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Binary Star

Binary star

A binary star, often called a double star, is a star system in which two stars linked by their mutual gravity orbit around a central point of mass. Binary stars are quite common. A recent survey of 123 nearby Sun-like stars showed that 57 percent had one or more companions.

English astronomer William Herschel (17381822) made the first discovery of a true binary system in the 1700s. He observed the motion of a pair of stars and concluded that they were in orbit around each other. Herschel's discovery provided the first evidence that gravity exists out-side our solar system.

Herschel discovered more than 800 double stars. He called these star systems binary stars. His son, John Herschel (17921871), continued the search for binaries and catalogued over 10,000 systems of two or more stars.

Words to Know

Astrometric binary: Binary system in which only one star can be seen, but the wobble of its orbit indicates the existence of another star in orbit around it.

Eclipsing binary: Binary system in which the plane of the binary's orbit is nearly edgewise to our line of sight, so that each star is partially of totally hidden by the other as they revolve around a common point of gravity.

Mass: The quantity of matter in the star as shown by its gravitational pull on another object.

Radiation: Energy in the form of waves or particles.

Spectroscopic binary: A binary system that appears as one star producing two different light spectra.

Spectrum: Range of individual wavelengths of radiation produced when light is broken down by the process of spectroscopy.

Visual binary: Binary system in which each star can be seen directly, either through a telescope or with the naked eye.

Types of binary systems

Several kinds of binary stars exist. A visual binary is a pair in which each star can be seen directly, either through a telescope or with the naked eye. In an astrometric binary, only one star can be seen, but the wobble of its orbit indicates the existence of another star in orbit around it. An eclipsing binary is a system in which the plane of the binary's orbit is nearly edgewise to our line of sight. Thus each star is partially or totally hidden by the other as they revolve.

Sometimes a binary system can be detected only by using a spectroscope (a device for breaking light into its component frequencies). If a single star gives two different spectra (range of individual wavelengths of radiation), it is actually a pair of stars called a spectroscopic binary.

A binary star may be a member of one or more of these classes. For example, an eclipsing binary may also be a spectroscopic binary if it is bright enough so that its light spectrum can be photographed.

The only accurate way to determine a star's mass is by studying its gravitational effect on another object. Binary stars have proven invaluable for this purpose. The masses of two stars in a binary system can be determined from the size of their orbit and the length of time it takes them to revolve around each other.

[See also Black hole; Brown dwarf; Doppler effect; Gravity and gravitation; X-ray astronomy ]

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binary star

binary star or binary system, pair of stars that are held together by their mutual gravitational attraction and revolve about their common center of mass. In 1650 Riccioli made the first binary system discovery, that of the middle star in the Big Dipper's handle, Zeta Urase Majoris. True binary stars are distinct from optical doubles—pairs of stars that lie nearly along the same line of sight from the earth but are not physically associated. Binary stars are grouped into three classes. A visual binary is a pair of stars that can be seen by direct telescopic observation to be a distinct pair with shared motion. A spectroscopic binary cannot be seen as two separate stars, even with the most powerful telescopes, but spectral lines from the pair show a periodic Doppler effect that indicates mutual revolution. Some lines indicate motion toward the earth while others indicate motion away; later, as the stars revolve around in their orbit, this pattern reverses. An eclipsing binary has the plane of its orbit lying near the line of sight, and shows a periodic fluctuation in brightness as one star passes in front of the other. The more massive star (A) of a binary is called the primary, and the less massive (B) is called the secondary; e.g., Sirius A and Sirius B are the primary and secondary components of the Sirius system. It seems likely that more than two-thirds of the stars in our galaxy are binary or multiple (a system of more than two stars moving around their mutual center of mass), since many stars within 30 light-years of the sun are binary or multiple. The masses of the components of a spectroscopic binary can be determined from the observed motions and Newton's law of gravitation; binary stars are the only stars outside the solar system for which masses have been directly determined. Binary stars are thus important indicators from which the masses of all similar stars can be deduced. Measurements of the masses of some of the visual binary stars have been used to verify the mass-luminosity relation. Although most binary stars have distance between them, the components of W Ursae Majoris binaries are actually in contact with each other, their mutual gravity distorting their shapes into teardrops. There are binary systems in which one member is a pulsar: PSR 1913+16, for example, has an orbital period of 7 hr 45 min; in this case the other star is also a neutron star. The orbit period decreases as the system loses energy in the form of gravitational waves; used as a clock to measure the effect of the curvature of space-time on the binary's orbit, such a system confirms Einstein's theory of general relativity.

