Hubble image of the Sirius binary system, in which Sirius B can be clearly distinguished (lower left). The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into Sirius is the brightest star in the night sky with a visual Apparent magnitude of &minus1

A binary star is a stellar system consisting of two stars orbiting around their center of mass. A star system or stellar system is a small number of Stars which orbit each other bound by gravitational attraction. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star For each star, the other is its companion star. Recent research suggests that a large percentage of stars are part of systems with at least two stars. Binary star systems are very important in astrophysics, because observing their mutual orbits allows their mass to be determined. Astrophysics is the branch of Astronomy that deals with the Physics of the Universe, including the physical properties ( Luminosity, Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The masses of many single stars can then be determined by extrapolations made from the observation of binaries.

Binary stars are not the same as optical double stars, which appear to be close together as seen from Earth, but may not be bound noticeably by gravity. Double Star is a Science fiction Novel by Robert A Heinlein, first serialized in Astounding Science Fiction EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Gravitation is a natural Phenomenon by which objects with Mass attract one another Binary stars can either be distinguished optically (visual binaries) or by indirect techniques, such as spectroscopy. Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ If binaries happen to orbit in a plane containing our line of sight, they will eclipse each other; these are called eclipsing binaries. An eclipse is an astronomical event that occurs when one Celestial object moves into the shadow of another

Systems consisting of more than two components, known as multiple stars, are also not uncommon and are generally classified under the same name. A multiple star consists of three or more Stars which appear from the Earth to be close to one another in the sky The components of binary star systems can exchange mass, bringing their evolution to stages that single stars cannot attain. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime Examples of binaries are Algol (an eclipsing binary), Sirius, and Cygnus X-1 (of which one member is probably a black hole). Algol (β Per / Beta Persei known colloquially as the Demon Star, is a bright Star in the Constellation Perseus. Sirius is the brightest star in the night sky with a visual Apparent magnitude of &minus1 Cygnus X-1 (abbreviated Cyg X-1) in the Constellation Cygnus. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e

Terminology

The term binary star was coined by Sir William Herschel in 1802 to designate, in his definition, "a real double star - the union of two stars that are formed together in one system by the laws of attraction". A neologism (from Greek neo = "new" + logos = "word" is a word that although devised relatively recently in a specific time period has been Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British Any two closely-spaced stars might appear to be a double star, the most famous case being Mizar and Alcor in the Big Dipper (Ursa Major). Double Star is a Science fiction Novel by Robert A Heinlein, first serialized in Astounding Science Fiction This is an article about the star for other uses please see Mizar (disambiguation Mizar (ζ UMa / ζ Ursae Majoris is a Star in This article is about the asterism; for other uses see Big Dipper (disambiguation. Ursa Major ( is a Constellation visible throughout the year in most of the northern hemisphere It is however possible that a double star is merely a star pair that only looks like a binary system: the two stars can in reality be widely separated in space, but just happen to lie in roughly the same direction as seen from Earth. Such false binaries are termed optical binaries, or optical pairs. With the invention of the telescope, many such pairs were found. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. Herschel, in 1780, measured the separation and orientations of over 700 pairs that appeared to be binary systems, and found that about 50 pairs changed orientation over two decades of observation. ^[1][2]

A true binary is a pair of stars bound together by gravity. Gravitation is a natural Phenomenon by which objects with Mass attract one another When they can be resolved (distinguished) with a powerful enough telescope (if necessary with the aid of interferometric methods) they are known as visual binaries. Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged Interferometry is the technique of using the pattern of Interference created by the superposition of two or more Waves to diagnose the properties of ^[3][4] In other cases, the only indication is the Doppler shift of the emitted light. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Systems in which this is the case, known as spectroscopic binaries, consist of relatively close pairs of stars where the spectral lines in the light from each one shifts first toward the blue, then toward the red, as each moves first toward us, and then away from us, during its motion about their common center of mass, with the period of their common orbit. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared If the orbital plane is very nearly along our line of sight, the two stars partially or fully occult each other regularly, and the system is called an eclipsing binary, of which Algol is the best-known example. In Shia Islam The Occultation is a term used to designate the hidden state of the Imam of the Time. Algol (β Per / Beta Persei known colloquially as the Demon Star, is a bright Star in the Constellation Perseus. ^[5]

Binary stars that are both visual and spectroscopic binaries are rare, and are a precious source of valuable information when found. Visual binary stars often have large true separations, with periods measured in decades to centuries; consequently, they usually have orbital speeds too small to be measured spectroscopically. Conversely, spectroscopic binary stars move fast in their orbits because they are close together; usually too close to be detected as visual binaries. Binaries that are both visual and spectroscopic thus must be relatively close to Earth.

