Binary stars

Type of physical science: Astronomy; Astrophysics

Field of study: Binary star systems

Two stars that are gravitationally bound to each other are known as a binary star system. Observationally, there are subclasses of binary stars, which are defined by the physical properties of the binary star system under study. These classes will be discussed in detail below.

Overview

Used as an adjective, the word "binary" is defined in Webster's dictionary as "compounded or consisting of or marked by two things or parts." When this adjective is used to modify the noun "stars," it has a more specific definition as utilized by astronomers. As one looks at the three-dimensional sky, the eye transforms the image onto a two-dimensional space.

Astronomers differentiate between two stars that appear very close together and those that are physically connected through the force of gravitational attraction. Ptolemy used the term "double star" to define the former case and Sir William Herschel in 1802 was the first to use the term "binary star" in his paper "On the Construction of the Universe." Herschel's definition is consistent with the latter case and is quite specific as to the physical conditions on the two stars defined as binary stars. Herschel thus called a "real" double star a binary star.

Some double stars are known as "fixed," which means that the component members have not moved with respect to each other since they were first discovered. One of the most famous "fixed" doubles is the beautiful (the primary star is yellow gold and the secondary star is blue) double star in the constellation Cygnus known as Albireio (the nose of the swan), which was first discovered by Friedrich Georg Wilhelm von Struve in 1832. Most double stars, however, have moved with respect to each other since their discovery. The parameter that astronomers use to define the location of the secondary component relative to the primary component is the position angle, as shown in the figure.

Since the binary star system contains more than one object, one must differentiate the component members. With binary stars, the star of the system that appears to be the brightest (apparent magnitude--how bright the star appears as seen from Earth) is designated as the A component. The next brightest is the B component, and so on in the case of dimmer members of a system. For example, the bright star Sirius in the constellation Canis Major is a known binary star. Thus, the brightest component is known as Sirius A and its fainter, physical companion is known as Sirius B.

Double stars are either optical doubles or visual binaries. The former category are only a result of the viewing direction and are thus considered impostors. The best-known system is the optical double Alcor and Mizar in the handle of the Big Dipper (Ursa Major), but Mizar itself is a visual binary. Visual binaries are two stars that orbit each other. The time it takes two stars to orbit each other varies considerably. Typically, the time it takes for visual binaries to orbit each other (known as the orbital period) will range from a few years to tens of thousands of years. For other classes of binary stars, the orbital period is much shorter. Visual binaries are those that can be separated with the naked eye. By simple deduction, they are separated by reasonably large distances but are still gravitationally bound to each other, which results in a closed orbit of the binary star system.

As stars move around each other, at times the total light from the system is reduced as one star blocks (eclipses) all or part of the light from the other component. As a result, the total light from the binary star varies as a function of time and thus eclipsing binary star systems are a subset of the class of stars known as variables. The most interesting group of binary stars are those known as spectroscopic binaries. These systems are such that even with the most powerful optical telescope on Earth, the component members of the binary star system cannot be resolved.

Astronomers study the spectra (the dispersed light from an object by wavelength) of such systems that determines the nature of the component members. Minute blueshifts and redshifts in the spectra of the system occur as the component member star alternately approaches and recedes relative to observers on Earth.

In 1955, the binary-star astronomer Zdenek Kopal classified close binaries in three groups: detached, semi-detached, and contact. The closer together that component members are, the shorter the orbital period. Thus, the contact binaries have the shortest periods. Traditionally, the two subclasses of contact binaries are the A-type and the W-type systems. In 1981, a third, B-type, subclass was proposed by astronomer Stefan W. Mochnacki based on the work of Leon B. Lucy and Robert W. Wilson. The W-type systems are by far the most numerous of all the eclipsing variable stars. As the stars revolve, the W-type systems are those where the more massive component blocks the smaller, less massive component that is the brighter star of the pair. The prototype of this subclass is the system W Ursa Major. Most of these systems have orbital periods in the six- to twelve-hour range. The stars that make up these systems are older stars with spectral types late F and G. The total mass of these systems is small, with values typically only 0.8 to 3 solar masses. (The mass of the sun is used as a reference unit of mass for other stars.)

The A-type systems typically have component members with spectral type A (but sometimes B). The fact that they normally are A-type stars is the source of the name. In these systems, the deepest eclipse (the largest decrease in the total light from the system) occurs when the less massive star is in front and blocking the light of the more massive and brighter star. The stars that are members of these systems are younger and hotter than those stars in the W-type systems. The total mass of the systems is larger than the W-type systems, with total masses at 4 to 5 solar masses.

