Algol A is being regularly eclipsed by the dimmer Algol B every 2.87 days. (Imaged in theH-band by the CHARA interferometer. Sudden jumps in the animation are artifacts.)
Artist's impression of the orbits ofHD 188753, a triple star system.
Astar system orstellar system is a small number ofstars that orbit each other,[1] bound bygravitational attraction. It may sometimes be used to refer to a single star.[2] A large group of stars bound by gravitation is generally called astar cluster orgalaxy, although, broadly speaking, they are also star systems. Star systems are not to be confused withplanetary systems, which include planets and similar bodies (such ascomets).
A star system of two stars is known as abinary star,binary star system orphysicaldouble star.
Systems with four or more components are rare, and are much less commonly found than those with 2 or 3.[3] Multiple-star systems are calledtriple,ternary, ortrinary if they contain three stars;quadruple orquaternary if they contain four stars;quintuple orquintenary with five stars;sextuple orsextenary with six stars;septuple orseptenary with seven stars; andoctuple oroctenary with eight stars.
These systems are smaller thanopen star clusters, which have more complex dynamics and typically have from 100 to 1,000 stars.[4]
Binary and multiple star systems are also known as aphysical multiple stars, to distinguish them fromoptical multiple stars, which merely look close together when viewed from Earth.Multiple stars may refer to either optical or physical,[5][3][6][7] but optical multiples do not form a starsystem.
Triple stars that arenot all gravitationally bound (and thus do not form a triple star system) might comprise a physical binary and anoptical companion (such asBeta Cephei) or, in rare cases, a purelyoptical triple star (such asGamma Serpentis).
Binary stars are the most common non-single stars. With multiple star systems, the number of known systems decreases exponentially with multiplicity.[9] For example, in the 1999 revision of Tokovinin's catalog[3] of physical multiple stars, 551 out of the 728 systems described are triple. However, because of suspectedselection effects, the ability to interpret these statistics is very limited.[10]
There are variousmethods to detect star systems and distinguish them from optical binaries multiples. These include:
Make observations six months apart and look for differences caused byparallaxes. (Not feasible for distant stars.)
Directly observe the stars orbiting each other or an apparently empty space (such as a dim star orneutron star). (Not feasible for distant stars or those with long orbital periods.)
In systems that satisfy the assumptions of thetwo-body problem – including having negligibletidal effects, perturbations (from the gravity of other bodies), andtransfer of mass between stars – the two stars will trace out a stableelliptical orbit around thebarycenter of the system. Examples of binary systems areSirius,Procyon andCygnus X-1, the last of which probably consists of a star and ablack hole.
Multiple-star systems can be divided into two main dynamical classes:
Hierarchical systems are stable and consist of nested orbits that do not interact much. Each level of the hierarchy can be treated as atwo-body problem.
Trapezia have unstable, strongly interacting orbits and are modelled as ann-body problem, exhibitingchaotic behavior.[11] They can have 2, 3, or 4 stars.
Star system namedDI Cha. While only two stars are apparent, it is actually a quadruple system containing two sets of binary stars.[12]
Most multiple-star systems are organized in what is called ahierarchical system: the stars in the system can be divided into two smaller groups, each of which traverses a larger orbit around the system'scenter 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.[13] Each level of the hierarchy can be treated as atwo-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[3][14]Keplerian orbits around the system's center of mass.[15]
For example, stable trinary systems consist of two stars in a closebinary system, with a third orbiting this pair at a distance much larger than that of the binary orbit.[16][13] 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.[17]EZ Aquarii is an example of a physical hierarchical triple system, which has an outer star orbiting an inner binary composed of two morered dwarf stars.