Astronomers have discovered some stars that seem to orbit around an empty space. Astrometric binaries are relatively nearby stars which can be seen to wobble around a middle point, with no visible companion. With some spectroscopic binaries, there is only one set of lines shifting back and forth. The same mathematics used for ordinary binaries can be applied to infer the mass of the missing companion. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The companion could be very dim, so that it is currently undetectable or masked by the glare of its primary, or it could be an object that emits little or no electromagnetic radiation, for example a neutron star. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type ^[6] In some instances, there is strong evidence that the missing companion is in fact a black hole: a body with such strong gravity that no light is able to escape. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e Such binaries are known as high-mass X-ray binaries. Probably the best known example at present is Cygnus X-1, where the mass of the unseen companion is believed to be about nine times that of our sun; far exceeding the Tolman-Oppenheimer-Volkoff limit (the maximum theoretical mass of a neutron star, the only other likely candidate for the companion). Cygnus X-1 (abbreviated Cyg X-1) in the Constellation Cygnus. The Tolman-Oppenheimer-Volkoff ( TOV) limit is an upper bound to the mass of stars composed of neutron-degenerate matter ( Neutron stars. In this way, Cygnus X-1 became the first object that was widely accepted as being a black hole. ^[7][8]

Classifications

By methods of observation

Binary stars are classified into four types according to their observable properties. ^[9] Any binary star can belong to several of these classes; for example, several spectroscopic binaries are also eclipsing binaries.

Visual binaries

A visual binary star is a binary star for which the angular separation between the two components is great enough to permit them to be observed as a double star in a telescope. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth Double Star is a Science fiction Novel by Robert A Heinlein, first serialized in Astounding Science Fiction A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. The resolving power of the telescope is an important factor in the detection of visual binaries, and as telescopes become larger and more powerful an increasing number of visual binaries will be detected. Angular resolution describes the resolving power of any image forming device such as an optical or Radio telescope, a Microscope, a Camera The brightness of the two stars is also an important factor, as brighter stars are harder to separate due to their glare than dimmer ones are.

The brighter star of a visual binary is the primary star, and the dimmer is considered the secondary. In some publications (especially older ones), a faint secondary is called the comes; if the stars are the same brightness, the discoverer "chooses" the primary. ^[10] The position angle of the secondary with respect to the primary is measured, together with the angular distance between the two stars. Position angle, usually abbreviated PA, is a measurement derived from observing visual binary stars. The time of observation is also recorded. After a sufficient number of observations are recorded over a period of time, they are plotted in polar coordinates with the primary star at the origin, and the most probable ellipse is drawn through these points such that the Keplerian law of areas is satisfied. Coordinates are numbers which describe the location of points in a plane or in space In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a In Astronomy, Kepler's Laws of Planetary Motion are three mathematical laws that describe the motion of Planets in the Solar System. This ellipse is known as the apparent ellipse, and is the projection of the actual elliptical orbit of the secondary with respect to the primary on the plane of the sky. In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star From this projected ellipse the complete elements of the orbit may be computed, with the semi-major axis being expressed in angular units unless the stellar parallax, and hence the distance, of the system is known. In Geometry, the semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between ^[3]

