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 some instances, the spectrum shows the lines from both stars; this case is called a double-lined spectroscopic binary. 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. The variation in the brightness of the star is called its light curve.
Five to ten percent of the stars visible to us are visual binary stars. Careful spectroscopic studies of nearby solar-type stars show that about two thirds of them have stellar companions. We estimate that roughly half of all stars in the sky are indeed members of binaries.
One of the fundamental properties that we want to know about a star is its mass. The only way that we can determine the masses of stars is to study the orbital motions of binary stars. Application of the laws of celestial mechanics allows us to calculate the masses of the stars from measures of their orbital periods, sizes and velocities.
Link here to the Astro 101 binary star simulations.
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