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''For the band of the same name, see: Binary Star (band) '' image of the Sirius binary system, in which Sirius B can be clearly distinguished (lower left).]] A binary star is a Stellar System consisting of two Star s Orbit ing around their Center Of Mass . For each star, the other is its ''companion star''. Recent research suggests that a large percentage of stars are part of systems with at least two stars. Binary star systems are very important in Astrophysics , because observing their mutual orbits allows their Mass to be determined. The masses of many single stars can then be determined by extrapolations made from the observation of binaries. Binary stars are not the same as Optical Double Stars , which appear to be close together as seen from Earth , but may not be bound noticeably by Gravity . Binary stars can either be distinguished optically (visual binaries) or by indirect techniques, such as Spectroscopy . If binaries happen to orbit in a plane containing our line of sight, they will Eclipse each other; these are called eclipsing binaries. Systems consisting of more than two components, known as multiple stars, are also not uncommon and are generally classified under the same name. The components of binary star systems can exchange mass, bringing their Evolution to stages that single stars cannot attain. Examples of binaries are Algol (an eclipsing binary), Sirius , and Cygnus X-1 (of which one member is probably a Black Hole ). TERMINOLOGY The term ''binary star'' was , 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.12 A true binary is a pair of stars bound together by Gravity . When they can be Resolved (distinguished) with a powerful enough telescope (if necessary with the aid of Interferometric methods) they are known as ''visual binaries''.34 In other cases, the only indication of binarity is the Doppler Shift of the emitted Light . Systems in which this is the case, known as ''spectroscopic binaries'', consist of relatively close pairs of stars where 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.5 Binary stars that are both visual and spectroscopic binaries are rare, and are a precious source of valuable information when found. Visual binary stars often have large true separations, with periods measured in decades to centuries; consequently, they 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 thus must be relatively close to Earth. Astronomers have discovered some stars that seem to orbit around an empty space. ''Astrometric binaries'' are relatively nearby stars which can be seen to wobble around a middle point, with no visible companion. With some spectroscopic binaries, there is only one set of lines shifting back and forth. The same mathematics used for ordinary binaries can be applied to infer the . Probably the best known example at present is Cygnus X-1 , where the mass of the unseen companion is believed to be about nine times that of our sun; far exceeding the Tolman-Oppenheimer-Volkoff Limit (the maximum theoretical mass of a neutron star, the only other likely candidate for the companion). In this way, Cygnus X-1 became the first object that was widely accepted as being a black hole.78 CLASSIFICATIONS By methods of observation Binary stars are classified into four types according to their observable properties.9 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 . 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 dimmer ones are. 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''; if the stars are the same brightness, the discoverer "chooses" the primary.James Muirden, ed., "Sky-Watcher's Handbook", (New York: W.H. Freeman and Company Ltd., 1993), Glenn Chaple, "Observing Double Stars", p. 227. 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 recorded over a period of time, they are plotted in Polar Coordinate s 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, with the Semi-major Axis being expressed in angular units unless the Stellar Parallax , and hence the distance, of the system is known.10 Spectroscopic binaries A ''spectroscopic binary star'' is a binary star in which the separation between the stars is usually very small, and the Orbit al velocity very high. Unless the plane of the orbit happens to be perpendicular to the line of sight, the orbital velocities will have components in the line of sight and the observed Radial Velocity of the system will vary periodically. Since radial velocity can be measured with a Spectrometer by observing the Doppler Shift of the stars' Spectral Line s, the binaries detected in this manner are known as spectroscopic binaries. Most of these cannot be resolved as a visual binary, even with Telescope s of the highest existing Resolving Power . In some spectroscopic binaries, spectral lines from both stars are visible and the lines are alternately double and single. Such a system is known as a double-lined spectroscopic binary (often denoted "SB2"). In other systems, the spectrum of only one of the stars is seen and the lines in the spectrum shift periodically towards the blue, then towards red and back again. Such stars are known as single-lined spectroscopic The Orbit of a spectroscopic binary is determined by making a long series of observations of the radial velocity of one or both components of the system. The observations are plotted against time, and from the resulting curve a period is determined. If the orbit is Circular then the curve will be a Sine curve. If the orbit is Elliptical , the shape of the curve will depend on the Eccentricity of the ellipse and the orientation of the major axis with reference to the line of sight. It is impossible to determine individually the Semi-major Axis ''a'' and the inclination of the orbit plane ''i''. However, the product of the semi-major axis and the sine of the inclination (i.e. ''a'' sin ''i'') may be determined directly in linear units (e.g. kilometres). If either ''a'' or ''i'' can be determined by other means, as in the case of eclipsing binaries, a complete solution for the orbit can be found.11 Eclipsing binaries An ''eclipsing binary star'' is a binary star in which the Orbit plane of the two Star s lies so nearly in the line of sight of the observer that the components undergo mutual Eclipse s. In the case where the binary is also a spectroscopic binary and the Parallax of the system is known, the binary is quite valuable for stellar analysis.14 |
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