A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars using a telescope , in which case they are called visual binaries . Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known. They may also be detected by indirect techniques, such as spectroscopy ( spectroscopic binaries ) or astrometry ( astrometric binaries ). If a binary star happens to orbit in a plane along our line of sight, its components will eclipse and transit each other; these pairs are called eclipsing binaries , or, together with other binaries that change brightness as they orbit, photometric binaries .
68-528: LB-1 is a binary star system in the constellation Gemini . In 2019 a paper in Nature proposed the system contained an unusually massive stellar black hole outside of ordinary single stellar evolution parameters. However, analyses in 2020 found the original 2019 conclusion to be incorrect. Some researchers now believe the system consists of a stripped B-type star and a massive rapidly rotating Be star . The optically observed star, LB-1 A, or LS V+22 25 ,
136-406: A main-sequence star goes through an activity cycle, the outer layers of the star are subject to a magnetic torque changing the distribution of angular momentum, resulting in a change in the star's oblateness. The orbit of the stars in the binary pair is gravitationally coupled to their shape changes, so that the period shows modulations (typically on the order of ∆P/P ~ 10 ) on the same time scale as
204-493: 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
272-747: 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. Binary stars that are both visual and spectroscopic binaries are rare and are a valuable source of information when found. About 40 are known. 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 found to be both visual and spectroscopic thus must be relatively close to Earth. An eclipsing binary star
340-451: A supernova that destroys the entire star, another possible cause for runaways. An 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. 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 center of mass. From
408-406: 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
476-711: A "surprisingly circular" orbit. Follow-up observations using the Gran Telescopio Canarias in Spain and the W. M. Keck Observatory in the United States better defined the findings. The parallax to LB-1 has been published in Gaia Data Release 2 , implying a distance around 2,300 pc . The observed spectral properties of the star are inconsistent with those expected for an ordinary main sequence B-type star at this distance. A separate spectroscopic analysis of
544-582: A binary star was computed in 1827, when Félix Savary computed the orbit of Xi Ursae Majoris . Over the years, many more double stars have been catalogued and measured. As of June 2017, the Washington Double Star Catalog , a database of visual double stars compiled by the United States Naval Observatory , contains over 100,000 pairs of double stars, including optical doubles as well as binary stars. Orbits are known for only
612-503: A binary system is optical binary , which refers to objects that are so close together in the sky that they appear to be a binary system, but are not. Such objects merely appear to be close together, but lie at different distances from the Solar System. When binary minor planets are similar in size, they may be called " binary companions " instead of referring to the smaller body as a satellite. Good examples of true binary companions are
680-442: 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 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 , or even high-powered binoculars . The angular resolution of
748-481: A few thousand of these double stars. The term binary was first used in this context by Sir William Herschel in 1802, when he wrote: If, on the contrary, two stars should really be situated very near each other, and at the same time so far insulated as not to be materially affected by the attractions of neighbouring stars, they will then compose a separate system, and remain united by the bond of their own mutual gravitation towards each other. This should be called
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#1733092842455816-454: A gas flow can actually be seen. 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
884-610: A near star paired with a distant star so he could measure the near star's changing position as the Earth orbited the Sun (measure its parallax ), allowing him to calculate the distance to the near star. He would soon publish catalogs of about 700 double stars. By 1803, he had observed changes in the relative positions in a number of double stars over the course of 25 years, and concluded that, instead of showing parallax changes, they seemed to be orbiting each other in binary systems. The first orbit of
952-405: A rapidly rotating B-type star with a disk (a Be star ) and a slowly rotating stripped helium star. The unseen companion to the star was discovered by measuring the radial velocity shifts of its companion star. If it is a black hole, this would mark the first time a stellar black hole was discovered without observation of its X-ray emissions. If the distance from parallax is ignored, and the star
1020-400: A real double star; and any two stars that are thus mutually connected, form the binary sidereal system which we are now to consider. By the modern definition, the term binary star is generally restricted to pairs of stars which revolve around a common center of mass. Binary stars which can be resolved with a telescope or interferometric methods are known as visual binaries . For most of
1088-456: A small telescope. Eclipsing binaries are where the objects' orbits are at an angle that when one passes in front of the other it causes an eclipse , as seen from Earth. Astrometric binaries are objects that seem to move around nothing as their companion object cannot be identified, it can only be inferred. The companion object may not be bright enough or may be hidden in the glare from the primary object. A related classification though not
