   #copyright

Binary star

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)


   This is a featured article. Click here for more information.
   Artist's impression of a binary system consisting of a black hole, with
   an accretion disc around it, and a main sequence star.
   Enlarge
   Artist's impression of a binary system consisting of a black hole, with
   an accretion disc around it, and a main sequence star.

   A binary star is a stellar system consisting of two stars orbiting
   around their centre of mass. 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. 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 by gravity.
   Binary stars can either be distinguished optically (visual binaries) or
   by indirect techniques, such as spectroscopy. If binaries happen to
   orbit in a plane containing our line of sight, they will eclipse each
   other; these are called eclipsing binaries.

   The components of binary star systems can exchange mass, bringing their
   evolution to stages that single stars cannot attain. Examples of
   binaries are Algol (an eclipsing binary), Sirius, and Cygnus X-1 (of
   which one member is probably a black hole).

Terminology

   Hubble image of the Sirius binary system, in which Sirius B can be
   clearly distinguished (lower left).
   Enlarge
   Hubble image of the Sirius binary system, in which Sirius B can be
   clearly distinguished (lower left).

   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". Any two closely-spaced stars might appear to be a double
   star, the most famous case being Mizar and Alcor in the Big Dipper. 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 our vantage point. Such false binaries are
   termed optical binaries. With the invention of the telescope, many such
   pairs were found. 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.

   A true binary is a pair of stars bound together by gravity. 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. In other cases, the only indication of binarity is the
   Doppler shift of the emitted light. 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 it moves first toward us, and
   then away from us, during its motion about their common centre of mass,
   with the period of their common orbit. 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.

   Binary stars that are both visual and spectroscopic binaries are rare,
   and are a precious source of valuable information when found. Visual
   binary stars have a large true separation, and consequently usually
   have orbital speeds too small to be measured spectroscopically from far
   away. 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. 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. 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. 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). In this way, Cygnus X-1 became the first
   object that was widely accepted as being a black hole.

Classifications

By methods of observation

   Binary stars are classified into four types according to their
   observable properties. 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. 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. 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. The position angle of the secondary
   with respect to the primary is measured, together with the angular
   distance between the two 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. 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. 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.

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. 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. 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.
   Most of these cannot be resolved as a visual binary, even with
   telescopes of the highest existing resolving power.

   In some spectroscopic binaries the spectra of both stars are visible
   and the lines are alternately double and single. Such stars are known
   as double-line binaries. In others, 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-line spectroscopic binaries.

   The orbit of a spectroscopic binary is determined by making a long
   series of observations of the radial velocity of one or more component
   of the binary. 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. 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.

   It is impossible to determine individually the semi-major axis a and
   the inclination of the orbit plane i. 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.

Eclipsing binaries

   An eclipsing binary, with an indication of the variation in intensity.
   An eclipsing binary, with an indication of the variation in intensity.

   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 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.

   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. Recently, they have
   been used to give direct distance estimates to the LMC, SMC, Andromeda
   Galaxy and Triangulum Galaxy. Eclipsing binaries offer a direct method
   to gauge the distance to galaxies to a new improved 5% level of
   accuracy.

   Eclipsing binaries are variable stars, not because the light of the
   individual components vary but because of the eclipses. The light curve
   of an eclipsing binary is characterized by periods of practically
   constant light, with periodic drops in intensity. 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. 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.

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.
   Nearby stars often have a relatively high proper motion, so astrometric
   binaries will appear to follow a sinusoidal path across the sky.

   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. Even though the
   companion is not visible, the characteristics of the system can be
   determined from the observations using Kepler's laws.

   This method of detecting binaries is also used to locate extrasolar
   planets orbiting a star. 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 blurring effect of the Earth's atmosphere, resulting in more
   precise resolution.

By configuration of the system

   Artist's conception of a cataclysmic variable system.
   Enlarge
   Artist's conception of a cataclysmic variable system.

   Another classification is based on the distance of the stars, relative
   to their sizes:

   Detached binaries are a kind of 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. 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 star (accretor). The mass transfer dominates the evolution of
   the system. In many cases, the inflowing gas forms an accretion disc
   around the accretor. Examples of this type are X-ray binaries and
   Cataclysmic variable stars.

   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. As the
   friction of the envelope brakes the orbital motion, the stars may
   eventually coalesce.

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. Also, the observation of binaries consisting of pre
   main sequence stars, supports the theory that binaries are already
   formed during star formation. Fragmentation of the molecular cloud
   during the formation of protostars is an acceptable explanation for the
   formation of a binary or multiple star system.

   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.

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 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. The mathematical point through which this transfer happens is
   called the first Lagrangian point. 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 the Beta Lyrae system, an eclipsing binary including an
   accretion disc.
   An animation of the Beta Lyrae system, an eclipsing binary including an
   accretion disc.

   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. 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.

   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. 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. In some binaries
   similar to Algol, a gas flow can actually be seen.

Runaways and novae

   A Chandra X-ray Observatory image of the remnants of the SN 1572
   supernova.
   Enlarge
   A Chandra X-ray Observatory image of the remnants of the SN 1572
   supernova.

   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.

   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. 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. 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. The result is an extremely bright outburst of
   light, known as a nova.

   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. A famous example of
   such an event is the supernova SN 1572, which was observed by Tycho
   Brahe. The Hubble Space Telescope recently took a picture of the
   remnants of this event.

