Binary and multiple- stars are very important in astrophysics because calculations of their orbits allow the masses of their component stars to be directly determined, which in turn allows other stellar parameters, such as radius and density, to be indirectly estimated. This also determines an empirical mass-luminosity relationship (MLR) from which the masses of single stars can be estimated..
.In a physical triple star system, each star orbits the center of mass of the system. Usually, two of the stars form a close binary system, and the third orbits this pair at a distance much larger than that of the binary orbit. This arrangement is called hierarchical.The reason for this is that if the inner and outer orbits are comparable in size, the system may become dynamically unstable, leading to a star being ejected from the system. Triple stars that are not all gravitationally bound might comprise a physical binary and an optical companion, such as Beta Cephei, or rarely, a purely optical triple star, such as Gamma Serpentis..
Most multiple-star systems are organized in what is called a hierarchical system: the stars in the system can be divided into two smaller groups, each of which traverses a larger orbit around the system's center of mass. Each of these smaller groups must also be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, and so on.Each level of the hierarchy can be treated as a Two-body problem by considering close pairs as if they were a single star. In these systems there is little interaction between the orbits and the stars' motion will continue to approximate stable. Keplerian orbits around the system's center of mass, unlike the unstable Trapezia systems or the even more complex dynamics of the large number of stars in star clusters and galaxies.
Hierarchical multiple star systems with more than three stars can produce a number of more complicated arrangements, which can be illustrated as a mobile diagram (Evans 1968). These are similar to ornamental mobiles hung from the ceiling. .
Each level of the diagram illustrates the decomposition of the system into two or more systems with smaller size. Evans calls a diagram multiplex if there is a node with more than two children, i.e., if the decomposition of some subsystem involves two or more orbits with comparable size. Since, for triple stars, this may be unstable, multiple stars are expected to be simplex, meaning that at each level there are exactly two children. Evans calls the number of levels in the diagram its hierarchy. A simplex diagram of hierarchy 1, as in (b), describes a binary system. A simplex diagram of hierarchy 2 may describe a triple system, as in (c), or a quadruple system, as in (d). A simplex diagram of hierarchy 3 may describe a system with anywhere from four to eight components. The mobile diagram in (e) shows an example of a quadruple system with hierarchy 3, consisting of a single distant component orbiting a close binary system, with one of the components of the close binary being an even closer binary. A real example of a system with hierarchy 3 is Castor, also known as Castor (Alpha Geminorum) or α Gem.
It consists of what appears to be a visual binary star which, upon closer inspection, can be seen to consist of two spectroscopic binary stars. By itself, this would be a quadruple hierarchy 2 system as in (d), but it is orbited by a fainter more distant component, which is also a close red dwarf binary. This forms a sextuple system of hierarchy 3. The maximum hierarchy occurring in A. A. Tokovinin's Multiple Star Catalogue, as of 1999, is 4. For example, the stars Gliese 644A and Gliese 644B form what appears to be a close visual binary star; since Gliese 644B is a spectroscopic binary, this is actually a triple system. The triple system has the more distant visual companion Gliese 643 and the still more distant visual companion Gliese 644C, which, because of their common motion with Gliese 644AB, are thought to be gravitationally bound to the triple system. This forms a quintuple system whose mobile diagram would be the diagram of level 4 appearing in (f).Higher hierarchies are also possible. One of examples is Beta Tucanae (β Tuc, β Tucanae),it is a group of six stars which appear to be at least loosely bound into a system in the constellation Tucana.Another , Kappa Tucanae (κ Tuc, κ Tucanae) is a star system in the constellation Tucana.
Polar Star is a multiple star, consisting of the main star α UMi Aa, two smaller companions, α UMi B and α UMi Ab, and two distant components α UMi C and α UMi D. α UMi B
A ternary system – containing three stars – may have all three stars orbiting a shared point however this seems to be uncommon as such set ups would not remain gravitationally stable for long.
Trinary Star System
the announcement of a planet in the Alpha Centauri trinary system
An artist's impression of the Gliese 570 System
Gliese 570,wich is also known as 33 G. Librae, is a ternary star system approximately 19 light-years away. The primary star is an orange dwarf star (much dimmer and smaller than the Sun). The other secondary stars are themselves a binary system, two red dwarfs that orbit one another. A brown dwarf has been confirmed to be orbiting in the system. In 1998, an extrasolar planet was thought to orbit the primary star, but it was discounted in 2000.The primary star of the system (component A) is an orange dwarf star that may just have over three fourths the mass of the Sun, about 77 percent of its radius, and only 15.6 percent of its visual luminosity. It has a separation of 190 astronomical units from the binary components B and C, moving in an eccentric orbit that takes at least 2130 years to complete. Gliese 570 A is spectral type K4V and emits X-rays.A binary system in their own right, components B and C are both rather dim red dwarf stars that have less mass, radius, and luminosity than the Sun. Component B is spectral type M1V, component C is spectral type M3V, and both emit X-rays.. Catalogued as Gliese 570 D (or rarely Gliese 570 d), it was observed at a wide separation of more than 1,500 astronomical unit from the triple star system.It has an estimated mass of 50 times that of Jupiter.The star, BD +20 2457, is a K2 giant — an old bloated star nearing the end of its life. Seeing a pair of brown dwarfs around a K-type giant is a first for astronomers and offers a unique window into how they can be produced.Brown dwarfs are dim, elusive objects that straddle the dividing line between planets and stars. They are too massive to be planets, but not massive enough to generate the fusion-powered energy of a star. These stellar cousins represent a kind of “missing link” between planets and stars, but little is known about how they are made.
