A binary star is actually a name for a star system that is made up of two or more stars that orbit a common center of mass. The brightest star of a binary pair is called the primary, while the other star is called companion, or secondary.
In astrophysics, binary stars easily allow for the calculation of mass of the individual stars. This is done though using their mutual orbit to precisely calculate the mass of the individual stars through Newtonian calculations. Through this, the radius and density of the individual stars can be calculated indirectly. This collected data from various binary stars also allow the calculation of mass for similar single stars through extrapolation. It is estimated that a third of the stars in our galaxy are binary and multiple star systems.
Binary stars are classified by the method of observation used to discover them. These include eclipsing, visual, spectroscopic, and astrometric binaries.
An eclipsing binary is a binary system where the orbital plain is close enough to the line of sight of an observer that the individual stars will eclipse each other.
Eclipsing binaries are also classified as variable stars, stars that have regular and periodic changes in the apparent magnitude. This variation in brightness may be from a pair of binary stars as one passes in front of the other as seen from the point of the observer. If the two stars are of different sizes, the larger star will block the other through a total eclipse, while the smaller star will partially dim the large one via an annular eclipse. This change in brightness over regular periods of time is known as the light curve.
The first observed eclipsing binary discovered was the star system Algol. The star was known to periodically change in magnitude since ancient times, but in 1881, it was discovered this was due to the fact Algol was not one, but two stars in close orbit.
A visual binary is a binary pair where the individual stars are visible through a telescope that has the appropriate resolving power. Brighter stars are more difficult to resolve as visible binaries then dimmer ones, due to glare. A binary may also be difficult to resolve visually if the primary (brighter star) is significantly more visually luminous then its companion, effectively washing out the other star.
By measuring the position angle of the companion star relative to the primary and the angler distance between the two stars over time, the ellipse, called the apparent ellipse, which is the orbit of the secondary in respect to the primary, can be plotted out. From this measurement of the semi-major axis and orbital period of binary stars' orbit, the mass of the stars can be determined.
Visual binaries are quite common and in fact make up many of the most well-known and prominent stars int he night sky, such as Castor. Three of the six closest known stars are visual binaries: Alpha Centauri, Luyten 726-8, and Sirius.
If it is impossible to resolve the star as a binary visually, then one method determining whether or not a star is part of a binary pair is through analyzing the light emitted from the star system using a spectrograph, binaries observed this way are known a spectroscopic binaries.
When a Spectrograph is used to indirectly observe a star, it spreads the light from that star into a full spectrum of colors superimposed with dark absorption lines. If the star observed in this method is suspected to be a binary star, the spectrum that is analyzed is from both stars together. If in observing this spectrum, a Doppler effect is observed over time, that determines there is indeed a binary pair. The Doppler effect is caused when the two stars orbit their common center of mass, one star moves closer to us while its companion moves away. As this happens, the spectral lines in the spectrum for the star moving close will be blue shifted, while the spectral lines of its companion moving away will be red shifted though the Doppler effect. As the stars move across out line of site in their orbit, the location of absorption lines or each star will become the same. As the first star moves away and its companion approaches, the opposite shifting of wavelengths will occur, with the companion star's spectral lines blue shifting as it approaches, while the first star's spectral lines red shift as it moves away in its orbit.
One of the most well known spectroscopic binaries is the star system Mizar. The star system was already known as a visual binary, when in 1889 it was discovered the primary star Mizar A of the visual binary pair, had its own close companion. This companion was the first star to be found using spectroscopy. Later it was observed that Mizar B also had its own spectroscopic companion.
An astrometric binary is a star system where only one star can be observed but an unseen companion is inferred through a perturbation ("wobble") of the star's proper motion in space. This perturbation is caused by the companion's gravitational influence on the observable primary star. Sirius is the best known example of an astrometric binary, when in 1844 Friedrich Bessell observed a wobble in the motion of Sirius A. He theorized that the star had an unseen companion. Later with improving telescope technology, the companion white dwarf was confirmed visually.
An optical binary are two stars that visually appear next to each other from the point of the observer using the unaided eye. The reason for this is the two stars are usually along the line of sight of the observer, giving the illusion they are part of a binary pair. However in reality the two stars are actually a great distance from each other and are not gravitationally bound as a single system. A prime example of this is Alpha Capricorni, traditionally seen as a binary with the individual stars referred to as α1 Capricorni and α2 Capricorni, however the former is 690 light years away, while the latter is only 109 light years away from Earth respectively.
