What is a Binary Star System?

Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Our solar system, with its single sun, is actually quite rare. Most star systems contain solar twins, or binary stars. Read on to learn more about these interesting star systems and then check your understanding with a short quiz.

Twin Stars

You know that planets revolve around stars just like our planet Earth orbits the Sun. But did you know that stars can also revolve around other stars? Scientists estimate that over 80% percent of the points of light in the night sky are actually multiple star systems. These systems can have two, three, four, or even more stars. There is evidence that the star system Jabbah in the Scorpius constellation has as many as seven stars! That means that our solar system, with its single star, is actually quite rare.

Most of these multiple star systems are binary stars; the prefix bi- is Latin in origin and means two. Binary stars are two stars that share a gravitational link and simultaneously orbit their common center of mass. The center of mass of an object (or objects) is its balancing point. Imagine that you could stick the two stars on either end of a long pole. The center of mass is the point where you could balance that pole on your finger without it tipping in one direction or the other.

In a binary star star system, two stars orbit their common center of mass.
A diagram showing the center of mass of two objects.

Binary stars are classified as either 'wide' or 'close'. In wide binaries, as the name suggests, the orbits of the two stars keep them far apart from each other. The stars move along their life course separately and have little effect on one another. Close binaries, however, are near enough to each other that the gravitational pull of one star can deform and sometimes devour the other star. Since stars are classified based on their masses, this transfer of matter from one star to another can completely alter the stars' life courses.

In this close binary system, the gravity of the highly dense white dwarf is pulling in the outer atmosphere of the blue giant.
Drawing of a close binary system.

Observing Binary Stars

In addition to being classified as either wide or close, binary stars are placed into four categories based on how they are observed from Earth. These categories include visual binaries, spectroscopic binaries, eclipsing binaries and astrometric binaries. The groupings are not exclusive, as it is possible for a binary star system to fit into more than one. For example, a spectroscopic binary can also be an eclipsing binary.

Visual binaries are pairs that have enough space in between them to be seen as two separate stars through a telescope. Be careful, though, that you are not looking at a double star. A double star refers to a pair of stars that, from our position here on Earth, appear to be visual binaries. However, they only seem close to each other because of the angle at which we happen to be looking at them. They may not be anywhere near one another space.

Spectroscopic binaries are too close to be observed as separate stars even through a telescope. Astronomers observe tell-tale patterns in the wavelengths of light emitted by the linked stars. They are then able to deduce the presence of a binary system based on these observations. Spectroscope is a Latin-Greek hybrid using the Latin term spectrum and the Greek term scope. Thus, the term literally means looking at the spectrum, though the wavelengths of observed light do not necessarily have to be in the visible range. The binary system pictured below, for example, was discovered through the emission of x-rays.

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