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binary star

binary star Two stars in orbit around a common centre of mass. Visual binaries can be seen as separate stars. In an eclipsing binary, one star periodically passes in front of the other, so that the total light output appears to fluctuate. Most eclipsing binaries are also spectroscopic binaries. A spectroscopic binary is a system too close for their separation to be measured visually and must be measured spectroscopically.

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binary star

bi·na·ry star • n. a system of two stars in which one star revolves around the other or both revolve around a common center.

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Binary star

Binary star

Techniques of observation

Importance

Visual binaries

Study of orbital motion

Astrometric binaries

Spectroscopic binaries

Eclipsing binaries

Mass exchange binaries

X-ray binaries

Resources

Binary stars, often called double stars, refer to pairs of stars sufficiently close to each other in space to be gravitationally bound together. Following the laws of gravitation, each of the components revolves around the common center of mass of the system. At least 50% of stars are found to exist as binary systems, according to conservative statistics. There seems to be no obvious preference for particular combinations of brightness, size, or mass differences and the wide range in periods of revolution from less than a day to thousands of years. Historically, visual binaries, those that appear as double stars when seen through a telescope, were discovered to be gravitationally bound by German-born English astronomer William Herschel (17381822) around 1800.

Techniques of observation

There are a number of telescopic techniques used to discover and study binary stars. No one telescopic method can be used because of the wide range in the

separations exhibited in the systems. The desired information about the orbital motion and the physical quantities of the stars themselves must come from different ways of observing. Hence, there are descriptive classifications of binary stars as determined by the various modes of study discussed below.

Importance

The importance of binaries lies in a number of areas. First, the analysis of a visual binary (where the two components can be seen visually through a telescope) leads to the only direct method for the evaluation of stellar mass, one of the most important parameters of the physical universe. In some cases the mass of the system is found, in other situations the mass ratio of the components, and in some the individual masses can be determined. Second, stellar duplicity plays an important role in the study of the physical aspects of stars, such as relative diameters, surface brightness, the genesis and evolution of stars, and the study of stellar-mass loss.

Visual binaries

Visual binaries are those stellar systems that appear as two stars in the eyepiece of a telescope. They are observed traditionally by manually measuring, at the eyepiece of a telescope, the angular separation of the two components. The angle the fainter stars make with respect to the brighter component is also

taken into account, and refers to the north direction on the plane of the sky. If the measurements of each component are made and photographically compared to background stars, the masses of each star can be found. Many observations are needed over a period of time commensurate to the period of revolution of the pair.

Study of orbital motion

The first goal in the study of orbital motion, generally, is to determine the period of revolution. Then, when feasible, the second goal is to determine the geometric elements (relating the apparent orbit on the plane of the sky to that on the true plane of the orbit) and the dynamical orbital elements of the system. These values will lead to the physical characteristics of the stellar components. The simple laws that govern the dynamics of orbital motion of double stars stem from the three Kepler laws that were originally formulated by German astronomer and mathematician Johannes Kepler (15711630) to describe the motions in the solar system. They had far-reaching implications for gravitational forces, which were explained later by English physicist and mathematician Sir Isaac Newton (16421727). The laws of Kepler, as used in binary star analysis, are: 1) The orbit described by the fainter component (often called B) around the brighter (A component) is an ellipse with the A component at one of the foci; 2) The component sweeps over equal areas, throughout its orbital path, in equal lengths of time; and 3) The sum of the masses of the two components (in units of the solar mass) is equal to the scale or semi-major axis cubed (in units of the Earth-sun distance) divided by the period squared (in unit of years). The mass of the binary system from Keplers third law is the only direct way stellar mass can be determined.