How can two stars create such a strange and intricate structure? Most stars are members of multiple-star systems. Some stars are members of close binary systems where material from one star swirls around the other in an accretion disk. Only a handful of stars, however, are members of an intermediate polar, a system featuring a white dwarf star with a magnetic field that significantly pushes out the inner accretion disk, only allowing material to fall down its magnetic poles. . The foreground white dwarf is so close to the normal star that it strips away its outer atmosphere. As the white dwarf spins, the columns of infalling gas rotate with it. The name intermediate polar derives from observations of emitted light polarized at a level intermediate to non-disk binary systems known as polars. Intermediate polars are a type of cataclysmic variable star system.
a DQ Hercules system
What is a double or binary star ?
A double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e. a binary system of stars in mutual orbit, gravitationally bound to each other, or because it is an optical double, a chance alignment of two stars in the sky that lie at different distances . A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes , or secondary.. The term double star may be used synonymously with binary star, but more generally, a double star may be either a binary star or an optical double star which consists of two stars with no physical connection but which appear close together in the sky as seen from the Earth. A double star may be determined to be optical if its components have sufficiently different proper motions or radial velocities, or if parallax measurements reveal its two components to be at sufficiently different distances from the Earth. Most known double stars have not yet been determined to be either bound binary star systems or optical doubles.Many double-star systems throughout the galaxy are wide binaries, in which 1,000 AU or more separate the stellar companions on average. Since the beginning of the 1780s, both professional and amateur double star observers have telescopically measured the distances and angles between double stars to determine the relative motions of the pairs. If the relative motion of a pair determines a curved arc of an orbit, or if the relative motion is small compared to the common proper motion of both stars, it may be concluded that the pair is in mutual orbit as a binary star. Otherwise, the pair is optical.Multiple stars are also studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars.
the bright double star Albireo
At 380 light years distant, the two bright stars of Albireo are comparatively far from each other and take about 75,000 years to complete a single orbit. The brighter yellow star is itself a binary star system, but too close together to be resolved even with a telescope. Albireo, pictured above, is the fifth brightest star system toward the constellation of the Swan (Cygnus) and easily visible to the unaided eye.
There are three types of paired stars:
optical doubles — unrelated stars which appear close together through chance alignment with Earth.
Optical doubles are not real binary stars because they do not orbit each other. These are just pair of stars that appear exceptionally close together in the sky even though they have different distances on earth. They are also an illusion made by a possible alignment of stars. They are considered rarer than true binaries..
non-visual binaries — stars whose binary status was deduced through more esoteric means such as occultation (eclipsing binaries), spectroscopy (spectroscopic binaries), or anomalies in proper motion (astrometric binaries)
Binary stars are classified into four types according to the way in which they are observed: visually, by observation; spectroscopically, by periodic changes in spectral lines; photometrically, by changes in brightness caused by an eclipse; or astrometrically, by measuring a deviation in a star's position caused by an unseen companion. Any binary star can belong to several of these classes; for example, several spectroscopic binaries are also eclipsing binaries.A binary star whose dual nature is only apparent through measurements made with a spectroscope. All close binaries and most eclipsing binaries fall into this category.
Due to the large masses of the system, they provide an ideal opportunity to test aspects of G R: The double pulsar system, PSR J0737-3039A and B, is 2000 light-years away in the direction of the constellation Puppis. It consists of two massive, highly compact neutron stars. It is the only known system of two detectable radio pulsars orbiting each other. Objects of this kind enable precise testing of Albert Einstein's general relativity theory , because the precise and consistent timing of the pulsar pulses allows relativistic effects to be seen when they would otherwise be too small. In particular, the predictions for energy loss due to gravitational waves appear to match the theory. Gravitational waves are ripples in the curvature of spacetime that propagate as a wave, travelling outward from the source. The gravitational field created by the pulsar is so strong that the scientists suspected they might notice deviations from the predictions of general relativity in the motions of the white dwarf around it. General relativity posits that massive objects warp the space and time around them, causing other objects, and even light, to travel along curved paths when they pass nearby. General relativity also predicts that a close binary system such as this one will radiate gravitational energy in the form of ripples in space-time called gravitational waves. This loss of energy would cause the orbital period of the system to change slightly over time. Alternative theories of gravity offer slightly different predictions for the white dwarf's motions. The effects of a passing gravitational wave can be visualized by imagining a perfectly flat region of spacetime with a group of motionless test particles lying in a plane (the surface of your screen) , the particles will follow the distortion in spacetime, as shown in the animations.
