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12/19/11 Binary star - Wikipedia, the free encyclopedia Binary star From Wikipedia, the free encyclopedia (Redirected from Binary stars) A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes,[1] or secondary. Research between the early 19th century and today suggests that many stars are part of either binary star systems or star systems with more than two stars, called multiple star systems. The term double star may be used synonymously with binary star, but Hubble image of the Sirius binary more generally, a double star may be system, in which Sirius B can be either a binary star or an optical clearly distinguished (lower left) double star which consists of two stars with no physical connection but which appear close together in the sky as seen from the Earth. A double star may be determined to be optical if its components have sufficiently different proper motions or radial velocities, or if parallax measurements reveal its two components to be at sufficiently different distances from the Earth. Most known double stars have not yet been determined to be either bound binary star systems or optical doubles. Binary star systems are very important in astrophysics because calculations of their orbits allow the masses of their component stars to be directly determined, which in turn allows other stellar parameters, such as radius and density, to be indirectly estimated. This also determines an empirical mass- luminosity relationship (MLR) from which the masses of single stars can be estimated. en.wikipedia.org/wiki/Binary_stars 1/27 12/19/11 Binary star - Wikipedia, the free encyclopedia Binary stars are often detected optically, in which case they are called visual binaries. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known. They may also be detected by indirect techniques, such as spectroscopy (spectroscopic binaries) or astrometry (astrometric binaries). If a binary star happens to orbit in a plane along our line of sight, its components will eclipse and transit each other; these pairs are called eclipsing binaries, or, as they are detected by their changes in brightness during eclipses and transits, photometric binaries. If components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these close binary systems can exchange mass, which may bring their evolution to stages that single stars cannot attain. Examples of binaries are Sirius and Cygnus X-1 (of which one member is probably a black hole). Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors of both novae and type Ia supernovae. Contents 1 Discovery 2 Classifications 2.1 Methods of observation 2.1.1 Visual binaries 2.1.2 Spectroscopic binaries 2.1.3 Eclipsing binaries 2.1.4 Astrometric binaries 2.2 Configuration of the system 2.3 Cataclysmic variables and X-ray binaries 3 Orbital period 4 Designations 4.1 A and B 4.2 1 and 2 4.3 Hot and cold 5 Evolution en.wikipedia.org/wiki/Binary_stars 2/27 12/19/11 Binary star - Wikipedia, the free encyclopedia 5.1 Formation 5.2 Mass transfer and accretion 5.3 Runaways and novae 6 Astrophysics 6.1 Research findings 6.1.1 Planets 7 Examples 8 Multiple star examples 9 See also 10 Notes and references 11 External links Discovery The term binary was first used in this context by Sir William Herschel in 1802,[2] when he wrote:[3] If, on the contrary, two stars should really be situated very near each other, and at the same time so far insulated as not to be materially affected by the attractions of neighbouring stars, they will then compose a separate system, and remain united by the bond of their own mutual gravitation towards each other. This should be called a real double star; and any two stars that are thus mutually connected, form the binary sidereal system which we are now to consider. By the modern definition, the term binary star is generally restricted to pairs of stars which revolve around a common centre of mass. Binary stars which can be resolved with a telescope or interferometric methods are known as visual binaries.[4][5] For most of the known visual binary stars one whole revolution has not been observed yet, they are observed to have travelled along a curved path or a partial arc.[6] The more general term double star is used for pairs of stars which are seen to be close together in the sky.[2] This distinction is rarely made in languages en.wikipedia.org/wiki/Binary_stars 3/27 12/19/11 Binary star - Wikipedia, the free encyclopedia other than English.[4] Double stars may be binary systems or may be merely two stars that appear to be close together in the sky but have vastly different true distances from the Sun. The latter are termed optical doubles or optical pairs.[7] Since the invention of the telescope, many pairs of double stars have been found. Early examples include Mizar and Acrux. Mizar, in the Big Dipper (Ursa Major), was observed to be double by Giovanni Battista Riccioli in 1650[8][9] (and probably earlier by Benedetto Castelli and Galileo).[10] The bright southern star Acrux, in the Southern Cross, was discovered to be double by Father Fontenay in 1685.[8] John Michell was the first to suggest that double stars might be physically attached to each other when he argued in 1767 that the probability that a double star was due to a chance alignment was small.[11][12] William Herschel began observing double stars in 1779 and soon thereafter published catalogs of about 700 double stars.[13] By 1803, he had observed changes in the relative positions in a number of double stars over the course of 25 years, and concluded that they must be binary systems;[14] the first orbit of a binary star, however, was not computed until 1827, when Félix Savary computed the orbit of Xi Ursae Majoris.[15] Since this time, many more double stars have been catalogued and measured. The Washington Double Star Catalog, a database of visual double stars compiled by the United States Naval Observatory, contains over 100,000 pairs of double stars,[16] including optical doubles as well as binary stars. Orbits are known for only a few thousand of these double stars,[17] and most have not been ascertained to be either true binaries or optical double stars.[18] This can be determined by observing the relative motion of the pairs. If the motion is part of an orbit, or if the stars have similar radial velocities and the difference in their proper motions is small compared to their common proper motion, the pair is probably physical.[19] One of the tasks that remains for visual observers of en.wikipedia.org/wiki/Binary_stars 4/27 12/19/11 Binary star - Wikipedia, the free encyclopedia double stars is to obtain sufficient observations to prove or disprove gravitational connection. Classifications Methods of observation Binary stars are classified into four types according to the way in which they are observed: visually, by observation; spectroscopically, by periodic changes in spectral lines; photometrically, by changes in brightness caused by an eclipse; or astrometrically, by measuring a deviation in a star's position caused by an unseen companion.[4][20] Any binary star can belong to several of these classes; for example, several spectroscopic binaries are also eclipsing binaries. Visual binaries A visual binary star is a binary star for which the angular separation between the two components is great enough to permit them to be observed as a double star in a telescope, or even high-powered binoculars. The resolving power of the telescope is an important factor in the detection of visual binaries, and as telescopes become larger and more powerful an increasing number of visual binaries will be detected. The brightness of the two stars is also an important factor, as brighter stars are harder to separate, due to their glare, than the dimmer ones. The brighter star of a visual binary is the primary star, and the dimmer is considered the secondary. In some publications (especially older ones), a faint secondary is called the comes (plural comites; English: companion.) If the stars are the same brightness, the discoverer designation for the primary is customarily accepted.[21] The position angle of the secondary with respect to the primary is measured, together with the angular distance between the two stars. The time of observation is also recorded. After a sufficient number of observations are en.wikipedia.org/wiki/Binary_stars 5/27 12/19/11 Binary star - Wikipedia, the free encyclopedia recorded over a period of time, they are plotted in polar coordinates with the primary star at the origin, and the most probable ellipse is drawn through these points such that the Keplerian law of areas is satisfied. This ellipse is known as the apparent ellipse, and is the projection of the actual elliptical orbit of the secondary with respect to the primary on the plane of the sky. From this projected ellipse the complete elements of the orbit may be computed, where the semi-major axis can only be expressed in angular units unless the stellar parallax, and hence the distance, of the system is known.[5] Spectroscopic binaries Sometimes, the only evidence of a binary star comes from the Doppler effect on its emitted light.

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