A binary star is a star system that includes two stars that orbit a common barycenter, which is the center of mass and point around which two or more objects orbit. When more than two stars orbit a common barycenter, the system is referred to as a multiple star system. A binary star system is different than a double star, which occurs when two stars appear very close together when observed from Earth, but are actually very distant. Additionally, a binary star system occurs when two stars are trapped in a barycenter in which mass is most concentrated. Binary and multiple star systems produce about four-fifths of the light seen in the sky.
Most binary stars contain a primary star, which is brighter, and a secondary star, which is slightly dimmer. When the stars are equal in brightness, the discoverer reserves the right to determine which is the primary and secondary star. Binary stars can be classified based on their orbit. For example, wide binaries have orbits that keep the stars apart, and they form and evolve independently with minimal impact on one another. On the other hand, close binaries form and evolve nearby, and can transfer mass back and forth. Some stars in close binaries can pull mass from the other star, and in some cases, engulf it. Visual binaries are wide enough for telescopes on Earth to view them as different stars, but spectroscopic binaries are so incredibly close that telescopes view them as a single star and astronomers must measure the wavelength of the light they emit to differentiate between them. Eclipsing binaries orbit at an angle such that they form an eclipse when one star passes in front of the other.
Discovery and Evolution
Visual binaries were the first binary systems to be observed from Earth. Astronomers and mathematicians such as Giovanni Battista Riccioli, Galileo Galilei, and Benedetto Castelli observed double stars during the 17th century, but it was Sir William Herschel who coined the term "binary star" in 1802 to refer to two stars that orbited a common center. A binary system can occur when a larger star captures a smaller star in its orbit, creating a binary pair. Alternatively, an envelope of gas and dust that collapses on itself splits and forms multiple stars.
Is the Sun a Binary Star?
In the early 1980s, some astronomers suggested that a second star that was either a white dwarf, brown dwarf, or a dim red dwarf formed a binary system with a primary star. The suggestion was made in an attempt to explain the periodic mass extinctions in Earth’s history, which some researchers and paleontologists suggested occurs every 26 million years. Although Earth’s space and ground telescopes have not detected the presence of a second sun in the Solar System, studies conducted in 2017 revealed that most stars, such as the sun, had companions during the early stages of their formation. Experimental models have shown that some stars in a binary system combine to form a single star or drift away from each other. It is unclear if the Sun had a binary partner that it engulfed or drifted away billions of years ago.