Pair of stars moving in orbit around their common centre of mass. Observations show that most stars are binary, or even multiple – for example, the nearest star system to the Sun, Rigil Kent (Alpha Centauri).
One of the stars in the binary system Epsilon Aurigae may be the largest star known. Its diameter is 2,800 times that of the Sun. If it were in the position of the Sun, it would engulf Mercury, Venus, Earth, Mars, Jupiter, and Saturn. A spectroscopic binary is a binary in which two stars are so close together that they cannot be seen separately, but their separate light spectra can be distinguished by a spectroscope.
Another type is the eclipsing binary, a double star in which the two stars periodically pass in front of each other as seen from Earth. When one star crosses in front of the other, the total light received on Earth from the two stars declines. The first eclipsing binary to be noticed was Algol, in 1670, by Italian astronomer Geminiano Montanari.
Alpha Centauri, for example, consists of a star almost identical to the Sun with another star about a third as bright closer to it than Neptune is to the Sun, i.e., closer than 4.4 billion km/2.79 billion mi. Each of these stars appears to describe an ellipse about the other in about 80 years. A third, much fainter star, Proxima Centauri, is too far away to disturb their mutual orbit appreciably. The study of such systems has been rewarding and has provided the only reliable information about the masses of stars. For a few stars it has also yielded direct measures of their dimensions, shapes, and effective temperatures.
The precision observation of binary stars did not start until the time of German-born British astronomer William Herschel in the 18th century. Herschel compiled the most thorough catalogue of binary stars up to that time; he recorded 848 ‘double stars’. Binaries can now be detected in a number of ways: by direct telescopic observation (‘visual binaries’); by suitable interferometers (‘interferometric binaries’); by periodic variations in proper motion (‘astrometric binaries’) and in radial velocity (‘spectroscopic binaries’); and as variable stars (‘eclipsing binaries’). Each method of observing picks out different samples of binaries. Visual observation selects those of long period, as these are the only ones sufficiently well separated; the interferometer can resolve stars with smaller separations, but is limited to those pairs in which the components are nearly equal in brightness; close pairs in which changes of velocity are large and take place in a short period are more likely to be noticed spectroscopically; photometric detection is limited to those systems in which the orbital plane happens to pass close to the Earth. The more ways in which a particular binary can be observed, the more detailed is the information that can be obtained about the individual components.
The stellar masses found from well observed visual binaries range from 0.16 that of the Sun for the fainter component of Krueger 60 to 2.14 for the brighter component of Sirius. Spectroscopic binaries indicate still greater masses for the very luminous O and B type stars, the most massive being Plaskett's star, with a mass 50 times that of the Sun. Even so, this range in mass is very small when compared with the corresponding range in luminosity, from 0.0004 that of the Sun to over 100,000. The number of known astrometric binaries is not large though the binary nature of both Sirius and Procyon was first established by this method. Another case of particular interest is Barnard's star, which is only 5.9 light years from the Sun. Intensive astrometric observations by van de Kamp and his colleagues suggest that it has two companions of planetary mass, the one slightly more massive, the other slightly less, than Jupiter.
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