1156-416: A visual binary, even with telescopes of the highest existing resolving power . 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
1224-598: A well-known black hole ). Binary stars are also common as the nuclei of many planetary nebulae , and are the progenitors of both novae and type Ia supernovae . Double stars , a pair of stars that appear close to each other, have been observed since the invention of the telescope . Early examples include Mizar and Acrux . Mizar, in the Big Dipper ( Ursa Major ), was observed to be double by Giovanni Battista Riccioli in 1650 (and probably earlier by Benedetto Castelli and Galileo ). The bright southern star Acrux , in
1292-456: A young, early-type , high-mass donor star which transfers mass by its stellar wind , while low-mass X-ray binaries are semidetached binaries in which gas from a late-type donor star or a white dwarf overflows the Roche lobe and falls towards the neutron star or black hole. Probably the best known example of an X-ray binary is the high-mass X-ray binary Cygnus X-1 . In Cygnus X-1, the mass of
1360-601: Is a B-type star nine times the mass of the Sun and located at least 7,000 light-years (2,100 pc) from Earth. It was found to exhibit radial velocity variations by Chinese astronomers using the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and the radial-velocity method to search for such wobbly stars. The astronomers observed the star orbiting an unseen companion every 78.9 days, in what researchers described as
1428-487: Is a sine curve. If the orbit is elliptical , the shape of the curve depends 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
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#17330928424551496-492: Is a binary star system in which the orbital 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. Algol , a triple star system in the constellation Perseus , contains the best-known example of an eclipsing binary. Eclipsing binaries are variable stars, not because
1564-420: Is a system of two astronomical bodies of the same kind that are comparable in size. Definitions vary, but typically require the center of mass to be located outside of either object. (See animated examples .) The most common kinds of binary system are binary stars and binary asteroids , but brown dwarfs , planets , neutron stars , black holes and galaxies can also form binaries. A multiple system
1632-493: 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 Earth's atmosphere , resulting in more precise resolution. Another classification
1700-406: Is assumed to be an ordinary main sequence B-type star, the unseen companion LB-1 B or LB-1 *, could be hypothesized to be a black hole , with a mass of about 70 solar masses , more than twice the mass as the maximum predicted by most current theories of stellar evolution . It would be in the stellar-mass black hole range, below the size of intermediate-mass black holes ; however, it would fall in
1768-471: Is based on the distance between the stars, relative to their sizes: Detached binaries are binary stars where each component is within its Roche lobe , i.e. the area where the gravitational pull of the star itself is larger than that of the other component. While on the main sequence 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
1836-444: 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. 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
1904-409: Is designated RHD 1 . These discoverer codes can be found in the Washington Double Star Catalog . The secondary star in a binary star system may be designated as the hot companion or cool companion , depending on its temperature relative to the primary star. Examples: While it is not impossible that some binaries might be created through gravitational capture between two single stars, given
1972-668: Is similar but consists of three or more objects, for example trinary stars and trinary asteroids . In a binary system, the brighter or more massive object is referred to as primary , and the other the secondary . Binary stars are also classified based on orbit. Wide binaries are objects with orbits that keep them apart from one another. They evolve separately and have very little effect on each other. Close binaries are close to each other and are able to transfer mass from one another. They can also be classified based on how we observe them. Visual binaries are two stars separated enough that they can be distinguished through binoculars or
2040-418: Is the primary star, and the dimmer is considered the secondary. In some publications (especially older ones), a faint secondary is called the comes (plural comites ; companion). If the stars are the same brightness, the discoverer designation for the primary is customarily accepted. The position angle of the secondary with respect to the primary is measured, together with the angular distance between
2108-424: The 90 Antiope and the 79360 Sila–Nunam systems. Pluto and its largest moon Charon are sometimes described as a binary system because the barycenter (center of mass) of the two objects is not inside either of them, but Charon is small enough compared to Pluto that it is usually classified as a moon. Orcus and its moon Vanth also orbit around a barycenter not inside either of them. The Sun and Jupiter orbit
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2176-522: The LMC , SMC , Andromeda Galaxy , and Triangulum Galaxy . Eclipsing binaries offer a direct method to gauge the distance to galaxies to an improved 5% level of accuracy. Nearby non-eclipsing binaries can also be photometrically detected by observing how the stars affect each other in three ways. The first is by observing extra light which the stars reflect from their companion. Second is by observing ellipsoidal light variations which are caused by deformation of
