A binary star system consists of two stars both orbiting around their barycenter. For each star, the other is its "companion star".
The term "binary star" was apparently first 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". A binary star is actually a pair of stars that are held together by the force of gravity. Systems in which the individual stars that compose a binary star can be distinguished are known as visual binaries. In systems in which the images of the individually resolved, astronomers use the Doppler shift of the spectrum to infer the presence of a binary. These systems are known as spectroscopic binaries.
Binaries are particularly crucial as one of the primary methods by which astronomers can directly measure the mass of a distant star. The gravitational pull between the individual stars of a binary star causes each to orbit around the other. 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 therefore be determined.
Scientists have discovered some stars that seem to orbit around an empty space. The same arguments for ordinary binaries can be used to infer the mass of the missing companion, and in some instances, one can make a very strong case that the missing companion is in fact a black hole -- a star with such strong gravitational force that no light is able to get out. Perhaps the best example of such a system is Cygnus_X-1, where the mass of the unseen companion is about nine times the mass of our sun -- far exceeding the maximum mass of a neutron star, the other likely candidate for the companion.
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 angular momentum is a conserved quantity in physics, binaries give us important clues about the conditions in which the stars were themselves formed.
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 :
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 the gravitational attraction can be determined are binary stars, binaries constitute 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 obtained, 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 make a complete solution for the specifications (mass, density, size, luminosity, and approximate shape) of both members of the system.
Binary star examples
See also
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Last updated: 10-15-2005 07:28:33
