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The term binary star is a misnomer because it is actually a star system made up of usually two stars that orbit around one center of mass – where the mass is most concentrated. A binary star is not to be confused with two stars that appear close together to the naked eye from Earth, but in reality are very far apart – Carl Sagan far!
Astrophysicists find binary systems to be quite useful in determining the mass of the individual stars involved. When two objects orbit one another, their mass can be calculated very precisely by using Newton’s calculations for gravity. The data collected from binary stars allows astrophysicists to extrapolate the relative mass of similar single stars.
There are several subcategories of binary stars, classified by their visual properties including eclipsing binaries, visual binaries, spectroscopic binaries and astrometric binaries.
Eclipsing binary stars are those whose orbits form a horizontal line from the point of observation; essentially, what the viewer sees is a double eclipse along a single plane; Algol for example.
A visual binary system is a system in which two separate stars are visible through a telescope that has an appropriate resolving power. These can be difficult to detect if one of the stars’ brightness is much greater, in effect blotting out the second star.
Spectroscopic binary stars are those systems in which the stars are very close and orbiting very quickly. These systems are determined by the presence of spectral lines – lines of color that are anomalies in an otherwise continuous spectrum and are one of the only ways of determining whether a second star is present. It is possible for a binary star system to be both a visual and a spectroscopic binary if the stars are far enough apart and the telescope being used is of a high enough resolution.
Astrometric binary stars are systems in which only one star can be observed, and the other’s presence is inferred by the noticeable wobble of the first star. This wobble happens as a result of the smaller star’s slight gravitational influence on the larger star.
So now you can answer the question, “what is a binary star?”
We have written many articles about binary stars on Universe Today. Here’s an article about a new class of binary stars discovered, and a situation where one star was ejected out of a binary partnership.