Astrometric binaries

Astrometric binaries are double star systems visible on astrometric photographs as single stars. They have a telltale wavy motion across the sky indicating that the visible star is revolving around the center of mass of the visible star and its invisible companion, and thus its motion over an interval of time is analyzed for gravitational orbital motion. This process is a slightly modified form of the method for visual binaries. Generally, the companion star is either too faint to be seen or too close to the primary star to be resolved as two stars. The largest ground telescopes and space telescopes, such as the Hubble space telescope, are used to observe the fainter component that might turn out to be a brown dwarf or a planet. The star Sirius is a fine example of a visual binary, discovered first as an astrometric binary in 1844 by German astronomer Friedrich Wilhelm Bessel (17841846).

Spectroscopic binaries

Spectroscopic binaries are pairs that are too close to each other, as seen from the Earth, to be resolved into two stars. However, when the light from the star is analyzed with a spectrograph, which spreads the light into a continuous spectrum of colors with dark absorption lines superimposed, the spectral lines are alternately shortened or lengthened indicating Doppler motion, a to-and-fro motion as seen from Earth. This shift in the wavelengths results from the periodic motion, in the line of sight, of the visible star revolving around the center of mass of the system. When only the brighter component has sufficient light to show on the spectrogram the system is known as a single-lined spectroscopic binary. When the spectra of the fainter component is also recorded, the name double-line spectroscopic binary is used.

Eclipsing binaries

Eclipsing binaries are those systems, seen as a single star, which show periodic changes in brightness. This occurs when one component eclipses the other during their orbital motions around the center of mass of the system. The plane of the orbital motion must necessarily be close to perpendicular to Earths line of sight; eclipses are further facilitated when their separation is small. By analyzing the brightness with the passage of time, the resulting light curve can indicate some geometric and dynamical components of orbital motion. Astrophysical properties can be determined such as the relative brightness of the two components, their relative diameters, and some aspects of their atmospheres.

Mass exchange binaries

Mass exchange binaries are short period pairs whose components are virtually in contact with each other so that they interact with each other; their large gaseous atmospheres may touch forming a figure eight in three dimensions. Sometimes the atmospheres overlap to such an extent that they make an envelope around the entire system. One components mass flows into the other, which results in mass loss and exchange. This in turn affects a change in the period of revolution of the pair. Generally, the components are at different stages in their evolutionary track. X-ray binaries described below are also binaries that exchange mass. Much remains to be done to understand the details of the physics of the interaction.

X-ray binaries

X-ray binaries are discovered through space telescopes, which focus on very short-wave energy radiation sources. The International Explorer, the Einstein X-Ray Observatory, and other satellites have been used for this purpose. Some semi-detached pairs emit xrays provided by mass transfer in a common atmospheric envelope. Close pairs with one component, a neutron star or a black hole, are likely indicated from enormous energy output in the form of ultraviolet and x-rays that are generated around the massive star as gas from the companion, unevolved star, is sucked toward the massive central degenerate component. This type of binary may have a period of revolution around two days or less.

See also Brown dwarf; Gravity and gravitation; X-ray astronomy.

KEY TERMS

Astrometric photographs Photos taken with telephoto-type telescopes yielding large scale portrayal suitable for accurate measurements of positions of stellar images.

Dark absorption lines Part of the stellar spectrum coming from different atomic elements in the atmosphere of the star.

Doppler shift The change in frequency or wavelength resulting from the relative motion of the source of radiation and the observer. A motion of approach between the two will result in a compression of the waves as they pass the observer and a rise in pitch in the frequency of the wave and a shortening of the relative wavelength called a blue shift. A relative motion of recession leads to a lowering of the pitch and a shift to longer redder wavelengths.

Dynamical orbital elements Used in equations to describe the true orbital path of a binary star component in the plane of the orbit.

Geometrical orbital elements Used in equations to describe the orbital path of a binary star component as seen on the plane of the sky.

Mass The quantity of matter in the star as exhibited by its gravitational pull on another object. Stellar mass is usually measured in units of the suns mass.

Resources

BOOKS

Chaisson, Eric. Astronomy: A Beginners Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Hilditch, R.W. An Introduction to Close Binary Stars. Cambridge and New York: Cambridge University Press, 2001.

Krumenaker, Larry, ed. The Characteristics and the Life Cycle of Stars: An Anthology of Current Thought. New York: Rosen Publishing Group, 2006.

Zelik, Michael. Astronomy: The Evolving Universe. Cambridge and New York: Cambridge University Press, 2002.