Spectroscopic binaries

A spectroscopic binary star is a binary star in which the separation between the stars is usually very small, and the orbital velocity very high. In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star Unless the plane of the orbit happens to be perpendicular to the line of sight, the orbital velocities will have components in the line of sight and the observed radial velocity of the system will vary periodically. Radial velocity is the Velocity of an object in the direction of the line of sight (i Since radial velocity can be measured with a spectrometer by observing the Doppler shift of the stars' spectral lines, the binaries detected in this manner are known as spectroscopic binaries. A spectrometer is an Optical instrument used to measure properties of Light over a specific portion of the Electromagnetic spectrum, typically used The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared Most of these cannot be resolved as a visual binary, even with telescopes of the highest existing resolving power. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. Resolving power may refer to Angular resolution Spectral resolution

In some spectroscopic binaries, spectral lines from both stars are visible and the lines are alternately double and single. Such a system is known as a double-lined spectroscopic binary (often denoted "SB2"). In other systems, the spectrum of only one of the stars is seen and the lines in the spectrum shift periodically towards the blue, then towards red and back again. Such stars are known as single-lined spectroscopic binaries ("SB1").

The orbit of a spectroscopic binary is determined by making a long series of observations of the radial velocity of one or both components of the system. In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star The observations are plotted against time, and from the resulting curve a period is determined. If the orbit is circular then the curve will be a sine curve. Circles are simple Shapes of Euclidean geometry consisting of those points in a plane which are at a constant Distance, called the If the orbit is elliptical, the shape of the curve will depend on the eccentricity of the ellipse and the orientation of the major axis with reference to the line of sight. In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape

It is impossible to determine individually the semi-major axis a and the inclination of the orbit plane i. In Geometry, the semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae However, the product of the semi-major axis and the sine of the inclination (i. e. a sin i) may be determined directly in linear units (e. g. kilometres). If either a or i can be determined by other means, as in the case of eclipsing binaries, a complete solution for the orbit can be found. ^[11]

Eclipsing binaries

An eclipsing binary, with an indication of the variation in intensity. ^[12][13]

An eclipsing binary star is a binary star in which the orbit plane of the two stars lies so nearly in the line of sight of the observer that the components undergo mutual eclipses. In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth An eclipse is an astronomical event that occurs when one Celestial object moves into the shadow of another In the case where the binary is also a spectroscopic binary and the parallax of the system is known, the binary is quite valuable for stellar analysis. Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between ^[14]

In the last decade, measurement of eclipsing binaries' fundamental parameters has become possible with 8 meter class telescopes. This makes it feasible to use them as standard candles. A standard candle is an astronomical object that has a known Luminosity. Recently, they have been used to give direct distance estimates to the LMC, SMC, Andromeda Galaxy and Triangulum Galaxy. The Large Magellanic Cloud (LMC is a nearby Satellite galaxy of our own galaxy the Milky Way. The Small Magellanic Cloud (SMC is a Dwarf galaxy. It contains several hundred million stars The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a Spiral galaxy approximately 3 million light-years away in the Eclipsing binaries offer a direct method to gauge the distance to galaxies to a new improved 5% level of accuracy. ^[15]

Eclipsing binaries are variable stars, not because the light of the individual components vary but because of the eclipses. For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson The light curve of an eclipsing binary is characterized by periods of practically constant light, with periodic drops in intensity. In Astronomy, a light curve is a graph of light intensity of a Celestial object or region as a function of time If one of the stars is larger than the other, one will be obscured by a total eclipse while the other will be obscured by an annular eclipse.

The period of the orbit of an eclipsing binary may be determined from a study of the light curve, and the relative sizes of the individual stars can be determined in terms of the radius of the orbit by observing how quickly the brightness changes as the disc of the near star slides over the disc of the distant star. In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star If it is also a spectroscopic binary the orbital elements can also be determined, and the mass of the stars can be determined relatively easily, which means that the relative densities of the stars can be determined in this case. The elements of an orbit are the parameters needed to specify that Orbit uniquely given a model of two point-masses obeying the Newtonian laws of motion and the ^[16]

Astrometric binaries

An astrometric binary star is a binary star for which only one of the component stars can be visually observed. The visible star's position is carefully measured and detected to have a wobble, due to the gravitational influence from its counterpart. The position of the star is repeatedly measured relative to more distant stars, and then checked for periodic shifts in position. Typically this type of measurement can only be performed on nearby stars, such as those within 10 parsecs. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 Nearby stars often have a relatively high proper motion, so astrometric binaries will appear to follow a sinusoidal path across the sky. The proper motion of a Star is the measurement of its change in position in the sky over time after Improper motions are accounted for