Hierarchical arrangements can be organized by what Evans (1968) calledmobile diagrams, which look similar to ornamental mobiles hung from the ceiling. Each level of the mobile illustrates the decomposition of the system into two or more systems with smaller size. Evans calls a diagrammultiplex if there is a node with more than twochildren, i.e. if the decomposition of some subsystem involves two or more orbits with comparable size. Because multiplexes may be unstable, multiple stars are expected to besimplex, meaning that at each level there are exactly twochildren. Evans calls the number of levels in the diagram itshierarchy.[13]
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 isCastor, also known as Alpha Geminorum or α Gem. It consists of what appears to be avisual binarystar which, upon closer inspection, can be seen to consist of twospectroscopic 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.[18]
The maximum hierarchy occurring in A. A. Tokovinin's Multiple Star Catalogue, as of 1999, is 4.[3] For example, the starsGliese 644A and Gliese 644B form what appears to be a close visualbinary star; because Gliese 644B is aspectroscopic 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).[19]
Higher hierarchies are also possible.[13][20] Most of these higher hierarchies either are stable or suffer from internalperturbations.[21][22][23] Others consider complex multiple stars will in time theoretically disintegrate into less complex multiple stars, like more common observed triples or quadruples.[24][25]
Trapezia are usually very young, unstable systems. These are thought to form in stellar nurseries, and quickly fragment into stable multiple stars, which in the process may eject components as galactichigh-velocity stars.[26][27] They are named after the multiple star system known as theTrapezium Cluster in the heart of theOrion Nebula.[26] Such systems are not rare, and commonly appear close to or within brightnebulae. These stars have no standard hierarchical arrangements, but compete for stable orbits. This relationship is calledinterplay.[28] Such stars eventually settle down to a close binary with a distant companion, with the other star(s) previously in the system ejected into interstellar space at high velocities.[28] This dynamic may explain therunaway stars that might have been ejected during a collision of two binary star groups or a multiple system. This event is credited with ejectingAE Aurigae,Mu Columbae and53 Arietis at above 200 km·s−1 and has been traced to theTrapezium cluster in theOrion Nebula some two million years ago.[29][30]
The components of multiple stars can be specified by appending the suffixesA,B,C, etc., to the system's designation. Suffixes such asAB may be used to denote the pair consisting ofA andB. The sequence of lettersB,C, etc. may be assigned in order of separation from the componentA.[31][32] Components discovered close to an already known component may be assigned suffixes such asAa,Ba, and so forth.[32]
Subsystem notation in Tokovinin's Multiple Star Catalogue
A. A. Tokovinin's Multiple Star Catalogue uses a system in which each subsystem in a mobile diagram is encoded by a sequence of digits. In the mobile diagram (d) above, for example, the widest system would be given the number 1, while the subsystem containing its primary component would be numbered 11 and the subsystem containing its secondary component would be numbered 12. Subsystems which would appear below this in the mobile diagram will be given numbers with three, four, or more digits. When describing a non-hierarchical system by this method, the same subsystem number will be used more than once; for example, a system with three visual components, A, B, and C, no two of which can be grouped into a subsystem, would have two subsystems numbered 1 denoting the two binaries AB and AC. In this case, if B and C were subsequently resolved into binaries, they would be given the subsystem numbers 12 and 13.[3]
The current nomenclature for double and multiple stars can cause confusion as binary stars discovered in different ways are given different designations (for example,discoverer designations for visual binary stars andvariable star designations for eclipsing binary stars), and, worse, component letters may be assigned differently by different authors, so that, for example, one person'sA can be another'sC.[33] Discussion starting in 1999 resulted in four proposed schemes to address this problem:[33]
KoMa, a hierarchical scheme using upper- and lower-case letters and Arabic and Roman numerals;
The Sequential Designation Method, a non-hierarchical scheme in which components and subsystems are assigned numbers in order of discovery;[35] and
WMC, the Washington Multiplicity Catalog, a hierarchical scheme in which the suffixes used in theWashington Double Star Catalog are extended with additional suffixed letters and numbers.
