Astrometric Binary

A binary star system in which the fainter component can not be seen

In an astrometric binary, the secondary component is invisible to the eye but is observable from the gravitational effects on the proper motion of the brighter companion. The components of a binary system revolve around a common centre of mass. As the pair move through space, in an astrometric binary, the visible star will be seen to move with a wobble. The path through space will be a ‘wave’ shape instead of a straight path.

There can be various reasons why the secondary component might not be seen. It may be too far away for the light to be detected, it could be a very cold star; the two stars could be too close and too far away to be resolved by telescopes or the primary may be much brighter and simply drown out the light from the secondary.

This method of looking at the movements of stars has been applied to the search for extrasolar planets. As instruments become ever more sensitive, smaller and smaller perturbations can be detected meaning smaller and smaller orbiting masses can be ‘seen’.

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