2244-467: The Pleiades cluster, and calculated that the likelihood of finding such a close grouping of stars was about one in half a million. He concluded that the stars in these double or multiple star systems might be drawn to one another by gravitational pull, thus providing the first evidence for the existence of binary stars and star clusters. William Herschel began observing double stars in 1779, hoping to find
2312-549: The Southern Cross , was discovered to be double by Father Fontenay in 1685. Evidence that stars in pairs were more than just optical alignments came in 1767 when English natural philosopher and clergyman John Michell became the first person to apply the mathematics of statistics to the study of the stars, demonstrating in a paper that many more stars occur in pairs or groups than a perfectly random distribution and chance alignment could account for. He focused his investigation on
2380-401: 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 . The visible star's position is carefully measured and detected to vary, due to the gravitational influence from its counterpart. The position of
2448-419: 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 , in which 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. As
2516-406: The pair-instability gap of black hole sizes, whereby sufficiently massive black hole progenitor stars undergo pair-instability supernovae and completely disintegrate, leaving no remnant behind. LB-1 would be the first black hole discovered in the mass gap range. The companion mass would be high enough that anything other than a black hole would be expected to be easily detected. According to one of
2584-522: 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
2652-460: The activity cycles (typically on the order of decades). Another phenomenon observed in some Algol binaries has been monotonic period increases. This is quite distinct from the far more common observations of alternating period increases and decreases explained by the Applegate mechanism. Monotonic period increases have been attributed to mass transfer, usually (but not always) from the less massive to
2720-491: 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, where the semi-major axis can only be expressed in angular units unless the stellar parallax , and hence the distance, of the system is known. Sometimes, the only evidence of a binary star comes from the Doppler effect on its emitted light. In these cases,
2788-422: The binary consists of a pair of stars where the spectral lines in the light emitted from each star shifts first towards the blue, then towards the red, as each moves first towards us, and then away from us, during its motion about their common center of mass , with the period of their common orbit. In these systems, the separation between the stars is usually very small, and the orbital velocity very high. Unless
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2856-425: 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 merge . W Ursae Majoris is an example. When a binary system contains a compact object such as a white dwarf , neutron star or black hole , gas from the other (donor) star can accrete onto
2924-420: The binary star has a Bayer designation and is widely separated, it is possible that the members of the pair will be designated with superscripts; an example is Zeta Reticuli , whose components are ζ Reticuli and ζ Reticuli. Double stars are also designated by an abbreviation giving the discoverer together with an index number. α Centauri, for example, was found to be double by Father Richaud in 1689, and so
2992-406: 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. The observations are plotted against time, and from the resulting curve a period is determined. If the orbit is circular , then the curve
3060-447: The compact object. This releases gravitational potential energy , causing the gas to become hotter and emit radiation. Cataclysmic variable stars , where the compact object is a white dwarf, are examples of such systems. In X-ray binaries , the compact object can be either a neutron star or a black hole . These binaries are classified as low-mass or high-mass according to the mass of the donor star. High-mass X-ray binaries contain
3128-494: The components fills the binary star's Roche lobe and the other does not. In this interacting binary star , 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. In many cases, the inflowing gas forms an accretion disc around the accretor. A contact binary is a type of binary star in which both components of
3196-454: The evolution of a star is determined by its mass, the process influences the evolution of both companions, and creates stages that cannot 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
3264-449: The hotter star causes the primary eclipse. An eclipsing binary's period of orbit may be determined from a study of its 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 nearest star slides over the disc of the other star. If it is also a spectroscopic binary, the orbital elements can also be determined, and
3332-442: The known visual binary stars one whole revolution has not been observed yet; rather, they are observed to have travelled along a curved path or a partial arc. The more general term double star is used for pairs of stars which are seen to be close together in the sky. This distinction is rarely made in languages other than English. Double stars may be binary systems or may be merely two stars that appear to be close together in
3400-426: 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 when one star passes in front of the other. The brightness may drop twice during the orbit, once when the secondary passes in front of the primary and once when the primary passes in front of the secondary. The deeper of
3468-421: The mass estimate of the compact object to as little as ~2–3 M ☉ and raises the possibility of a neutron star . A revised multiepoch spectroscopic study of LB-1 has revealed that LB-1 does not contain a black hole at all. Instead, it comprises a rapidly rotating Be star and a slowly-rotating helium star. The system was proposed to have formed through a past mass-transfer event. In this framework,
