Binary Star
A binary star is a double star system in which two stars orbit each other around a central point of gravity.
Before the early 1800s, binary stars were thought to be an optical illusion. An observer looking into the sky might see two stars that appeared to be side by side, but it was assumed they were really very far apart, and just happened to be in the same direction of view. For some stars this is indeed true, but for many others it is not.
English astronomer William Herschel made this first identifications of true binary stars. Like James Bradley, Herschel had been trying to measure the parallax of a star; that is, its apparent shift in position resulting from the motion of the earth around the sun. A successful parallax measurement would finally make it possible to determine how far away a star was.
Following advice Galileo had given long before, Herschel concentrated on pairs of stars that appeared to be close together. This way, the movement of the closer star could be compared against the position of the farther star. But by 1793, he realized that in many cases the pairs of stars he was observing were not merely in the same line of sight, but were actually very close together. The movement of many of these pairs of stars could only be explained if it was assumed they were actually in orbit around each other. Like Bradley, Herschel was never able to measure parallax, but he discovered over eight hundred double stars in the attempt, which he called binary stars. Scine then, literally thousands of binary stars have been discovered. A recent survey of 123 sun-like stars showed 57 percent were members of a multiple star system. Our Sun, moving through space on its own, is in a distinct minority.
There are several kinds of binary stars. If the pair of stars is far enough apart for each to be seen with the eye or a telescope, the pair is called a visual binary. When only one star can be seen, but the existence of the other is inferred by the wobble it produces on the visible star, the pair is called an astrometric binary. When one star passes in front of the other periodically, the pair is called an eclipsing binary. This can happen only if we are looking directly into the plane of the binary star's orbit. If a spectroscope (a device that spreads light into its component wavelenghts) can detect two different spectra from one apparent star, the pair is called a spectroscopic binary. The spectrum of the approaching star is Doppler-shifted toward the blue end of the spectrum, while that of the receding star is red-shifted.
By observing how one star in a binary pair affects the other, an accurate determination of their masses can be made. Mass is the most important quantity one can determine for a star, so binaries have played an important role in the development of our understanding of the stars.
In some binary systems, the so-called wide binaries, the stars may be separated by distances greater than that from the Sun to Pluto, but in close binaries, the stars are sometimes so close together that their atmospheres may actually be touching. In certain cases, material can actually flow from one star onto the other; such systems are called semidetached binaries. In some systems, the star receiving the material is the burned-out remnant of a former star, the material draining onto it can become hot enough to ignite in a massive thermonuclear explosion called a nova. Occasionally, the burned-out stellar remnant appears to be a black hole. By their nature, black holes are extremely difficult to identify, and binary systems are one of the few places where these exotic objects can be observed.
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