A binary star system consists of two stars which orbit around a common point, called the center of mass following Kepler's Laws .
Visual binaries are systems in which the individual stars can be seen through a telescope.
Spectroscopic binaries are systems in which the stars are so close together that they appear as a single star even in a telescope. The binary nature of the system is deduced from the periodic doppler shifts of the wavelengths of lines seen in the spectrum, as the stars move through their orbits around the center of mass. In other cases, only one set of lines is seen, the other star being too faint, and we call the system a single -lined spectroscopic binary . Eclipsing binaries are systems in which the orbital plane is oriented exactly edgewise to the plane of the sky so that the one star passes directly in front of the other, blocking out its light during the eclipse. Eclipsing binaries may also be visual or spectroscopic binaries.