If the companion is sufficiently massive to cause an observable shift in position of the star, then its presence can be deduced. From precise astrometric measurements of the movement of the visible star over a sufficiently long period of time, information about the mass of the companion and its orbital period can be determined. Astrometry is the branch of Astronomy that relates to precise measurements and explanations of the positions and movements of Stars and other celestial bodies ^[17] Even though the companion is not visible, the characteristics of the system can be determined from the observations using Kepler's laws. Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer In Astronomy, Kepler's Laws of Planetary Motion are three mathematical laws that describe the motion of Planets in the Solar System. ^[18]

This method of detecting binaries is also used to locate extrasolar planets orbiting a star. Any Planet is an extremely faint light source compared to its parent Star. An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 However, the requirements to perform this measurement are very exacting, due to the great difference in the mass ratio, and the typically long period of the planet's orbit. Detection of position shifts of a star is a very exacting science, and it is difficult to achieve the necessary precision. Space telescopes can avoid the bluring effect of the Earth's atmosphere, resulting in more precise resolution. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five

By configuration of the system

Artist's conception of a cataclysmic variable system. Cataclysmic variable stars ( CV) are stars which irregularly increase in brightness by a large factor then drop back down to a quiescent state

Another classification is based on the distance of the stars, relative to their sizes:^[19]

Detached binaries are a kind of binary stars where each component is within its Roche lobe, i. 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 e. the area where the gravitational pull of the star itself is larger than that of the other component. Gravitation is a natural Phenomenon by which objects with Mass attract one another The stars have no major effect on each other, and essentially evolve separately. Most binaries belong to this class.

Semidetached binary stars are binary stars where one of the components fills the binary star's Roche lobe and the other does not. Gas from the surface of the Roche lobe filling component (donor) is transferred to the other, accreting star. The mass transfer dominates the evolution of the system. Mass transfer is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of Atoms and Molecules In many cases, the inflowing gas forms an accretion disc around the accretor. An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body Examples of this type are X-ray binaries and Cataclysmic variable stars. X-ray binaries are a class of Binary stars that are luminous in X-rays The X-rays are produced by matter falling from one component (usually a relatively normal Cataclysmic variable stars ( CV) are stars which irregularly increase in brightness by a large factor then drop back down to a quiescent state

A contact binary is a type of binary star in which both components of the binary fill their Roche lobes. The uppermost part of the stellar atmospheres forms a common envelope that surrounds both stars. The stellar atmosphere is the outer region of the volume of a Star, lying above the Stellar core, Radiation zone and Convection zone. As the friction of the envelope brakes the orbital motion, the stars may eventually merge. In Physics, orbital motion is the either a motion of a planet in a Planetary orbit, or a motion of an Electron around the nucleus of an ^[20]

Binary star evolution

Formation

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. Gravitation is a natural Phenomenon by which objects with Mass attract one another Also, the observation of binaries consisting of pre main sequence stars, supports the theory that binaries are already formed during star formation. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star. Fragmentation of the molecular cloud during the formation of protostars is an acceptable explanation for the formation of a binary or multiple star system. A protostar is a large object that forms by contraction out of the gas of a Giant molecular cloud in the Interstellar medium. ^[21][22]

The outcome of the three body problem, where the three stars are of comparable mass, is that eventually one of the three stars will be ejected from the system and, assuming no significant further perturbations, the remaining two will form a stable binary system. The n -body problem is the problem of finding given the initial positions masses and velocities of n bodies their subsequent motions as determined by

Mass transfer and accretion

As a main sequence star increases in size during its evolution, it may at some point exceed its Roche lobe, meaning that some of its matter ventures into a region where the gravitational pull of its companion star is larger than its own. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime 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 Gravitation is a natural Phenomenon by which objects with Mass attract one another ^[23] The result is that matter will transfer from one star to another through a process known as Roche Lobe overflow (RLOF), either being absorbed by direct impact or through an accretion disc. An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body The mathematical point through which this transfer happens is called the first Lagrangian point^[24]. Mathematics is the body of Knowledge and Academic discipline that studies such concepts as Quantity, Structure, Space and It is not uncommon that the accretion disc is the brightest (and thus sometimes the only visible) element of a binary star.