Applications

Stars are typically measured for a specific parameter. For example, an astronomer may wish to measure how bright the stars are in a given range of wavelengths. This collection of measurements would then be a photometric catalog. Another observer may wish to determine the distance to a given set of nearby stars. This list would then be a parallax catalog. The majority of binary stars are the result of spectral classification catalogs.

The majority of measured stars in the sky are other than single stars. Gravitational forces are in evidence for the double and multiple-star systems. Binary stars are the most common type followed by single stars. Triple-star systems are quite unusual and the Castorian system (to the unaided eye, the star known as Castor in the constellation Gemini) is even more unusual. Each of the members of this triple-star system is a spectroscopic binary star system.

Thus, the Castorian system is really a six-star system. The Castor A binary star system has an orbital period of 9.2 days, the Castor B system has an orbital period of 2.9 days, and the Castor C system has an orbital period of 0.8 days. The measurements of this fascinating system began in 1826 by Struve. The orbital period of the Castor A-Castor B systems around each other is in the range of 340 to 477 years. There is much error involved when only a small portion of the orbit (the path defined by one object gravitationally bound to another) has been measured and with varying degrees of precision over the years since the initial observations in 1826. The situation is worse for the orbital period of the Castor C system around the quadruple members of Castor A plus Castor B: The revolution period must be on the order of 25,000 years.

Sirius, the brightest star in the sky other than the sun, has intrigued observers for almost two centuries. Early observations by Friedrich Wilhelm Bessel in 1834 showed some variability in the motion of the star. Since Sirius is so bright, it became an observational challenge to try to detect the "unseen" companion that was causing the variability in the motion of Sirius. In addition, since the primary component is so bright, it became difficult to determine accurately the spectral class of the secondary component. As observing methods and equipment became more sophisticated, it has been determined that Sirius B is one of the most famous examples of a class of stars known as white dwarfs.

Context

The history of observational astronomy is flush with examples of the discovery of the prototype of a new class of object. Astronomers are now searching for other examples of such objects. In the seventeenth century, the Italian astronomer Giambattista Riccioli discovered the first double star in the constellation Ursa Major. Mizar was the first double star measured on the photographic plates taken by G. P. Bond at the Harvard College Observatory in 1857. One of the members of Mizar was determined to be another class of binary star by Edward Charles Pickering in 1889. The early history of binary stars is full of chance encounters with this class of object found often while looking for another object. It was not until Edmund Halley's discovery in 1718 that some of the brighter stars were in motion that caused a major interest in the field of astrometry (the branch of astronomy that deals with the precise measurement of positions of stars). Astrometry was one of the newest areas of astronomy because the technology required for precise measurement lagged the requirement for such technology.

The commonly recognized "fathers of double-star astronomy" are Christian Mayer and Herschel. In 1779, Mayer first speculated that there were small suns revolving around larger suns. These speculations were based on observations at Mannheim (a city in southwest Germany near the French border) in 1777 and 1778. His work contains the first known catalog of double stars ever published. This first catalog is a tabular listing of eighty entries. Included in this list of eighty double stars is Castor, one of the most famous stars in the constellation Gemini.

Herschel started his study of the planets and stars in May of 1773, but it was not until 1779 that he initiated a systematic search for the class of objects known as double stars. In all, Herschel prepared three lists of double stars that he observed with his own telescopes (probably the finest at the time). In 1782, he presented his first list to the Royal Society, which contained 269 double stars, 227 of which had not been noticed previously. In 1784, Herschel presented his second list with 434 additional objects. Both of these lists included a position for the major component, a position angle and a measured estimate of the angular separation of the two components. Herschel's third and final list was published in 1821 (about a year prior to his death) and included a final list of 145 pairs but without the detailed measurements of the first two lists.

The position angle as defined today is different from that used by Herschel in his early lists of double stars. It is important to remember that this parameter is one that typically changes (except for the "fixed" binaries) as a function of time. The changes vary with the orbital period of the binary system, the distance of the system to Earth, and the orientation of the orbital plane in the sky. Since a circle by definition has 360 degrees, a complete orbit will result in a 360-degree change in the position angle. Nevertheless, the orbits of binary star systems are typically ellipses, thus a change in the position angle of 180 degrees does not mean half of the orbital period. For example, the position angle of Sirius B in 1990 is 2 degrees (almost due north of Sirius A) and 182 degrees in 1998 (almost due south of Sirius A), yet the orbital period of the system is 49.94 years. Thus, Sirius B uses only about eight years to change its position angle by 180 degrees and requires the remaining forty-two years of its orbital period to change the remaining 180 degrees.

