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A binary star system consists of two Star s both Orbit ing around their Center Of Mass . For each star, the other is its ''companion star''. Recent research suggests that the majority of stars is comprised in a binary (or multiple) star system. Binary star systems are very important in Astrophysics : most of the properties of any star are deduced from observations of binary star systems. Without these, for example, it would be impossible to compute the mass of any random star. Care should be taken not to confuse binary stars with Optical Double Stars , which appear to be close together as seen from Earth , but are in reality not bound by Gravity . Binary stars can either be optically distinguishable ( Visual Binaries ) or only by indirect techniques such as Spectroscopy . If binaries happen to orbit in a plane Perpendicular to our line of sight, it is possible for them to Eclipse each other ( Eclipsing Binary ). Binary stars are also interesting as it is possible for the companions to exchange mass, bringing their Evolution to stages that single Star s cannot attain. The most famous examples of binaries are Algol (an Eclipsing Binary ), Sirius and Cygnus X-1 (of which one member is probably a Black Hole ). BACKGROUND Terminology photograph of the Sirius binary system is the first one in which Sirius B can clearly be distinguished (lower left).]] The term 'binary star' was apparently coined by Sir William Herschel in 1802 to designate ''a real double star; the union of two stars that are formed together in one system by the laws of attraction''. Any two closely-spaced stars might appear to be a '' Double Star '', the most famous case being Mizar and Alcor in the Big Dipper . Odds are though, that a double star is probably a foreground/background star pair that only looks like a binary system; the two stars are in reality widely separated in space, but just happen to lie in roughly the same direction as seen from our vantage point. Such false binaries are termed '' Optical Binaries ''. With the invention of the Telescope , many such pairs were found. Herschel, in 1780, measured the separation and orientations of over 700 pairs that appeared to be binary systems and found that about 50 pairs changed orientation over two decades of observation. Formation of Binary Star Systems , University of Tennessee A true binary is a pair of stars bound together by of the emitted Light . These systems, known as ''spectroscopic binaries'', consist of relatively close pairs of stars such that the Spectral Line s in the light from each one shifts first toward the blue, then toward the red, as it moves first toward us, and then away from us, during its motion about their common Center Of Mass , with the period of their common orbit. If the orbital plane is very nearly along our line of sight, the two stars partially or fully Occult each other regularly, and the system is called an '' Eclipsing Binary '', of which Algol is the best-known example. Eclipsing Binary Stars , D. Bruton, Stephen F. Austin State University Binary stars that are both visual and spectroscopic binaries are rare, and are a precious source of valuable information when found. Unless they are relatively close to Earth , visual binary stars have a large true separation, and consequently usually have orbital speeds too small to be measured spectroscopically. Conversely, spectroscopic binary stars move fast in their orbits because they are close together—usually too close to be detected as visual binaries. Binaries that are both visual and spectroscopic are thus usually relatively close to us. Scientists have discovered some stars that seem to orbit around an empty space. '', where the mass of the unseen companion is about nine times that of our sun; far exceeding the Maximum Theoretical Mass of a neutron star, the other likely candidate for the companion. X-ray Binary Stars , NASA Binary Star Systems , NASA Use in astrophysics Binaries provide the best method for Astronomer s to determine the mass of a distant star. The gravitational pull between them causes them to orbit around their common center of mass. From the orbital pattern of a visual binary, or the time variation of the spectrum of a spectroscopic binary, the mass of its stars can be determined. Because a majority of stars exist in binary systems, binaries are particularly important to our understanding of the processes by which stars form. In particular, the period and masses of the binary tell us about the amount of Angular Momentum in the system. Because this is a Conserved Quantity in Physics , binaries give us important clues about the conditions under which the stars were formed. In a binary system, the more massive star is usually designated 'A' and its companion 'B'. Thus the bright Main Sequence Star of the Sirius system is Sirius A, while the smaller White Dwarf