Binary star

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This article is about a star system. For the rap group, see Binary Star (rap).

A binary star system consists of two stars both orbiting around their barycenter. For each star, the other is its "companion star".

1 Background
2 Binary star classifications
3 Research findings
4 Binary star examples
5 Binary stars in fiction
6 See also
7 External links

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Background

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.

A true binary is a pair of stars bound together by gravity. When they can be resolved (distinguished) with a powerful enough telescope (with the aid of interferometric methods) they are known as visual binaries. In other cases, the only indication of binarity is the Doppler shift of the emitted light. These systems, known as spectroscopic binaries, consist of relatively close pairs of stars such that the spectral lines 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 occultation each other regularly, and the system is called an eclipsing binary of which Algol is the best-known example.

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. Astrometric binaries, for example, 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 mass of the missing companion. The companion could be very dim, so that it is currently undetectable or masked by the glare of its primary, or it could be an object that does not emit visible light, or in fact any electromagnetic radiation, like a neutron star. In some instances, one can make a strong case that the missing companion is in fact a black hole—a body with such strong gravity that no light is able to escape. Perhaps the best known example at present is Cygnus X-1, 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.

Binaries provide the best method for astronomers 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),

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Binary star classifications

At present, binary stars are classified into four types according to their observable properties:

Any star can belong to several of these classes, e.g., several spectroscopic binaries are also eclipsing binaries.

Another three-category classification is based on the distance of the stars, relative to their sizes :

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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 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.

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.

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.

Science fiction has often featured planets of binary or ternary stars as a setting. In reality, some orbital ranges are impossible for dynamical reasons (the planet would be expelled from its orbit relatively quickly, being either ejected from the system altogether or transferred to a more inner or outer orbital range), whilst other orbits present serious challenges for eventual biospheres because of likely extreme variations in surface temperature during different parts of the orbit. Detecting planets around multiple star systems introduces additional technical difficulties, which may be why so far (as of July 2005) only one such planet has been found: HD 188753 Ab.

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Binary star examples

Albireo
Algol (triple, eclipsing binary)
Alpha Centauri (triple)
Castor (sextuple)
Procyon
Sirius

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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 Star Trek: Enterprise episode "Singularity" the ship visits a three-sun (Trinary or Ternary) star system.

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