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Binary Star

(system of two stars co-orbiting)

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:

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

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. Binaries formed together are known as a Primordial Binaries, another sign being aligned spin axes. A capture requires the coincidence of stars passing close to each other, plus something to change their velocity, such as 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.

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:

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".

(star type,binary stars)

Referenced by:
Aitken Double Star Catalogue (ADS)
Alpha Centauri
Am Star
Astrometric Binary
Black Widow Pulsar (B1957+20)
Black Hole Binary (BHB)
Black Hole Merger
Blended Spectra
Binary Neutron Star (BNS)
Methylidyne (CH)
Circumbinary Planet
Contact Binary
Double-line Spectroscopic Binary
Eclipsing Binary
Extra-Solar Planet
Galactic Binary
GG Tau
Globular Cluster (GC)
Gravitational Lensing
Guide Star Catalog (GSC)
GW Detection (GW)
Heggie-Hills Law
High-Velocity Star
Instability Region
Kepler Telescope
Kepler Radius
Luhman 16
Luyten 726-8
Mass Function
Mass Loss
Gravitational Microlensing
Multiplicity Fraction
Optical Double
Orbital Inclination
Post-common Envelope Binary (PCEB)
Hulse-Taylor Binary (PSR B1913+16)
Radial Velocity (RV)
Redshift (z)
Rossiter-McLaughlin Effect (RM Effect)
Solar System
Spectroscopic Binary
Spectrum Binary
Stellar Population Synthesis (SPS)
SS 433
Stellar Distance Determination
Stellar Parameter Determination
Symbiotic Binary
Tidal Capture
Transiting Planet
Transit Timing Variations (TTV)
Turn-off Point (TO)
Variable Star
Visual Binary