Astronomy - Binary Systems

Binary Stars

Since the invention of the telescope, many pairs of double stars have been found.

John Michell was the first to suggest that double stars might be physically attached to each other when he argued in 1767 that the probability that a double star was due to a chance alignment was small.

The term binary was first used in this context by Sir William Herschel in 1802, when he wrote.

"If, on the contrary, two stars should really be situated very near each other, and at the same time so far insulated as not to be materially affected by the attractions of neighbouring stars, they will then compose a separate system, and remain united by the bond of their own mutual gravitation towards each other".

William Herschel began observing double stars in 1779 and soon thereafter published catalogues of about 700 double stars. By 1803, he had observed changes in the relative positions in a number of double stars over the course of 25 years, and concluded that they must be binary systems;

The first orbit of a binary star, however, was not computed until 1827, when Félix Savary computed the orbit of Xi Ursae Majoris.

Since this time, many more double stars have been catalogued and measured. The Washington Double Star Catalogue, a database of visual double stars compiled by the United States Naval Observatory, contains over 100,000 pairs of double stars, including optical doubles as well as binary stars.

A binary system  is an astronomical term referring to two objects in space (usually stars, but also planets, black holes, galaxies or asteroids) which are so close that their gravitational interaction causes them to orbit about a common center of mass called a barycenter.

In a binary system the brightest object is referred to as primary, and the dimmer the secondary.

Two Stars Orbiting Barycentre

More than four-fifths of the single points of light we observe in the night sky are actually two or more stars orbiting together. The most common of the multiple star systems are binary stars, systems of only two stars together. These pairs come in an array of configurations that help scientists to classify stars, and could have impacts on the development of life.

Wide binaries are objects with orbits that keep them a part from one another. They evolve separately and have very little effects on each other.

Close binaries are close to each other and are able to transfer mass from one another. They may also exert a gravitational force on each other.

Visual binaries are two stars separated enough that they can be viewed through a telescope or binoculars. The relative brightness of the two stars is an important factor, as glare from a bright star may make it difficult to detect the presence of a fainter component.

Eclipsing binaries are where the object's orbits are at an angle that when one passes in front of the other it causes an eclipse or transit, as seen from Earth.

Astrometric binaries are objects that seem to move around nothing as their companion object cannot be identified, it can only be inferred.

The companion object may not be bright enough or may be hidden in the glare from the primary object.

Spectroscopic binaries

Sometimes, the only evidence of a binary star comes from the Doppler effect on its emitted light. In these cases, the binary consists of a pair of stars where the spectral lines in the light emitted from each star shifts first towards the blue, then towards the red, as each moves first towards us, and then away from us, during its motion about their common center of mass, with the period of their common orbit.

While it is not impossible that some binaries might be created through gravitational capture between two single stars, given the very low likelihood of such an event (three objects are actually required, as conservation of energy rules out a single gravitating body capturing another) and the high number of binaries, this cannot be the primary formation process.

Also, the observation of binaries consisting of pre main-sequence stars, supports the theory that binaries are already formed during star formation.

Fragmentation of the molecular cloud during the formation of protostarsis an acceptable explanation for the formation of a binary or multiple star system.