An animation of an eclipsing binary system undergoing mass transfer.

If a star grows outside of its Roche lobe too fast for all abundant matter to be transferred to the other component, it is also possible that matter will leave the system through other Lagrange points or as stellar wind, thus being effectively lost to both components. Matter is commonly defined as being anything that has mass and that takes up space. A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star. ^[25] Since the evolution of a star is determined by its mass, the process influences the evolution of both companions, and creates stages that can not be attained by single stars. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth ^[26][27]

Studies of the eclipsing ternary Algol led to the Algol paradox in the theory of stellar evolution: although components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones, it was observed that the more massive component Algol A is still in the main sequence, while the less massive Algol B is a subgiant star at a later evolutionary stage. Algol (β Per / Beta Persei known colloquially as the Demon Star, is a bright Star in the Constellation Perseus. In Stellar astronomy, the Algol paradox is an apparently Paradoxical situation when elements of a Binary star seem to evolve in discord with the established Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness Subgiant star is a class of Stars that are slightly brighter than normal Main sequence (dwarf stars of the same spectral class but not as bright as The paradox can be solved by mass transfer: when the more massive star became a subgiant, it filled its Roche lobe, and most of the mass was transferred to the other star, which is still in the main sequence. Mass transfer is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of Atoms and Molecules 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 In some binaries similar to Algol, a gas flow can actually be seen. ^[28]

Runaways and novae

A Chandra X-ray Observatory image of the remnants of the SN 1572 supernova. The Chandra X-ray Observatory is a Satellite launched on STS-93 by NASA on July 23, 1999. SN 1572 ( Tycho's Supernova, Tycho's Nova) "B Cassiopeiae" (B Cas or 3C 10 was a Supernova of Type Ia in the

It is also possible for widely separated binaries to lose gravitational contact with each other during their lifetime, as a result of external perturbations. The components will then move on to evolve as single stars. A close encounter between two binary systems can also result in the gravitational disruption of both systems, with some of the stars being ejected at high velocities, leading to runaway stars. A runaway star is one which is moving through space with an abnormally high Velocity compared to other stars around it ^[29][30]

If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gases from the star's outer atmosphere. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. 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 These are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of degenerate matter, and so is largely unresponsive to heat, while the accreted hydrogen is not. Degenerate matter is matter which has sufficiently high Density that the dominant contribution to its Pressure rises from the Pauli Exclusion Hydrogen fusion can occur in a stable manner on the surface through the CNO cycle, causing the enormous amount of energy liberated by this process to blow the remaining gases away from the white dwarf's surface. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus The CNO cycle (for Carbon - Nitrogen - Oxygen) or sometimes Bethe-Weizsäcker-cycle, is one of two sets of fusion reactions The result is an extremely bright outburst of light, known as a nova. A nova (pl novae or novas) is a Cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a White ^[31]

In extreme cases this event can cause the white dwarf to exceed the Chandrasekhar limit and trigger a supernova that destroys the entire star, and is another possible cause for runaways. The Chandrasekhar limit limits the mass of bodies made from Electron-degenerate matter, a dense form of matter which consists of nuclei immersed in a gas of Electrons A supernova (plural supernovae or supernovas) is a stellar Explosion. ^[32][33] A famous example of such an event is the supernova SN 1572, which was observed by Tycho Brahe. SN 1572 ( Tycho's Supernova, Tycho's Nova) "B Cassiopeiae" (B Cas or 3C 10 was a Supernova of Type Ia in the Tycho Brahe, born Tyge Ottesen Brahe ( December 14 1546 &ndash October 24 1601) was a Danish nobleman The Hubble Space Telescope recently took a picture of the remnants of this event. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into

Use in astrophysics

A simulated example of a binary star, where two bodies with similar mass orbit around a common barycenter in elliptic orbits. In Astrodynamics or Celestial mechanics an elliptic orbit is a Kepler orbit with the eccentricity greater than 0 and less than 1

