An astrometric binary star is a binary star for which only one of its component stars could be visually observed. The visible star's position is carefully measured and detected to have a wobble, due to the gravitational influence from its counterpart.
From astrometric measurements of the movement of the visible star, the size and period of its orbit can be deduced. This can then be used to estimate the mass of the system.
This method of detecting binaries is also used to locate planets orbiting a star.
Binary stars are not the same as optical double stars, which appear to be close together as seen from Earth, but may not be bound by gravity.
Binary stars may be found with any conceivable separation, from pairs orbiting so closely that they are practically in contact with each other, to pairs so distantly separated that their connection is indicated only by their common proper motion through space.
In the case of a visual binary star, after the orbit has been determined and the stellar parallax of the system determined, the combined mass of the two stars may be obtained by a direct application of the Keplerian harmonic law.
The ratio of the masses of the stars in the binary is obtained from this curve, along with the orbital period of the binary.
Note that in the case of astrometricbinaries, it is usually possible to measure the orbital period of the binary and the average distance of separation of the two stars.
An astrometricbinary is observed to have a period of 44.5 years and an orbit with an average distance of separation between the component stars of 100 AU.
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