Binary star

Examples of binaries are Sirius, and Cygnus X-1 (Cygnus X-1 being a well-known black hole).

Sirius (, a latinisation of Greek Σείριος, Seirios, lit."glowing" or "scorching") is a binary star and the brightest star in the night sky.

The term double star is often used synonymously with binary star; however, double star can also mean optical double star.

This occurs because the pair either forms a binary star (i.e. a binary system of stars in mutual orbit, gravitationally bound to each other) or is an optical double, a chance line-of-sight alignment of two stars at different distances from the observer.

Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors of both novae and type Ia supernovae.

All observed novae involve closely located binary stars (the progenitors), either a pair of red dwarfs in the process of merging, or a white dwarf and another star.

If a binary star happens to orbit in a plane along our line of sight, its components will eclipse and transit each other; these pairs are called eclipsing binaries, or, as they are detected by their changes in brightness during eclipses and transits, photometric binaries.

A binary star system can also produce eclipses if the plane of the orbit of its constituent stars intersects the observer's position.

Examples of binaries are Sirius, and Cygnus X-1 (Cygnus X-1 being a well-known black hole).

In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.

A binary star is a star system consisting of two stars orbiting around their common barycenter.

This is the case for the Earth–Moon system, in which the barycenter is located on average 4671 km from Earth's center, 75% of Earth's radius of 6378 km. When the two bodies are of similar masses, the barycenter will generally be located between them and both bodies will orbit around it. This is the case for Pluto and Charon, one of Pluto's natural satellites, as well as for many binary asteroids and binary stars.

Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors of both novae and type Ia supernovae.

The mechanisms that produce such a wide variety of shapes and features are not yet well understood, but binary central stars, stellar winds and magnetic fields may play a role.

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.

On May 2, 1780, Sir William Herschel discovered that this was a binary star system, making it the first such system ever discovered.

Early examples include Mizar and Acrux. The bright southern star Acrux, in the Southern Cross, was discovered to be double by Father Fontenay in 1685.

α 1 Crucis is itself a spectroscopic binary with components designated α Crucis Aa (also officially named Acrux ) and α Crucis Ab. Its two component stars orbit every 76 days at a separation of about 1 astronomical unit (AU).

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

He soon discovered many more binary and multiple stars than expected, and compiled them with careful measurements of their relative positions in two catalogues presented to the Royal Society in London in 1782 (269 double or multiple systems) and 1784 (434 systems).

Early examples include Mizar and Acrux.

Mizar is a visual double with a separation of 14.4 arcseconds, each of which is a spectroscopic binary.

The light curve of an eclipsing binary is characterized by periods of practically constant light, with periodic drops in intensity when one star passes in front of the other.

Light curves can be periodic, as in the case of eclipsing binaries, Cepheid variables, other periodic variables, and transiting extrasolar planets, or aperiodic, like the light curve of a nova, a cataclysmic variable star, a supernova or a microlensing event or binary as observed during occultation events.

Sometimes, the only evidence of a binary star comes from the Doppler effect on its emitted light.

Doppler first proposed this effect in 1842 in his treatise "Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels" (On the coloured light of the binary stars and some other stars of the heavens).

They may also be detected by indirect techniques, such as spectroscopy (spectroscopic binaries) or astrometry (astrometric binaries).

He made the first measurement of stellar parallax: 0.3 arcsec for the binary star 61 Cygni.

Algol, a triple star system in the constellation Perseus, contains the best-known example of an eclipsing binary.

Rather than being an intrinsically variable star, it is an eclipsing binary.

The bright southern star Acrux, in the Southern Cross, was discovered to be double by Father Fontenay in 1685.

Iota Crucis is a binary star 125 light-years from Earth.

Examples of binaries are Sirius, and Cygnus X-1 (Cygnus X-1 being a well-known black hole).

The compact object and blue supergiant star form a binary system in which they orbit around their center of mass every 5.599829 days.

Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors of both novae and type Ia supernovae.

One model for the formation of this category of supernova is a close binary star system.

Detached binaries are binary stars where each component is within its Roche lobe, i.e. the area where the gravitational pull of the star itself is larger than that of the other component. A contact binary is a type of binary star in which both components of the binary fill their Roche lobes.

The Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star.

When a binary system contains a compact object such as a white dwarf, neutron star or black hole, gas from the other (donor) star can accrete onto the compact object.

The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star.

Algol, a triple star system in the constellation Perseus, contains the best-known example of an eclipsing binary.

Thus Algol became one of the first known spectroscopic binaries.

W Ursae Majoris is an example.

W Ursae Majoris (W UMa) is the variable star designation for a binary star system in the northern constellation of Ursa Major.

These binaries are classified as low-mass or high-mass according to the mass of the donor star.

X-ray binaries are a class of binary stars that are luminous in X-rays.

Cataclysmic variable stars, where the compact object is a white dwarf, are examples of such systems.

Cataclysmic variable stars are binary stars that consist of two components; a white dwarf primary, and a mass transferring secondary.

A contact binary is a type of binary star in which both components of the binary fill their Roche lobes.

In astronomy, a contact binary is a binary star system whose component stars are so close that they touch each other or have merged to share their gaseous envelopes.