Astrometric Binary Stars

In astrometric binaries, only the orbit of the brighter component about the centre of mass can be observed. If the mass of the visible component is estimated, e. g. from its luminosity, the mass of the invisible companion can also be estimated.

The first astrometric binary was Sirius, which in the 1830's was observed to have an undulating proper motion. It was concluded that it had a small companion, which was visually discovered a few decades later (Figs. 9.4 and 14.1). The companion, Sirius B, was a completely new type of object, a white dwarf.

The proper motions of nearby stars have been carefully studied in the search for planetary systems. Although e. g. Barnard's star may have unseen companions, the existence of planetary systems around other stars was not established by proper motion studies but with spectroscopic observations (see below).

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