Use in astrophysics

   A simulated example of a binary star, where two bodies with similar
   mass orbit around a common barycenter in elliptic orbits.
   Enlarge
   A simulated example of a binary star, where two bodies with similar
   mass orbit around a common barycenter in elliptic orbits.

   Binaries provide the best method for astronomers to determine the mass
   of a distant star. The gravitational pull between them causes them to
   orbit around their common centre 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.

   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. Because
   this is a conserved quantity in physics, binaries give us important
   clues about the conditions under which the stars were formed.

   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.
   However, if the pair is very widely separated, they may be designated
   with superscripts as with Zeta Reticuli (ζ^1 Ret and ζ^2 Ret).

Research findings

   It is believed that a quarter to half of all stars are in binary
   systems, with as many as 10% of these systems containing more than two
   stars (triples, quadruples, etc.).

   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. 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. 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.
   This is supporting evidence for the theory that binary systems are
   formed during star formation.

   In pairs where the two stars are of equal brightness, they are also of
   the same spectral type. 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.
   Artist's impression of the sight from a (hypothetical) moon of planet
   HD 188753 Ab (upper left), which orbits a triple star system. The
   brightest companion is just below the horizon.
   Enlarge
   Artist's impression of the sight from a (hypothetical) moon of planet
   HD 188753 Ab (upper left), which orbits a triple star system. The
   brightest companion is just below the horizon.

   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. In the
   case of a visual binary star, after the orbit has been determined and
   the stellar parallax of the system determined, the combined mass of the
   two stars may be obtained by a direct application of the Keplerian
   harmonic law.

   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.

   Science fiction has often featured planets of binary or ternary stars
   as a setting. 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. 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.

Multiple star examples

   The two visibly distinguishable components of Albireo.
   The two visibly distinguishable components of Albireo.

   The large distance between the components, as well as their difference
   in colour, make Albireo one of the easiest observable visual binaries.
   The brightest member, which is the third brightest star in the
   constellation Cygnus, is actually a close binary itself. Also in the
   Cygnus constellation is Cygnus X-1, an X-ray source considered to be a
   black hole. It is a high-mass X-ray binary, with the optical
   counterpart being a variable star. Another famous binary is Sirius, the
   brightest star in the night time sky, with a visual apparent magnitude
   of −1.46. It is located in the constellation Canis Major. In 1844
   Friedrich Bessel deduced that Sirius was a binary. In 1862 Alvan Graham
   Clark discovered the companion (Sirius B; the visible star is Sirius
   A). In 1915 astronomers at the Mount Wilson Observatory determined that
   Sirius B was a white dwarf, the first to be discovered. 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.

   An example of an eclipsing binary is Epsilon Aurigae in the
   constellation Auriga. The visible component belongs to the spectral
   class F0, the other (eclipsing) component is not visible. 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. 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.

   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).

   Algol is the most famous ternary (long thought to be a binary), located
   in the constellation Perseus. Two components of the system eclipse each
   other, the variation in the intensity of Algol first being recorded in
   1670 by Geminiano Montanari. The name Algol means "demon star" (from
   Arabic الغول al-ghūl), which was probably given due to its peculiar
   behaviour. 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 −0.01. 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.

   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. 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.

Fictional usage

   Many writers in the science fiction genre have explored the
   possibilities of binary star systems. One of the more famous examples
   is the planet Tatooine in the Star Wars movies, which orbits the two
   stars Tatoo 1 and Tatoo 2. Other instances include the anime series
   Simoun and the 1994 computer game Little Big Adventure, the latter set
   on a planet stabilised between two stars. In David Weber's Honorverse,
   the Manticore system is a binary star with three habitable planets, two
   of them ( Sphinx and the capital world Manticore) orbiting Manticore A
   and one ( Gryphon) orbiting Manticore B. In the Stargate fictional
   universe, the planet Chulak is located in a binary star system. At the
   start of the film " Starship Troopers", a FedNet news graphic describes
   the Arachnid home world as orbiting "a twin star system whose brutal
   gravitational forces produce an unlimited supply of Bug meteorites."
   The Battlestar Galactica episode " The Captain's Hand" is set in a
   binary star system.

   There are also fiction uses of even more exotic stellar configurations.
   In the Star Trek: The Next Generation episode " Night Terrors" the
   Enterprise-D becomes trapped within an unusual binary star system, and
   in Star Trek: Enterprise episode " Singularity," the Enterprise visits
   a ternary star system, as do the heroes of Futurama in the episode " My
   Three Suns". The home star system of the planet Vulcan in Star Trek is
   likely to be 40 Eridani, a triple star system, 16 light-years from
   Earth. The movie Pitch Black takes place on a planet in a ternary
   system, whilst Isaac Asimov's renowned short story " Nightfall" is set
   in a six-sun system. Another example comes from the re-imagined
   Battlestar Galactica, in which the Battlestar Pegasus fights off three
   Cylon base ships in a binary star system. Less well known but notable
   for its complexity is the Marune system, Number 993 in Jack Vance's
   "Alastor Cluster", in which the planet is lit by a combination of four
   suns whose combination of lighting patterns controls local culture:
   Marune orbits one star of a central binary pair and the second binary
   pair orbits the first, further out; nor are any of the orbits
   co-planar.
   Retrieved from " http://en.wikipedia.org/wiki/Binary_star"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