Cataclysmic binaries, sometimes referred to as cataclysmic variable stars or cataclysmic variables, are very close binary pair that will suddenly and irregularly increase in brightness before returning to their normal magnitude. The two components of a cataclysmic binary consist of a white dwarf primary and an M class secondary (ranging from a main sequence star to a giant). The two stars are sufficiently close that the white dwarf distorts and draws off material from the secondary. This infalling matter of mostly hydrogen forms an accretion disk around the white dwarf. Instabilities in this accretion disk can lead to what is known as a dwarf nova.
There are two types of cataclysmic binaries, non-magnetic and magnetic. The non-magnetic types are by far the most common, these include U Geminorum stars as well as those that are the source of classical and recurrent novae. Much more rare are the magnetic types, where a powerful magnetic field surrounds the primary white dwarf star, greatly affecting how the material flows from the secondary star, as well as locking the two stars into a synchronous rotation.
The observations of binary stars began with the invention of the telescope, with the first known recordings in the 17th century. Giovanni Battista Riccioli discovered in 1650 that Mizar was actually a binary. Christiaan Huygens found that that Theta Orionis was actually three stars in 1656, Robert Hooke made the same observation about Gamma Arietis in 1664, while in 1685 Father Fontenay observed that the star Acrux was really a binary pair.
William Herschel was the first person to coin the term binary star. He defined the term in 1802 as:
- The union of two stars, that are formed together in one system, by the laws of attraction.
Herschel began his observation of binaries in 1779. The result was a cataloging of over 700 double stars systems as recorded in his book Catalogue of 500 new Nebulae ... and Clusters of Stars; with Remarks on the Construction of the Heavens in 1802. By the next year, he concluded that these double stars must be binary systems. It was not until 1827 though that an actual orbit of a binary star system was calculated. This was completed by Félix Savary of the star Xi Ursae Majoris. Today over 100,000 binary star systems have been cataloged, although the actual orbits of only a few thousand of these are known, with some cataloged stars possibly being only optical binaries.
Evolution of Binary Stars
Binary pairs have been observed in protostars that have yet to reach the main sequence, suggesting that binaries form in the early stages of star formation. This could be due to the fragmentation of the molecular cloud as the stars first form, allowing for multiple stars to be created in the same system.
Because most binary stars are of different masses, one will evolve off the main sequence before its companion. In this scenario, several different events may happen. The two stars may remain detached if the companion is far enough away in distance, and not very gravitationally massive. The two stars will be semi-detached if the star that has evolved into the giant star is gravitationally close enough to its companion and exceeds its Roche lobe, losing mass to its companion through accretion. In this situation, much of the giant's mass may transfer to the companion star, actually making it the more massive of the two, despite still being in the main sequence. Algol is the prime example of this.
In some situations, the two stars of the binary system are so close that the expanding giant may actually come in atmospheric contact with its companion. This is known as a contact binary. In this situation, the very close companion may cause the atmosphere of the giant to literally "splash away," leaving a naked core. The companion, meanwhile, may spiral towards the core of the once giant from the friction of their atmospheres. The result will be a merged core that becomes a white dwarf.
Type I supernova
If one of the stars on a binary system is a white dwarf that is close enough to its companion when the companion star exceeds its Roche lobe, the white dwarf will steadily accrete gases from its companion's atmosphere. This accreted gases will build up and compress on the surface of the white dwarf, due to its high gravity at its surface. The gas will then steadily heat up as more material accumulates. The result is that hydrogen fusion may occur on the surface of the dwarf and, through the tremendous release in energy, throw the rest of the collected gas off in a brilliant flash. This extremely bright event is called a nova. 
The above event will occur as long as the additional mass the white dwarf accreted from its companion doesn't cause the star to exceed the Chandrasekhar limit, which is 1.4 solar masses. If the material buildup on the white dwarf exceeds this limit, the electron degeneracy of the white dwarf itself will no longer be able to hold the star up against gravity. In this scenario a type I supernova will occur that destroys the star. The most famous example of this is the supernova SN 1572, considered one of the foremost events in astronomy. Tycho Brahe observed this event extensively.
- Orbits of 150 Visual Binaries
- The Radial Velocity Equation in the Search for Exoplanets ( The Doppler Spectroscopy or Wobble Method )
- D. Prialnik, Novae, Encyclopaedia of Astronomy and Astrophysics (2001, p.1846-1856)