companion Carbon Star.
• X-ray Binary (XRB) or Binary X-ray System - includes an accreting Neutron Star or Black Hole, whose high gravitational potential energy provides the energy to produce X-rays (through shock). With a strong Magnetic Field, it might be an X-ray Pulsar.
• Algol.
• Post-common Envelope Binary.

The commonly-used system for referring to the individual stars of a binary/multiple star system are indicated by following the name with "A" for the brightest, "B" for the second brightest, then "C" and so on. For example, the two stars making up Sirius are termed "Sirius A" and "Sirius B". If two are very close and a third is distant, the two close stars might use lower-case suffixes, i.e., "Aa" and "Ab", with the further member called "B".