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A visual binary is one in which the two stars can be observed in a
telescope to orbit their common centre of mass, as shown in this animation.
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©Richard Pogge (OSU)
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A well-known example of a visual binary is
Mizar in the Plough.
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Sometimes only one of the components
is observed to "wobble" in the sky, the other star being too faint to see,
in which case we have an astrometric binary. Sirius, the brightest
star in the sky, was classified as an astrometric binary until its faint
companion was discovered in 1862 by Alvan Clark.
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Really good orbits are available for only about 50 visual binaries,
although another 2000 are known to some degree of accuracy. Combined
with a knowledge of their distances, visual binary orbits provide one
of the best means of determining stellar masses.
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