"Exotic" Types of binary star systems.
Gravitational waves spread out from a binary star system (two stars orbiting each other).
. The area enclosed by the test particles does not change and there is no motion along the direction of propagation. The investigation of double pulsars is a great opportunity as the environment created by warped space-time due to the shift of intense masses is extremely rare, and thus perfect for the testing of Einstein's theory and the observation of theoretical gravitational waves . Several Earth-based tests are underway to look for perturbations in space-time distances caused by passing gravitational waves. And one can consider that current efforts to detect gravitational waves, based on Einstein's predictions, are on the right track.
Due to the large masses of the system, they provide an ideal opportunity to test aspects of G R:.
The double pulsar system, PSR J0737-3039A and B, is 2000 light-years away in the direction of the constellation Puppis. It consists of two massive, highly compact neutron stars. It is the only known system of two detectable radio pulsars orbiting each other.
Objects of this kind enable precise testing of Albert Einstein's general relativity theory , because the precise and consistent timing of the pulsar pulses allows relativistic effects to be seen when they would otherwise be too small. In particular, the predictions for energy loss due to gravitational waves appear to match the theory. Gravitational waves are ripples in the curvature of spacetime that propagate as a wave, travelling outward from the source. The gravitational field created by the pulsar is so strong that the scientists suspected they might notice deviations from the predictions of general relativity in the motions of the white dwarf around it. General relativity posits that massive objects warp the space and time around them, causing other objects, and even light, to travel along curved paths when they pass nearby. General relativity also predicts that a close binary system such as this one will radiate gravitational energy in the form of ripples in space-time called gravitational waves. This loss of energy would cause the orbital period of the system to change slightly over time. Alternative theories of gravity offer slightly different predictions for the white dwarf's motions. The effects of a passing gravitational wave can be visualized by imagining a perfectly flat region of spacetime with a group of motionless test particles lying in a plane (the surface of your screen) , the particles will follow the distortion in spacetime, as shown in the animations.
e two provide physicists with an unprecedented natural, cosmic "laboratory" for studying the nature of gravity.
Artist's illustration of the PSR J0348+0432
Artist's illustration of the PSR J0348+0432
In many cases a binary system is too far away, or the stars are too close, or one star is so much brighter than the other that we cannot distinguish the two stars visually. In that case we may still infer that the system is binary by several indirect methods. One such method is to detect the presence of an unseen companion by its gravitational influence on the primary star. A binary discovered in this way is termed an astrometric binary.
Motion around the Center of Mass
m1 r1 = m2 r2
where R is the total separation between the centers of the two objects.
Modification of Kepler's Third LawThis requires that Kepler's 3rd Law be modified to read,
( m1 + m2 ) P2 = ( r1 + r2 ) 3 = R3(The equation in this form is only valid if the orbits are approximately circular; for more elliptical orbits a somewhat more complicated equation must be used.) Because the two stars revolve around the common center of mass, even if we cannot see one of them we may still infer its presence by observing the motion of the star that we can see around a center of mass between the two stars. .
The combination of the motion around the center of mass and the proper motion on the celestial sphere gives rise to a wobbling motion on the celestial sphere, as illustrated in the following diagram for the Sirius system.
A Binary Star System with a Massive Primary and Low Mass Secondary :
A binary system can have several configurations – The two can orbit a common centre of gravity (termed a barycentre) which lies between the two, or the lower mass star can orbit the higher mass star just as Earth orbits the sun
UMa star or contact binary
A semi-detached binary is a pair with a more involved interaction. One star fills it Roche Lobe – This is the area of space within which the star’s gravity is stronger than that its partner – the other does not fill its Roche Lobe. This results in a process of matter transfer off the larger star to the smaller one. This matter forms a disk as it spirals round its new star. Eventually friction reduces its speed and it falls on to the surface of the star – it does not fall directly onto the star due to the conservation of angular momentum. A semi-detached binary may evolve form a detached binary after the more massive star swells as it leaves the main sequence by giant formation.
Contact binaries occur when both stars fill their Roche Lobes, this results in the outer atmospheres of both stars joining to form a shared ‘common envelope’. This envelope causes friction which slows the orbits of both stars. This decrease in orbital velocity can eventually (but not in every case) cause both stars to merge entirely forming a higher mass, single star.
The process of mass transfer from a Roche-lobe filling star to its companion can be likened to a rising water level in the plot below of Roche potential as a function of distance between the two stars - when the right-hand well is full, the water will spill into the left-hand well. This process finally provided an explanation for a long-standing mystery known as the algol paradox.