Binary star
An astrometric binary is one where only one companion can be seen. For astrometric binaries fairly near the Earth (up to about 10 parsecs), it may be possible to see the visible companion "wobble" as it moves around its invisible companion. By making measurements over a long period of time, it may be possible to calculate the mass of the visible star and how long its orbit takes. This method is also used to detect the presence of large planets orbiting a star; as of 2007, over two hundred planets have been discovered in this way.