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Multiple Star Systems


Binary Stars


Planets revolve around stars because of gravity. However, gravity is not restricted to between large and small bodies, stars can revolve around stars as well. In fact, 85% of the stars in the Milky Way galaxy are not single stars, like the Sun, but multiple star systems. If two stars orbit each other at large separations, they evolve independently and are called a wide pair. If the two stars are close enough to transfer matter by tidal forces, then they are called a close or contact pair. The evidence is that most stars that we see in the sky are parts of multiple star systems revolving around a common center of mass, called the barycenter. If there are two stars in the system, it is called a binary star system. We shall concentrate on such binary systems as representative of multiple stars.


A binary star system consists of two stars which orbit around a barycenter; the objects follow Kepler's Laws. There are several subcategories of binary stars, classified by their visual properties including eclipsing binaries, visual binaries, spectroscopic binaries and astrometric binaries. 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.


Visual Binaries


Visual binaries are systems in which the individual stars can be seen through a telescope. Any two stars seen close to one another is a double star, the most famous being Mizar and Alcor in the Big Dipper. Odds are, though, that a double star is probably a foreground and background star pair that only looks near each other. With the invention of the telescope may such pairs were found. Herschel, in 1780, measured the separation and orientations of over 700 double stars and found that only about 50 pairs changed orientation over 2 decades of observation. One such example is Sirius A and B shown below. Their motion through the sky is a complex, twisted path which takes decades to map and plot. 

Eclipsing Binaries


In the late 1600's, Italian astronomers noticed that some stars occasionally drop in their brightness up to 1/3 their peak luminosity. Later measurements showed that these declines were periodic, ranging from hours to days. It is now recognized that these brightness changes are due to the eclipsing of one star by another (as they pass in front of each other).


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.


Eclipsing binaries are studied by monitoring their light curves (shown below), the changes in brightness with time. When the smaller, dimmer star passes in front of the brighter star, there is a deep minimum. When the dimmer star passes behind the bright star there is a second, less deep, minimum. Notice the transition zone at the start and end of each eclipse.

Spectroscopic binaries

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.

Most of the stars in our Galaxy are not isolated stars like the Sun, but exist in multiple star systems of 2, 3 or more stars orbiting around a common center of gravity called a BARYCENTER.


The exact center of all the material (that is, mass) that makes up an object—whether a planet or a pencil—is called its "center of gravity." For example, if you have a straight stick, like a ruler, there's a place at the middle where you can balance it on your finger. That's its center of gravity.

Ruler's center of gravity.

But the center of gravity may not be the point that looks like the middle of the object. Some parts of the object may be heavier (denser) than others. A sledge hammer is heavier on one end than the other. Its center of gravity is much closer to the heavy end than the lighter end.

Hammer's center of gravity.

To get an idea of where the center of gravity is, rest the ends of any object like the ruler or a pencil on one finger from each hand. Slowly move your fingers together without dropping the object. Your fingers will meet underneath the object's center of gravity. You can balance the object on one finger at that special place.

The actual center of gravity could be close to the surface if, for example, the object is flat like a ruler or a dinner plate. Or the center of gravity could be deep inside if the object is "three-dimensional," like a box or a ball. And if you let the object spin (like when you throw it), it will try to spin about that point.

We can take advantage of this bit of knowledge and look for large planets in other solar systems by learning to detect this type of tiny wobble in the star's position.


Animation show planet orbiting star from above, with star wobbling.

(Animation above) As seen from above, a large planet orbits a star–or rather the star and planet orbit their shared center of mass, or barycenter. 

(Animation above) As seen from the side, a large planet and star orbit their shared center of mass, or barycenter, with the star seeming to shift back and forth.  These different views of binary stars produce different light curves as well. 

 Direct link to video above:

Astronomers can use the orbits and, hence, the gravitational pull, of pairs of stars orbiting each other to determine the mass of the stars, a key stellar property.  It’s the star’s mass that controls all the other characteristics of the star: it’s luminosity, temperature, color, size, even lifetime. BUT…it’s not possible to directly measure the mass of a single, isolated star.  So what can we do?The good thing is that 50% to 75% of all stars are found in binary or multiple star systems. If we can watch 2 stars orbit each other and determine the period of revolution and the distance between the stars, we can calculate the total mass of the system and maybe even the individual masses of the two stars. Once we know the masses of many stars in binary star systems, we can infer the masses of single, isolated stars by comparing them to similar stars with known masses. 

Other Resources

Try the animation here and here!

Learn how these star systems form