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.

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.

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.

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.

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.

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.

Early examples include Mizar and Acrux.

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 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.

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.

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.

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.

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

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).

In cases where the binary star has a Bayer designation and is widely separated, it is possible that the members of the pair will be designated with superscripts; an example is Zeta Reticuli, whose components are ζ 1 Reticuli and ζ 2 Reticuli.

Zeta Reticuli, Latinized from ζ Reticuli, is a wide binary star system in the southern constellation of Reticulum.

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.

Orbital periods can be less than an hour (for AM CVn stars), or a few days (components of Beta Lyrae), but also hundreds of thousands of years (Proxima Centauri around Alpha Centauri AB).

Alpha Centauri A and B are Sun-like stars (Class G and K), and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus.

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.

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).

Symbiotic stars are binary star systems composed of a late-type giant star and a hotter companion object. Since the nature of the companion is not well-established in all cases, it may be termed a "hot companion".

A symbiotic binary is a type of binary star system, often simply called a symbiotic star.

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.

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.

Orbital periods can be less than an hour (for AM CVn stars), or a few days (components of Beta Lyrae), but also hundreds of thousands of years (Proxima Centauri around Alpha Centauri AB).

The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars.

Orbital periods can be less than an hour (for AM CVn stars), or a few days (components of Beta Lyrae), but also hundreds of thousands of years (Proxima Centauri around Alpha Centauri AB).

In these hot blue binary variables, a white dwarf accretes hydrogen-poor matter from a compact companion star.