For a designation system, identifying the hierarchy within the system has the advantage that it makes identifying subsystems and computing their properties easier. However, it causes problems when new components are discovered at a level above or intermediate to the existing hierarchy. In this case, part of the hierarchy will shift inwards. Components which are found to be nonexistent, or are later reassigned to a different subsystem, also cause problems.[36][37]
During the 24th General Assembly of theInternational Astronomical Union in 2000, the WMC scheme was endorsed and it was resolved by Commissions 5, 8, 26, 42, and 45 that it should be expanded into a usable uniform designation scheme.[33] A sample of a catalog using the WMC scheme, covering half an hour ofright ascension, was later prepared.[38] The issue was discussed again at the 25th General Assembly in 2003, and it was again resolved by commissions 5, 8, 26, 42, and 45, as well as the Working Group on Interferometry, that the WMC scheme should be expanded and further developed.[39]
The sample WMC is hierarchically organized; the hierarchy used is based on observed orbital periods or separations. Since it contains many visualdouble stars, which may be optical rather than physical, this hierarchy may be only apparent. It uses upper-case letters (A, B, ...) for the first level of the hierarchy, lower-case letters (a, b, ...) for the second level, and numbers (1, 2, ...) for the third. Subsequent levels would use alternating lower-case letters and numbers, but no examples of this were found in the sample.[33]
Alpha Centauri is a triple star composed of a main binaryyellow dwarf and anorange dwarf pair (Rigil Kentaurus andToliman), and an outlyingred dwarf,Proxima Centauri. Together, Rigil Kentaurus and Toliman form a physicalbinary star, designated as Alpha Centauri AB, α Cen AB, or RHD 1 AB, where the AB denotes this is abinary system.[40] The moderately eccentricorbit of the binary can make the components be as close as 11AU or as far away as 36 AU. Proxima Centauri, also (though less frequently) called Alpha Centauri C, is much farther away (between 4300 and 13,000 AU) from α Cen AB, and orbits the central pair with a period of 547,000 (+66,000/-40,000) years.[41]
Polaris or Alpha Ursae Minoris (α UMi), the north star, is a triple star system in which the closer companion star is extremely close to the main star—so close that it was only known from its gravitational tug on Polaris A (α UMi A) until it was imaged by theHubble Space Telescope in 2006.
Gliese 667 is a triple star system with two K-type main sequence stars and ared dwarf. The red dwarf, C, hosts between two and seven planets, of which one, Cc, alongside the unconfirmed Cf and Ce, are potentially habitable.
Fomalhaut (α PsA, α Piscis Austrini) is a triple star system in theconstellationPiscis Austrinus. It was discovered to be a triple system in 2013, when the K typeflare star TW Piscis Austrini and the red dwarf LP 876-10 were all confirmed to shareproper motion through space. The primary has a massive dust disk similar to that of the earlySolar System, but much more massive. It also contains a gas giant,Fomalhaut b. That same year, the tertiary star, LP 876-10 was also confirmed to house a dust disk.
HD 181068 is a unique triple system, consisting of ared giant and two main-sequence stars. The orbits of the stars are oriented in such a way that all three stars eclipse each other.
HD 98800 is a quadruple star system located in theTW Hydrae association.
Capella, a pair ofgiant stars orbited by a pair ofred dwarfs, around 42 light years away from the Solar System. It has anapparent magnitude of around 0.08, making Capella one of the brightest stars in the night sky.
ThePH1 system has the planetPH1 b (discovered in 2012 by thePlanet Hunters group, a part of theZooniverse) orbiting two of the four stars, making it the first known planet to be in a quadruple star system.[47]
KOI-2626 is the first quadruple star system with an Earth-sized planet.[48]
Xi Tauri (ξ Tau, ξ Tauri), located about 222light years away, is aspectroscopic and eclipsing quadruple star consisting of three blue-white B-typemain-sequence stars, along with anF-type star. Two of thestars are in a closeorbit and revolve around each other once every 7.15 days. These in turn orbit the third star once every 145 days. The fourth star orbits the other three stars roughly every fifty years.[49]
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