Binary Stars

By , 8th May 2009 in Astronomy

A binary star is a star system consisting of two stars orbiting around their common centre of mass. The brighter star is called the primary and the other is its companion star or secondary.

Binary stars were first discovered in 1767 by John Michell when he suggested that double stars might be physically attached to each other. In 1779 Sir William Herschel began observing double stars and soon thereafter published catalogues of double stars. He was the first to coin the phrase Binary Star in 1802. The modern definition restricts the term to pairs of stars which revolve around a common centre of mass (barycenter).

Hubble image of the Sirius binary system, in which Sirius B can be clearly distinguished (lower left).

Double Stars may include binary systems; however a double star is used to describe two stars which are apparently close together as observed from Earth. The stars may not interact with each other and may be separated by a considerable distance.

Importance of Binary Stars

Binary star systems are important to astrophysicists because their orbits allow the masses of two stars to be calculated. This allows other parameters such as radius and density to be estimated. These calculations allow an empirical mass-luminosity relationship to be created which allows the masses of single stars to be estimated.


There are a number of ways that a binary star can be detected. Luckily most stars are large enough and bright enough that they can be directly observed. These are called visual binaries and usually have very long orbital periods (centuries or millennia) which make orbital calculations difficult.

An eclipsing binary, with an indication of the variation in intensity.

If a binary star happens to orbit in a plane along our line of sight, its components will mutually eclipse and transit each other; these pairs are called eclipsing binaries or photometric binaries.

Mass Transfer and Accretion

Most binary stars orbit at large distances and there is relatively little gravitational pull from one to the other. In this situation the stars are said to be detached.

It is possible for two binary stars to be close enough together that the gravitational pull of one is enough to "pull" matter from the other. The Roche Lobe is the region of space around a star in a binary system within which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material outside of the lobe will fall into the other star through a process known as Roche Lobe overflow (RLOF). These are called Semidetached binary stars.

If the stars are very close to each other than their outer atmosphere will "merge" and both stars will fill their Roche lobes. The uppermost part of the stellar atmospheres forms a common envelope that surrounds both stars. These are contact binaries.


The image above right shows the most famous binary star Sirius A and B. This image was taken by the Hubble Space Telescope and was intentionality over exposed so that the much dimmer Sirius B can be seen in the lower left hand corner. While Sirius A and B can be observed with amateur telescopes, the difference in magnitude between the two means that it is very difficult.

Albireo photographed by Jim Spinner on October 26, 2004

One of the easiest binary systems to image is Albireo in the constellation of Cygnus. Albireo is the brightest star in Cygnus and the binary pair show a very striking difference in colour, with Albireo A glowing yellow and Albireo B a little fainter glowing in blue light. The image right was photographed using an 8" Yeovil SCT and Philips Toucam WebCam by Jim Spinner on October 26th, 2004.

Some must see double stars

Mizar & Alcor13h 23.9m+54° 56'11.8'
Albireo19h 30.7m+27° 58'34"
Epsilon Lyrae18h 44.3m+39° 40'3.5'
Almach02h 03.9m+42° 20'9.8"
Ras Algethi17h 14.6m+14° 23'4.6"
Trapezium05h 35.3m-05° 23'25"
Castor07h 34.6m+31° 53'3"
Izar14h 45.0m+27° 04'2.8"
Further Reading

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