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Since our solar system has a single star at its heart, it's logical to assume that all stars form independently and travel the galaxy alone. However, it turns out that about a third (or possibly even more) of all stars are born in our galaxy (and in other galaxies) exist in multiple-star systems. There can be two stars (called a binary), three stars, or even more.
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 stars in such a system 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. Another favorite is Albireo, part of the constellation Cygnus, the Swan. Both are easy to spot, but it requires telescope or binoculars to see the components of each binary system.
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 from 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.
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. That makes 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 as more observations are made with more powerful telescopes.
Spectroscopy is a powerful tool in astronomy. It allows astronomers to determine various properties of stars simply by studying their light in minute detail. However, in the case of binaries, spectroscopy can also reveal that a star system may, in fact, be composed of two or more stars.
How does this work? 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 (so close that even a good telescope can't "split" them apart, they are rarely also visual binaries. In the odd 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). Closeness and long periods make the partners of each system easier to spot.
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.
In eclipsing binary systems the orbital plane 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 scale 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.
Binary stars can teach astronomers a lot about their individual systems.They can also give clues to their formation, and the conditions under which they were born, since there had to be enough material in the birth nebula for both to form and not disrupt each other. In addition, there weren't likely large "sibling" stars nearby, since those would have "eaten up" the material needed for the formation of the binaries. The science of binaries is still very much an active topic in astronomy research.
Edited and updated by Carolyn Collins Petersen.