Binaries provide the best method for astronomers to determine the mass of a distant star. Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena The gravitational pull between them causes them to orbit around their common center of mass. From the orbital pattern of a visual binary, or the time variation of the spectrum of a spectroscopic binary, the mass of its stars can be determined. In this way, the relation between a star's appearance (temperature and radius) and its mass can be found, which allows for the determination of the mass of non-binaries. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Remote Authentication Dial In User Service ( RADIUS) is a networking protocol that provides centralized access authorization and accounting management for people or computers

Because a large proportion of stars exist in binary systems, binaries are particularly important to our understanding of the processes by which stars form. In particular, the period and masses of the binary tell us about the amount of angular momentum in the system. In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position Because this is a conserved quantity in physics, binaries give us important clues about the conditions under which the stars were formed. In Physics, a conservation law states that a particular measurable property of an isolated Physical system does not change as the system evolves Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion.

In a binary system, the more massive star is usually designated 'A', and its companion 'B'. Thus the bright main sequence star of the Sirius system is Sirius A, while the smaller white dwarf member is Sirius B. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness Sirius is the brightest star in the night sky with a visual Apparent magnitude of &minus1 A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. However, if the pair is very widely separated, they may be designated with superscripts as with Zeta Reticuli (ζ¹ Ret and ζ² Ret). Zeta Reticuli (ζ Ret / ζ Reticuli also called Bernard's Star is a binary Star system located about 39 Light years away from Earth. ^[34]

Research findings

It is believed that up to seventy-five percent of all stars are in binary systems, with as many as 10% of these systems containing more than two stars (triples, quadruples, etc. ). ^[35]

There is a direct correlation between the period of revolution of a binary star and the eccentricity of its orbit, with systems of short period having smaller eccentricity. The orbital period is the time taken for a given object to make one complete Orbit about another object In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape Binary stars may be found with any conceivable separation, from pairs orbiting so closely that they are practically in contact with each other, to pairs so distantly separated that their connection is indicated only by their common proper motion through space. The proper motion of a Star is the measurement of its change in position in the sky over time after Improper motions are accounted for Among gravitationally bound binary star systems, there exists a so called log normal distribution of periods, with the majority of these systems orbiting with a period of about 100 years. In Probability and Statistics, the log-normal distribution is the single-tailed Probability distribution of any Random variable whose This is supporting evidence for the theory that binary systems are formed during star formation. Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star. ^[36]

In pairs where the two stars are of equal brightness, they are also of the same spectral type. In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics In systems where the brightnesses are different, the fainter star is bluer if the brighter star is a giant star, and redder if the brighter star belongs to the main sequence. A giant star is a Star with substantially larger Radius and Luminosity than a Main sequence star of the same surface temperature. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness ^[37]

Artist's impression of the sight from a (hypothetical) moon of planet HD 188753 Ab (upper left), which orbits a triple star system. A star system or stellar system is a small number of Stars which orbit each other bound by gravitational attraction. The brightest companion is just below the horizon. The horizon ( Ancient Greek ὁ ὁρίζων, /ho horídzôn/ from ὁρίζειν, "to limit" is the apparent line that separates

Since mass can be determined only from gravitational attraction, and the only stars (with the exception of the Sun, and gravitationally-lensed stars), for which this can be determined are binary stars, these are a uniquely important class of stars. The Sun (Sol is the Star at the center of the Solar System. A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of In the case of a visual binary star, after the orbit and the stellar parallax of the system has been determined, the combined mass of the two stars may be obtained by a direct application of the Keplerian harmonic law. Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between In Astronomy, Kepler's Laws of Planetary Motion are three mathematical laws that describe the motion of Planets in the Solar System. ^[38]

Unfortunately, it is impossible to obtain the complete orbit of a spectroscopic binary unless it is also a visual or an eclipsing binary, so from these objects only a determination of the joint product of mass and the sine of the angle of inclination relative to the line of sight is possible. In the case of eclipsing binaries which are also spectroscopic binaries, it is possible to find a complete solution for the specifications (mass, density, size, luminosity, and approximate shape) of both members of the system. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Luminosity has different meanings in several different fields of science