PERIODICALS

Determination of the Ages of Close Binary Stars on the Main Sequence Astrophysics 45, no. 3 (2002) 342-357.

Sincell, M. Profile: Twin Stars Of Astrophysics Make Room For Two. Science 293, no. 5532 (2001): 1040-1041.

OTHER

Cornell University. Binary Star Simulation (Orbiting Binary Stars). <http://instruct1.cit.cornell.edu/courses/astro101/java/binary/binary.htm> (accessed October 2, 2006).

Sarah Lee Lippincott

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Binary Star

Binary star

Binary stars, often called double stars, refer to pairs of stars sufficiently close to each other in space to be gravitationally bound together. Following the laws of gravitation, each of the components revolves around the common center of mass of the system. At least 50% of stars are found to exist as binary systems, according to conservative statistics . There seems to be no obvious preference for particular combinations of brightness, size, or mass differences and a wide range in periods of revolution from less than a day to thousands of years. Likewise, there is a large range in separations from those stars in contact to those separated by thousands of times the Earth to Sun distance . Historically, visual binaries, those that appear as double stars when seen through a telescope , were discovered to be gravitationally bound by William Herschel around 1800.


Techniques of observation

There are a number of telescopic techniques used to discover and study binary stars. No one telescopic method can be used because of the wide range in the separations exhibited in the systems. The desired information about the orbital motion and the physical quantities of the stars themselves must come from different ways of observing. Hence, there are descriptive classifications of binary stars as determined by the various modes of study discussed below.


Importance

The importance of binaries lies in a number of areas: 1) The analysis of a visual binary (where the two components can be seen visually through a telescope) leads to the only direct method for the evaluation of stellar mass, one of the most important parameters of the physical universe. In some cases the mass of the system is found, in other situations the mass ratio of the components, and in some the individual masses can be determined; 2) Stellar duplicity plays an important role in the study of the physical aspects of stars, such as relative diameters, surface brightness, the genesis and evolution of stars, and the study of stellar-mass loss.


Visual binaries

Visual binaries are those stellar systems which appear as two stars in the eyepiece of a telescope. They are traditionally observed by manually measuring, at the eyepiece of a telescope, the angular separation of the two components. The angle the fainter stars make with respect to the brighter component is also taken into account, and refers to the north direction on the plane of the sky. If the measurements of each component are made and photographically compared to background stars, the masses of each star can be found. Many observations are needed over a period of time commensurate to the period of revolution of the pair.


Study of orbital motion

The first goal generally is to determine the period of revolution, then when feasible, the geometric elements (relating the apparent orbit on the plane of the sky to that on the true plane of the orbit) and the dynamical orbital elements of the system as far as possible, which will lead to the physical characteristics of the stellar components. The simple laws which govern the dynamics of orbital motion of double stars stem from the three Kepler laws which were originally formulated by Johann Kepler to describe the motions in our solar system . They had far-reaching implications of gravitational forces explained later by Newton. The laws of Kepler, as used in binary star analysis, are: 1) The orbit described by the fainter component (often called B) around the brighter (A component) is an ellipse with the A component at one of the foci; 2) The component sweeps over equal areas, throughout its orbital path, in equal lengths of time; 3) The sum of the masses of the two components (in units of the solar mass) is equal to the scale or semi-major axis cubed (in units of the earth-sun distance) divided by the period squared (in unit of years). The mass of the binary system from Kepler's third law is the only direct way stellar mass can be determined.

Astrometric binaries

Astrometric binaries are double star systems visible on astrometric photographs as single stars. They have a telltale wavy motion across the sky indicating that the visible star is revolving around the center of mass of the visible star and its invisible companion, and thus its motion over an interval of time is analyzed for gravitational orbital motion. This process is a slightly modified form of the method for visual binaries. Generally, the companion star is either too faint to be seen or too close to the primary star to be resolved as two stars. The largest ground telescopes and also the Hubble space telescope are used to try and "see" the fainter component which might turn out to be a brown dwarf or a planet . The star Sirius is a fine example of a visual binary, discovered first as an astrometric binary in 1844 by the German astronomer F. Bessel.


