binary star

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 or optical double stars. A star's binary companion is a star with which it shares such an orbit. A higher-order multiple star system has three or more stars and the terms multiple star system and multiple star are sometimes used with that meaning, but often meant to include binary stars as well.

Binary (or more) star systems are said to be common: the most convincingly cited 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. Their radial velocities' are sometimes both determinable, yielding their mass ratio (μ, the ratio of the two masses, i.e., 1 for stars of equal mass). In cases where the total mass can also be determined, e.g., from the orbital period and radius, the mass of each star is evident.

The commonly-used system for designating the individual stars of a binary/multiple star system consists of adding letters to the star system's designator, with letter "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".

Binary stars generally have similar composition (as shown by their spectra), as if they were formed together. Binaries formed together are known as primordial binaries, another sign being aligned rotation axes. A capture requires the coincidence of stars passing close to each other, plus something to change their velocity, such as a third star, or with just two, tidal forces between them (i.e., tidal-capture binaries). The "close pass" is more likely in areas with a very high density of stars such as the center of globular clusters or galaxies. Simulations have suggested that tidal capture binaries are often the result of an exchange between a single star and an existing binary resulting in the single star becoming a binary companion, with the replaced binary companion becoming a single star.

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 (i.e., not bright enough to be seen).
• spectroscopic binary - measurement of the spectrum over time shows an oscillating Doppler shift of some spectral line, indicating the type of radial motion that orbiting a companion would produce.

Another set of classes is based upon how close they are and how much they interact:

• compact binary - small orbits: one criteria is that they orbit 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.

Given these possibilities as well as the different sizes/spectral types 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 absorption lines, often apparently a red giant that previously received mass from a companion carbon star.
• X-ray binary (XRB) or binary X-ray system - producing X-rays, generally generated by shock during accretion by a neutron star. It could be an X-ray pulsar or an X-ray burster.
• VV Cephei systems - (like VV Cephei) consisting of a giant star and a hot star forming a long-period variable eclipsing binary with an eccentric orbit and which interact when they are near each other.
• ζ Aurigae systems - (or Zeta Aurigae systems, like the star ζ Aurigae) are analogous to VV Cephei systems but with a different class of giant.
• Algol.
• post-common-envelope binary (PCEB).