Planets around binary stars

Science fiction has often featured planets of binary or ternary stars as a setting. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is In reality, some orbital ranges are impossible for dynamical reasons (the planet would be expelled from its orbit relatively quickly, being either ejected from the system altogether or transferred to a more inner or outer orbital range), whilst other orbits present serious challenges for eventual biospheres because of likely extreme variations in surface temperature during different parts of the orbit. The biosphere is the broadest level of ecological study the global sum of all Ecosystems. Planets that orbit just one star in a binary pair are said to have "S-type" orbits, whereas those that orbit around both stars have "P-type" or "circumbinary" orbits. It is estimated that 50–60% of binary stars are capable of supporting habitable terrestrial planets within stable orbital ranges. ^[39]

Simulations have shown that the presence of a binary companion can actually improve the rate of planet formation within stable orbital zones by "stirring up" the protoplanetary disk, increasing the accretion rate of the protoplanets within. ^[39]

Detecting planets around multiple star systems introduces additional technical difficulties, which may be why they are only rarely found. Examples include PSR B1620-26c and HD 188753 Ab, the latter being the only known planet in a ternary system as of 2006. ^[40]

Multiple star examples

The two visibly distinguishable components of Albireo. Albireo (β Cyg / β Cygni / Beta Cyg / Beta Cygni is the fifth brightest star in the Constellation Cygnus.

The large distance between the components, as well as their difference in color, make Albireo one of the easiest observable visual binaries. Albireo (β Cyg / β Cygni / Beta Cyg / Beta Cygni is the fifth brightest star in the Constellation Cygnus. The brightest member, which is the third brightest star in the constellation Cygnus, is actually a close binary itself. In common usage a constellation is a group of celestial bodies that are connected together in some arrangement typically stars to form a visible figure or picture Cygnus (ˈsɪgnəs Swan, Κύκνος) is a northern Constellation. Also in the Cygnus constellation is Cygnus X-1, an X-ray source considered to be a black hole. Cygnus X-1 (abbreviated Cyg X-1) in the Constellation Cygnus. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e It is a high-mass X-ray binary, with the optical counterpart being a variable star. For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson ^[41] Another famous binary is Sirius, the brightest star in the night time sky, with a visual apparent magnitude of −1. Sirius is the brightest star in the night sky with a visual Apparent magnitude of &minus1 The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value 46. It is located in the constellation Canis Major. Canis Major ( greater Dog) is one of the 88 modern Constellations and was also in Ptolemy 's list of 48 constellations In 1844 Friedrich Bessel deduced that Sirius was a binary. Friedrich Wilhelm Bessel (22 July 1784 &ndash 17 March 1846 was a German Mathematician, Astronomer, and systematizer of the Bessel functions In 1862 Alvan Graham Clark discovered the companion (Sirius B; the visible star is Sirius A). Alvan Graham Clark ( July 10, 1832 &ndash June 9, 1897) born in Fall River, Massachusetts, was an American In 1915 astronomers at the Mount Wilson Observatory determined that Sirius B was a white dwarf, the first to be discovered. The Mount Wilson Observatory (MWO is an Astronomical observatory in Los Angeles County California. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. In 2005, using the Hubble Space Telescope, astronomers determined Sirius B to be 12,000 km in diameter, with a mass that is 98% of the Sun. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The Sun (Sol is the Star at the center of the Solar System. ^[42]

An example of an eclipsing binary is Epsilon Aurigae in the constellation Auriga. Epsilon Aurigae (ε Aur / ε Aurigae is a star in the Constellation Auriga. Auriga (ɔːˈraɪgə Charioteer) is a northern Constellation. The visible component belongs to the spectral class F0, the other (eclipsing) component is not visible. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics The next such eclipse occurs from 2009–2011, and it is hoped that the extensive observations that will likely be carried out may yield further insights into the nature of this system. Another eclipsing binary is Beta Lyrae, which is a contact binary star system in the constellation of Lyra. Beta Lyrae (β Lyr / β Lyrae is a Binary star system approximately 882 Light-years away in the Constellation Lyra. Lyra (ˈlaɪrə Lyre) is one of the 48 Constellations listed by Ptolemy, and is one of the 88 modern constellations recognized by the International Its two component stars are close enough that material from the photosphere of each is pulled towards the other, drawing the stars into an ellipsoid shape. Beta Lyrae is the prototype for this class of eclipsing binaries, whose components are so close together that they deform by their mutual gravitation. ^[43]

