A double star is a star that, upon closer or more sophisticated examination, proves to be two stars. The term can be loosely extended to include systems of three, four, or more stars, though a system with three or more stars is often called a multiple star. Astronomers believe that most stars are members of either double- or multiple-star systems. Our Sun seems to be single, but there is still a chance that it has a distant and very dim companion that we will someday discover by virtue of its having a huge proper motion in the heavens.
Not all apparent pairings of stars prove to be a real pairing in space. Optical doubles are apparent pairs of stars in which the two stars are really objects at greatly different distances from us that just happen to lie along
A diagram of a double star orbit. The companion star's position for 1994 is indicated by a filled-in circle. Note that this double is now opening from tightest separation.
nearly the same line of sight. Alpha Capricorni (for naked eye and binoculars) and Delta Herculis (for telescopes) are interesting examples.
The brighter, more massive star in a double system is called the primary. The less bright, less massive member of the system is the secondary or companion. Most double stars are binary stars—star systems in which the component stars actually orbit around each other (or, more strictly speaking, around a common center of gravity called the barycenter). But some companions are at great distances—even up to a light-year or more—from the primary and are called common motion doubles because they only share a common proper motion through space.
Some double stars are visual doubles, pairs of stars that can be resolved into two separate points of light by a telescope. Spectroscopic binaries are doubles too close together to split in telescopes but whose duplicity (doubleness) can be detected by its effect on the spectrum of the double star's light. A pair that is approaching us in space may show a redshift in its spectral lines due to the fact that one of the component stars is also moving away from us in the course of its circling around the other component.
Observers need to know the separation of a double star and the position angle of the companion. The position angle, or P.A. is the azimuth, with 0° as north, 90° as east and so on. In most binaries, the position angle changes at least slightly in the course of years or decades. But some stars have such vast orbits around each other that their separation and position angle stay relatively fixed over a few hundred years—leading to their being called relfixes.
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