member is Sirius B. However, if the pair is very widely separated, they might be designated with superscripts as with Zeta Reticuli (ζ1 Ret and ζ2 Ret). Binary and Multiple Star Systems , Lawrence Hall of Science at the University of California BINARY STAR CLASSIFICATIONS , with an indication of the variation in intensity. D. Gossman, ''Light Curves and Their Secrets'', Sky & Telescope (October 1989, p.410) Eclipsing Binary Simulation , Cornell Astronomy]] At present, binary stars are classified into four types according to their observable properties Binary Stars , Cornell Astronomy: Any star can belong to several of these classes, for example several spectroscopic binaries are also eclipsing binaries. Another three-category classification is based on the distance of the stars, relative to their sizes: RESEARCH FINDINGS During the past 200 years a large amount of research has been carried out on binary stars leading to some general conclusions. It is believed that at least a quarter of all stars are at least binary systems, with as many as 10% of these systems containing more than two stars (ternary etc.). There is a direct correlation between the Period Of Revolution of a binary star and the Eccentricity of its orbit, with systems of short period having smaller eccentricity. Binary stars may be found with any conceivable separation, from pairs orbiting so closely that they are practically in contact with each other, to pairs so distantly separated that their connection is indicated only by their common Proper Motion through space. Remarkably, among gravitationally-bound binary star sytems, there exists a so called Log Normal Distribution of periods, with the majority of these systems orbiting with a period of about 100 years. In pairs where the two stars are of equal Brightness , they are also of the same Spectral Type . In systems where the brightnesses are different, the fainter star is bluer if the brighter star is a Giant Star , and redder if the brighter star belongs to the Main Sequence . Birth and Death of Stars , J. Schombert, University of Oregon (upper left), which orbits a ternary star system. The brightest companion is just below the Horizon .]] Since mass can be determined only from gravitational attraction, and the only stars (with the exception of the Sun , and Gravitationally-lensed stars), for which this can be determined are binary stars, these are a uniquely important class of stars. In the case of a visual binary star, after the orbit has been determined and the Stellar Parallax of the system determined, the combined mass of the two stars may be obtained by a direct application of the Keplerian Harmonic Law . Binary Star Motions , Cornell Astronomy Unfortunately, it is impossible to obtain the complete orbit of a spectroscopic binary unless it is also a visual or an eclipsing binary, so from these objects only a determination of the joint product of mass and the Sine of the angle of inclination relative to the line of sight is possible. Therefore, without additional information regarding the angle of inclination, the mass can only be inferred in a Statistical sense. In the case of eclipsing binaries which are also spectroscopic binaries, it is possible to find a complete solution for the specifications (mass, Density , size, Luminosity , and approximate shape) of both members of the system. . Planets with Two Suns Likely Common , M. Schirber, Space.com MASS TRANSFER AND ACCRETION As a Main Sequence Star increases in size during its Evolution , it may at some point exceed its Roche Lobe , meaning that some of its matter ventures into a region where the Gravitational Pull of its companion star is larger than its own. Z. Kopal, ''The Roche Problem'' (Kluwer Academic, 1989) The result is that matter will transfer from one star to another, either being absorbed by direct impact or through an Accretion Disc . The process influences the evolution of both companions, and creates stages that can not be attained by single Star s. Mass Transfer in the Binary Star Algol , J. M. Blondin, M. T. Richards, M. L. Malinowski, American Museum of Natural History MULTIPLE STAR EXAMPLES
BINARY STARS IN FICTION '' Isaac Asimov '''s '' Nightfall '' is set in a six-sun system. The 1994 computer game '' Little Big Adventure '' was set on a planet stabilised between two stars. The planet Tatooine in the movie Star Wars orbits the binary star system of Tatoo. The names of the stars are Tatoo 1 and Tatoo 2. Manticore System is a binary star in Honorverse with three habitable planets, two of them (capital Manticore and Sphinx ) orbiting Manticore A and one ( Gryphon ) orbiting Manticore B . In the episode " Singularity " the ship visits a three-sun (Trinary or Ternary) star system. The Anime series '' Simoun '' is set on a planet in a binary star system. SEE ALSO REFERENCES EXTERNAL LINKS
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