Other interesting binaries include 61 Cygni (a binary in the constellation Cygnus, composed of two K class (orange) main sequence stars, 61 Cygni A and 61 Cygni B, which is known for its large proper motion), Procyon (the brightest star in the constellation Canis Minor and the eighth brightest star in the night time sky, which is a binary consisting of the main star with a faint white dwarf companion), SS Lacertae (an eclipsing binary which stopped eclipsing), V907 Sco (an eclipsing binary which stopped, restarted, then stopped again) and BG Geminorum (an eclipsing binary which is thought to contain a black hole with a K0 star in orbit around it). Not be confused with 16 Cygni, a more distant system containing two G-type stars harboring the Gas giant planet 16 Cygni Bb. Cygnus (ˈsɪgnəs Swan, Κύκνος) is a northern Constellation. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness The proper motion of a Star is the measurement of its change in position in the sky over time after Improper motions are accounted for This article is about the star Procyon is also the mammalian genus to which raccoons belong Canis Minor (ˌkeɪnɨs ˈmaɪnɚ smaller Dog) is one of the 88 modern Constellations and was also in Ptolemy 's list of 48 constellations A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. Page under construction BG Geminorum is long period Binary star system which contains a K0 I secondary star in orbit around a primary star

Algol is the most famous ternary (long thought to be a binary), located in the constellation Perseus. Algol (β Per / Beta Persei known colloquially as the Demon Star, is a bright Star in the Constellation Perseus. For other meanings see Perseus (disambiguation Perseus (ˈpɝsiːəs or /-sjuːs/ is a northern Constellation, named after the Greek Two components of the system eclipse each other, the variation in the intensity of Algol first being recorded in 1670 by Geminiano Montanari. Geminiano Montanari ( June 1 1633 - October 13 1687) was an Italian astronomer, lens -maker and proponent of the The name Algol means "demon star" (from Arabic الغول al-ghūl), which was probably given due to its peculiar behavior. Arabic (ar الْعَرَبيّة (informally ar عَرَبيْ) in terms of the number of speakers is the largest living member of the Semitic language A ghoul is a Monster from ancient Arabian folklore that dwells in burial grounds and other uninhabited places Another visible ternary is Alpha Centauri, in the southern constellation of Centaurus, which contains the fourth brightest star in the night sky, with an apparent visual magnitude of −0. Alpha Centauri (α Centauri / α Cen also known as Rigil Kentaurus, Rigil Kent, or Toliman, is the brightest Star in the southern Constellation Centaurus ( Centaur) is a bright constellation of the southern hemisphere. Bright stars are bright because they have high luminosities and/or they are nearby The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value 01. This system also underscores the fact that binaries need not be discounted in the search for habitable planets. Centauri A and B have an 11 AU distance at closest approach, and both should have stable habitable zones. ^[44]

There are also examples of systems beyond ternaries: Castor is a sextuple star system, which is the second brightest star in the constellation Gemini and one of the brightest stars in the nighttime sky. Castor (α Gem / α Geminorum / Alpha Geminorum is the second brightest star in the Constellation Gemini and one of the brightest stars in the Gemini (ˈgɛmɪnaɪ Twins, symbol, Unicode ♊ is one of the Constellations of the Zodiac known as "the twins" Astronomically, Castor was discovered to be a visual binary in 1719. Each of the components of Castor is itself a spectroscopic binary. Castor also has a faint and widely separated companion, which is also a spectroscopic binary.

See also

• Binary stars in fiction

References

External links

• List of the best visual binaries for amateurs, with orbital elements
• Pictures of binaries at Hubblesite.org
• Chandra X-ray Observatory
• Binary Stars at Open Directory
• An extensive simulation for the Algol system by North Carolina State University
• Selected visual double stars and their relative position as a function of time