Spectroscopic binaries

Spectroscopic binaries are pairs which are too close to each other as seen from the earth to be resolved into two stars. However, when the light from the star is analyzed with a spectrograph, which spreads the light into a continuous spectrum of colors with dark absorption lines superimposed, the spectral lines are alternately shortened or lengthened indicating Doppler motion, a to-andfro motion as seen from Earth. This shift in the wavelengths results from the periodic motion, in the line of sight, of the visible star revolving around the center of mass of the system. When only the brighter component has sufficient light to show on the spectrogram the system is known as a single-lined spectroscopic binary. When the spectra of the fainter component is also recorded the name double-line spectroscopic binary is used.


Eclipsing binaries

Eclipsing binaries are those systems, seen as a single star, which show periodic changes in brightness. This occurs when one component eclipses the other during their orbital motions around the center of mass of the system. The plane of the orbital motion must necessarily be close to perpendicular to our line of sight; eclipses are further facilitated when their separation is small. By analyzing the brightness with the passage of time, the resulting light curve can indicate some geometric and dynamical components of orbital motion. Astrophysical properties can be determined such as the relative brightness of the two components, their relative diameters, and some aspects of their atmospheres.

Mass exchange binaries

Mass exchange binaries are short period pairs whose components are virtually in contact with each other so that they interact with each other; their large gaseous atmospheres may touch forming a figure eight in three dimensions. Sometimes the atmospheres overlap to such an extent that they make an envelope around the entire system. Mass from one component flows into the other resulting in mass loss and exchange. This in turn affects a change in the period of revolution of the pair. Generally, the components are at different stages in their evolutionary track. X-ray binaries described below are also binaries that exchange mass. Much remains to be done to understand the details of the physics of the interaction.



X-ray binaries

X-ray binaries are discovered through space telescopes, which focus on very short-wave energy radiation sources. The International Explorer and the Einstein X-Ray Observatory and other satellites have been used. Some semi-detached pairs emit x–rays provided by mass transfer in a common atmospheric envelope. Close pairs with one component, a neutron star or a black hole , are likely indicated from enormous energy output in the form of ultraviolet and x-rays which are generated around the massive star as gas from the companion, unevolved star, is sucked toward the massive central degenerate component. This type of binary may have a period of revolution around two days or less.

See also Brown dwarf; Gravity and gravitation; X-ray astronomy.


Resources

books

Couteau, Paul. Observing Double Stars. Cambridge, MA: The MIT Press, 1981.

Introduction to Astronomy and Astrophysics. 4th ed. New York: Harcourt Brace, 1997.

Zeilik, M., and J. Gaustad. Astronomy, the Cosmic Perspective. New York: John Wiley, 1990.


periodicals

Degirmenci, L. "Formation, Structure And Evolution Of Stars." Astronomy And Astrophysics 363, no. 1 (2000): 244-252.


"Determination of the Ages of Close Binary Stars on the Main Sequence" Astrophysics 45, no. 3 (2002) 342-357.

Sincell, M. " Profile: Twin Stars Of Astrophysics Make Room For Two." Science 293, no. 5532 (2001): 1040-1041.


other

McAlister, H.A., and Wm. I. Hartkopf, eds. ComplementaryApproaches to Double and Multiple Star Research, IAV Colloquium 135. ASP Conference Series, Vol. 32, 1992. Published by the Astronomical Society of the Pacific.


Sarah Lee Lippincott

KEY TERMS

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Astrometric photographs

—Photos taken with "telephoto type" telescopes yielding large scale portrayal suitable for accurate measurements of positions of stellar images.

Dark absorption lines

—Part of the stellar spectrum coming from different atomic elements in the atmosphere of the star.

Doppler shift

—The change in frequency or wavelength resulting from the relative motion of the source of radiation and the observer. A motion of approach between the two will result in a compression of the waves as they pass the observer and a rise in "pitch" in the frequency of the wave and a shortening of the relative wavelength called a "blue shift." A relative motion of recession leads to a lowering of the "pitch" and a shift to longer "redder" wavelengths.

Dynamical orbital elements

—Used in equations to describe the true orbital path of a binary star component in the plane of the orbit.

Geometrical orbital elements

—Used in equations to describe the orbital path of a binary star component as seen on the plane of the sky.

Mass

—The quantity of matter in the star as exhibited by its gravitational pull on another object. Stellar mass is usually measured in units of the sun's mass.

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