Thus, Sirius B has changed its position angle by 180 degrees in only about one-sixth of the orbital period. Several graphically presented orbits of bright double-star systems have been published. The generation of these plots requires a basic knowledge of celestial mechanics and projection geometry.

Binaries are extremely useful systems for astronomers since their motions obey the well-understood laws of motion. In addition, a temporal study of these systems provides clues as to how these systems are changing or evolving, which in turn is important for the study of stellar evolution. The Thermal Relaxation Oscillation (TRO) model of Lucy, created independently also by Brian P. Flannery, is such that W-type contact binary star systems are unevolved even though the physical parameters that define the system's orbit may vary with time. In fact, this type of system appears to oscillate between a state of marginal contact and a state of no contact. The W-type contact binary star systems are ten to thirty times more common than all other eclipsing variables combined. A model that is to describe contact binaries must first deal with these systems effectively. Lucy also proposed that the A-type systems are in full contact and in equilibrium. Some evidence exists that stars in A-type systems evolved into a contact situation, which supports the TRO model. The problem is that the TRO model predicts that there should be broken contact systems, but there is no known system that satisfies that condition unambiguously.

Another major theoretical model was developed by Frank Shu, Stephen Lubow, and Lawrence Anderson and is called the contact discontinuity model (DSC). The basic tenet of this model is that binary star systems formed in contact and they are also in equilibrium. The end of the life of a contact system, as predicted by the DSC model, is that the fate of the systems is cataclysmic in nature. The shortest known orbital period of any binary star is eleven minutes. If this discovery holds, the system would provide direct proof of a collision between two stars.

Principal terms

BINARY STAR: two stars that are physically connected through the force of gravitational attraction

BLUESHIFTS: when the spectral lines have shifted toward shorter wavelengths than the laboratory reference set of lines; this is caused by the fact that the object is moving toward the observer

DOUBLE STAR: two stars that appear very close together

ELLIPSE: one of the geometrical figures known as a conic section

ORBITAL PERIOD: the time required for one object to execute a complete revolution around the other object

REDSHIFTS: when the spectral lines have shifted toward the longer wavelengths than the laboratory (at rest) reference set of lines; this is caused by the fact that the object is moving away from the observer

SPECTRAL TYPE: the Harvard College Observatory spectral classification of stars compiled by Annie Jump Cannon for the HENRY DRAPER CATALOGUE; the majority of stars (known as the main sequence) have spectral types O, B, A, F, G, K, M, which roughly defines the youngest, hottest stars (the O stars) to the oldest, coolest stars (the M stars)

VARIABLE STAR: any star or stellar system that produces a variable amount of light as seen by an observer on Earth as a function of time

VISUAL BINARIES: two stars that are gravitationally bound to each other but are either far enough apart or close enough to Earth to be able to differentiate the components with or without a visual aid (such as a telescope)

WHITE DWARF: a star with the surface temperature of a normal star but with a density thousands of times greater than that of water

Bibliography

Aitken, Robert G. THE BINARY STARS. Reprint. New York: Dover, 1964. A basic textbook on binary stars. Provides a good historical outline for the subject, but many of the analysis methods are now obsolete as used in modern astronomical computations.

Croswell, Ken. "Contact Binaries: Stars That Touch." ASTRONOMY 10 (December, 1982): 66-70. A very readable discussion of contact binary stars and the major theories that define their role in stellar evolution.

Goldstein, Alan. "Split a Star in Two." ASTRONOMY 17 (December, 1989): 88-91. A short discussion of observational techniques for observing five close double stars. The graphical presentation of the orbits for these systems is very useful and enlightening.

Harrington, Phil. "The Ten Best Double Stars." ASTRONOMY 17 (July, 1989): 78. A discussion of double stars for the general reader. "How Many Stars Are Binary?" SKY & TELESCOPE 74 (July, 1987): 8. Discusses binary stars. For a wide audience.

MacRobert, Alan. "Observing Double Stars." SKY & TELESCOPE 68 (November, 1984): 417. For the amateur astronomist. Discusses double stars.

Binary star pair and their relative orientation by position angle

Close binary system

Coordinate Systems Used in Astronomy