#### Astrometric Binaries

Not all binaries are visible as two separate stars. In fact, in most binary systems that are known only one of the stars can be seen clearly. How then do we know that these other systems are binaries if we can't see the two stars? The two most important ways in which an unseen companion star can be inferred are by the gravitational influence of the unseen star on the visible one, and by effects in the observed spectrum. A binary inferred from the perturbation of its motion by an unseen companion is called an astrometric binary and the precise measurement of stellar motion required to find such small perturbations is a part of the field called astrometry. Astrometric binaries are addressed here. Binary systems deduced from effects on the spectrum are called spectroscopic binaries; they are the subject of a later module.

##### Proper Motion for Binary Stars
The following figure illustrates the proper motion on the celestial sphere of the Sirius system.

 The Wobble of an Astrometric Binary The motion of the Sirius system about its center of mass causes the proper motion of the two stars across the celestial sphere to wobble, as illustrated in this animation. This can be used to detect the presence of an unseen companion. You may illustrate this by using the buttons in the binary star orbit applet to hide the orbits and to hide one of the stars. The corresponding motion of the other star is what we would see for Sirius A if it did not have proper motion on the celestial sphere. This motion, coupled with the proper motion of the Sirius system, gives the wobbling path noted above.
This diagram illustrates clearly that the motion of Sirius across the celestial sphere is a superposition of two motions: the revolution of the two stars around their center of mass, and the slow drift of the center of mass across the celestial sphere because of its proper motion. (In the image shown above the sizes of the stars are exaggerated to make them easy to see. In reality, Sirius A would be about 200 times larger than Sirius B, and each would just be points on this scale.)

##### If One Star Is Not Visible
Sirius is a visual binary, so we can see both stars and their true motion. If only the primary star of a binary star can be seen, it will appear to wobble in its proper motion across the celestial sphere because of the gravitational influence of the unseen companion. A binary system inferred from such wobbling motion of the primary is an astrometric binary.

##### Don't Confuse Binary Wobble with Parallax Wobble
This wobble produced by an unseen companion should not be confused with the apparent wobble in the motion caused by parallax for all stars because of the Earth's motion around the Sun. This animation illustrates the effect of parallax on proper motion. When we illustrate the wobbling motion of a binary system, the parallax effect has already been subtracted out of the motion.