Seeing Double: Binary Stars

Sirius binary star system
A Hubble Space Telescope image of Sirius A and B, a binary system 8.6 light-years away from Earth. NASA/ESA/STScI

Since our solar system has a single star at its heart, you might think that all stars form independently and travel the galaxy alone. It turns out, however, that about a third (or possibly even more) of all stars are born in multiple-star systems.

The Mechanics of a Binary Star

Binaries (two stars orbiting around a common center of mass) are very common in the sky. The larger of the two is called the primary star, while the smaller one is the companion or secondary star.

One of the best-known binaries in the sky is the bright star Sirius, which has a very dim companion. There are many other binaries you can spot with binoculars, as well. 

The term binary star system should not be confused with the term double star. Such systems are usually defined as two stars that appear to be interacting, but actually are very distant to each other and have no physical connection. It can be confusing to tell them apart, especially from a distance. 

It can also be quite difficult to identify the individual stars of a binary system, as one or both of the stars may be non-optical (in other words, not especially bright in visible light). When such systems are found though, they usually fall into one of four following categories.

Visual Binaries

As the name suggests, visual binaries are systems in which the stars can be identified individually. Interestingly, in order to do so it is necessary for the stars to be "not too bright".

(Of course, distance to the objects is also a determining factor if they will be individually resolved or not.)

If one of the stars is of high luminosity, then its brightness will "drown out" the view of the companion, making it difficult to see. Visual binaries are detected with telescopes, or sometimes with binoculars.

In many cases other binaries, like those listed below, could be determined to be visual binaries when observed with powerful enough instruments. So the list of systems in this class is continually growing with increased observation.

Spectroscopic Binaries

Spectroscopy is a powerful tool in astronomy, allowing us to determine various properties of stars. However, in the case of binaries, they can also reveal that a star system may, in fact, be composed of two or more stars.

As two stars orbit each other they will at times be moving towards us, and away from us at others. This will cause their light to be blueshifted then redshifted repeatedly. By measuring the frequency of these shifts we can calculate information about their orbital parameters.

Because spectroscopic binaries are often very close to each other, they are rarely also visual binaries. In the rare instances that they are, these systems are usually very close to Earth and have very long periods (the farther apart that they are, the longer it takes them to orbit their common axis).

Astrometric Binaries

Astrometric binaries are stars that appear to be in orbit under the influence of an unseen gravitational force. Often enough, the second star is a very dim source of electromagnetic radiation, either a small brown dwarf or perhaps a very old neutron star that has spun down below the death line.

Information about the "missing star" can be ascertained by measuring the orbital characteristics of the optical star.

The methodology for finding astrometric binaries is also used to find exoplanets (planets outside our solar system) by looking for "wobbles" in a star. Based on this motion the masses and orbital distances of the planets can be determined.

Eclipsing Binaries

In eclipsing binary systems the orbital plain of the stars is directly in our line of sight. Therefore the stars pass in front of each other as they orbit.

When the dimmer star passes in front of the brighter star there is a significant "dip" in the observed brightness of the system. Then when the dimmer star moves behind the other, there is a smaller, but still measurable dip in brightness.

Based on the time stale and magnitudes of these dips the orbital characteristics as well as information about the stars' relative sizes and masses can be determined.

Eclipsing binaries can also be good candidates for spectroscopic binaries, though, like those systems they are rarely if ever found to be visual binary systems.

Edited and updated by Carolyn Collins Petersen.

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Your Citation
Millis, John P., Ph.D. "Seeing Double: Binary Stars." ThoughtCo, Mar. 2, 2017, Millis, John P., Ph.D. (2017, March 2). Seeing Double: Binary Stars. Retrieved from Millis, John P., Ph.D. "Seeing Double: Binary Stars." ThoughtCo. (accessed March 24, 2018).