Finding Proper Motion Pairs
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About Binary Stars

Astronomers like to say that "two of every three stars" is a binary. A binary star is a pair of stars held together by the mutual gravity, and which orbit each other in a perpetual dance around the Galaxy.

Binary stars come in many types, characterized by their separation and orientation. Some binary stars are so close together as to be virtually touching, undergoing a process of mass transfer as gas flows from one star onto another. Most binaries are more widely separated, affecting each other only through the gravitational attraction keeping them each bound in orbit around the other. But even most of these binaries are so close together that they appear as a single star in the sky. In some cases, we can image both stars using telescopes and special imaging techniques. But if the two stars of a binary pair are sufficiently far apart, both stars appear in the sky as separate, individual stars, and astronomers must measure their motions and distances carefully to determine if two stars that appear close together in the sky are, in fact, a binary pair.

Binary stars can be detected with a variety of other techniques. If the Earth lies in the same plane as the orbit of the two stars, the stars will periodically pass one in front of the other, producing a diming of the light of the pair as one star blocks the light of its companion. The detection of periodic eclipses is one way to identify binary stars. For other binaries, we can detect the motion of the stars as they orbit each other by measuring the Doppler shift of their spectra. Similar to the changing pitch of the siren of a passing police car, the star's light shifts to higher or lower frequency as the star moves alternately toward or away from Earth, following its periodic orbit.

Binary stars that are sufficiently widely separated that we can see both stars are known as visual binaries, but not all stars that appear close together in the sky are true binaries. Many are merely double stars, which appear close together only by chance, but are actually widely separated in space. To determine if a double star is really a visual binary, we need to determine if the stars belong together.

Stars in a visual binary will appear to be at the same distance, and moving together in the same direction at the same speed. Double stars that are not visual binaries will most likely be at different distances, and moving in different directions at different speeds.

More information about Binary Stars


RBSEU is funded through a CCLI grant from the National Science Foundation



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