An astrometric binary is a type of binary star system where one star is visible, while the other is detected through its gravitational influence on the visible star. This method relies on observing the slight wobble or motion of the visible star as it orbits around the center of mass of the system, which is affected by the unseen companion. These systems are important for studying stellar masses and dynamics. By analyzing the astrometric data, astronomers can infer the presence of the hidden star, which may be a white dwarf, neutron star, or even a black hole. This technique enhances our understanding of stellar evolution and the structure of the universe.