Binary Stars
Binary stars, also called double stars, are composed of two stars that orbit around a common center of mass. Binary stars are quite common. Astronomers estimate that 85% of the stars in the galaxy are actually systems consisting of two or more stars.
Many double star systems can be detected with a small telescope and a few can even be seen as two stars by the unaided eye. English astronomer William Herschel first determined in 1801 that binary stars actually orbit each other, and are not two unrelated stars that happen to be in the same direction. Double stars that can be observed separately by telescope observation are called visual binary stars. These stars are generally separated by distances of several astronomical units or more. The orbital periods of visual binary stars can be several centuries, so many observations are needed to confirm that they are actually binary stars. Because the members of the system are so widely separated, there is very little transfer of matter between them and they have very little effect on each other.
A second type of binary star is detected by observing the spectrum of the system. These spectroscopic binary stars are generally much closer to each other than visual binaries, as close as a few hundredths of an astronomical unit. As they orbit each other, their spectrum as viewed from earth changes. As the stars orbit their center of gravity, the stars have a different velocity relative to the earth. This causes a periodic Doppler shift of the spectrum of one or both members of the pair. The amount and period of the shift is used to calculate the orbital velocity, size of the stars, and their distance from one another. The orbital periods of spectroscopic binaries are as short as a few days.
A third type of double stars involves a pair of stars revolving about their common center of mass in an orbit whose plane passes through or very near the Earth. An observer on the Earth sees one star pass in front of the other. As the stars alternately eclipse one another, there is a periodic decrease in their observed brightness. This type of system is called an eclipsing binary, or an eclipsing variable star. The star Algol was the first recognized as an eclipsing binary by Dutch-born astronomer John Goodricke (1764-1786) in 1782. Several thousand are now known. By comparing the observed duration of the eclipse to the period of the orbit, as determined spectroscopically, astronomers can find the diameters of the stars relative to the size of their orbits. From this they can calculate the densities of the stars and their temperatures. Most of the thousands of eclipsing binary stars that are known have orbital periods of a few days and are less than one astronomical unit apart.
Most stars are actually binary or multiple stars. Many binary stars are themselves part of larger multiple star systems. For example, Castor, in the constellation Gemini, is a visual binary with an orbital period of 467 years. Each of the components of the binary is itself a spectroscopic binary, with a period of several days.
Several mechanisms have been proposed to explain the formation of binary stars. Widely separated binaries, such as visual binary stars, may have formed by tidal capture. Two clouds of matter lose energy to tidal effects as they approach each other and then each cloud evolves into a star. An alternative is conucleation, in which the two stars form from the material of a single cloud after two fragments begin orbiting each other. Theories to account for the formation of closely separated binaries include fragmentation, in which a collapsing cloud breaks apart during star formation to produce two or more stars, and fission, in which the newly formed star splits into two pieces, each of which becomes a star.
Binary stars that are close together transfer mass between them. This transfer changes the angular momentum of the stars, and changes their separation and orbital periods. As matter builds in the space between the stars, the increase in energy can cause matter to be ejected from the system as a nova or supernova. One of the stars then becomes a neutron star or a black hole. Close binary systems that include a small, very dense star can become extremely energetic as the transferred mass is heated, creating pulses of x rays. These systems are known as x-ray pulsars. Binary pulsars include a neutron star or a white dwarf star. As their companions transfer mass, the rotation rate of the systems increase, shortening the orbital period. Some binary pulsars have orbital periods of only a few milliseconds.
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