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#17330928424553536-507: 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. Since about 1995, measurement of extragalactic eclipsing binaries' fundamental parameters has become possible with 8-meter class telescopes. This makes it feasible to use them to directly measure the distances to external galaxies, a process that is more accurate than using standard candles . By 2006, they had been used to give direct distance estimates to
3604-436: The more massive star The components of binary stars are denoted by the suffixes A and B appended to the system's designation, A denoting the primary and B the secondary. The suffix AB may be used to denote the pair (for example, the binary star α Centauri AB consists of the stars α Centauri A and α Centauri B.) Additional letters, such as C , D , etc., may be used for systems with more than two stars. In cases where
3672-402: 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, for example with the binary mass function . 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. Binary system A binary system
3740-442: The plane of the orbit happens to be perpendicular to the line of sight, the orbital velocities have components in the line of sight, and the observed radial velocity of the system varies 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
3808-453: The researchers, "This discovery forces us to re-examine our models of how stellar-mass black holes form [...] This remarkable result, along with the LIGO-Virgo detections of binary black hole collisions during the past four years, really points towards a renaissance in our understanding of black hole astrophysics." Alternatively, the evidence for the star to be a stripped helium star reduces
3876-407: The sky but have vastly different true distances from the Sun. The latter are termed optical doubles or optical pairs . 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
3944-451: 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 wobbly path across the sky. If the companion is sufficiently massive to cause an observable shift in position of
4012-467: The star suggests that instead of a B-type main sequence star as had been indicated, LB-1 A is more likely a stripped helium star (whose spectrum is very similar) with only ~1 M ☉ , if at the distance determined by the Gaia satellite. An additional spectroscopic analysis utilised multi-epoch spectroscopy and disentangling techniques and found that LB-1 comprises two non-degenerate stars:
4080-502: The star's shape by their companions. The third method is by looking at how relativistic beaming affects the apparent magnitude of the stars. Detecting binaries with these methods requires accurate photometry . Astronomers have discovered some stars that seemingly orbit around an empty space. Astrometric binaries are relatively nearby stars which can be seen to wobble around a point in space, with no visible companion. The same mathematics used for ordinary binaries can be applied to infer
4148-425: 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
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#17330928424554216-630: The stripped helium star was originally the more massive star and has therefore evolved faster than its companion. After leaving the main sequence, the progenitor star transferred mass to its companion, which became the massive rapidly rotating Be star we see today. Binary star If components in binary star systems are close enough, they can gravitationally distort each other's outer stellar atmospheres. In some cases, these close binary systems can exchange mass, which may bring their evolution to stages that single stars cannot attain. Examples of binaries are Sirius , and Cygnus X-1 (Cygnus X-1 being
4284-412: The telescope is an important factor in the detection of visual binaries, and as better angular resolutions are applied to binary star observations, an increasing number of visual binaries will be detected. The relative brightness of the two stars is also an important factor, as glare from a bright star may make it difficult to detect the presence of a fainter component. The brighter star of a visual binary
4352-403: The two eclipses is called the primary regardless of which star is being occulted, and if a shallow second eclipse also occurs it is called the secondary eclipse. The size of the brightness drops depends on the relative brightness of the two stars, the proportion of the occulted star that is hidden, and the surface brightness (i.e. effective temperature ) of the stars. Typically the occultation of
4420-451: 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
4488-663: The unseen companion is estimated to be about nine times that of the Sun, far exceeding the Tolman–Oppenheimer–Volkoff limit for the maximum theoretical mass of a neutron star. It is therefore believed to be a black hole; it was the first object for which this was widely believed. Orbital periods can be less than an hour (for AM CVn stars ), or a few days (components of Beta Lyrae ), but also hundreds of thousands of years ( Proxima Centauri around Alpha Centauri AB). The Applegate mechanism explains long term orbital period variations seen in certain eclipsing binaries. As
4556-428: The very low likelihood of such an event (three objects being actually required, as conservation of energy rules out a single gravitating body capturing another) and the high number of binaries currently in existence, this cannot be the primary formation process. The observation of binaries consisting of stars not yet on the main sequence supports the theory that binaries develop during star formation . Fragmentation of
4624-411: Was observed that the more massive component Algol A is still in the main sequence , while the less massive Algol B is a subgiant 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,
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