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A star system or stellar system is a small number of stars that orbit each other,[1] bound by gravitational attraction. A large group of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems. Star systems are not to be confused with planetary systems, which include planets and similar bodies (such as comets).

A star system of two stars is known as a binary star, binary star system or physical double star. If there are no tidal effects, no perturbation from other forces, and no transfer of mass from one star to the other, such a system is stable, and both stars will trace out an elliptic orbit around the barycenter of the system indefinitely. (See Two-body problem). Examples of binary systems are Sirius, Procyon and Cygnus X-1, the last of which probably consists of a star and a black hole.

A multiple star system consists of three or more stars that appear from Earth to be close to one another in the sky. This may result from the stars actually being physically close and gravitationally bound to each other, in which case it is a physical multiple star, or this closeness may be merely apparent, in which case it is an optical multiple star (meaning that the stars may appear to be close to each other when viewed from planet Earth, as they both seem to occupy the same point in the sky, but in reality, one star may be much farther away from Earth than the other, which is not readily apparent unless one can view them from a different angle). Physical multiple stars are also commonly called multiple stars or multiple star systems.[2][3][4][5][6]

Most multiple star systems are triple stars. Systems with four or more components are less likely to occur.[5] Multiple-star systems are called triple, trinary or ternary if they contain three stars; quadruple or quaternary if they contain four stars; quintuple or quintenary with five stars; sextuple or sextenary with six stars; septuple or septenary with seven stars. These systems are smaller than open star clusters, which have more complex dynamics and typically have from 100 to 1,000 stars.[7] Most multiple star systems known are triple; for higher multiplicities, the number of known systems with a given multiplicity decreases exponentially with multiplicity.[8] 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 selection effects, knowledge of these statistics is very incomplete.[9]

Multiple-star systems can be divided into two main dynamical classes: (1) hierarchical systems which are stable and consist of nested orbits that don't interact much and so each level of the hierarchy can be treated as a Two-body problem, or (2) the trapezia which have unstable strongly interacting orbits and are modelled as an n-body problem, exhibiting chaotic behavior.[10] They can have 2, 3, or 4 stars.

Hierarchical systems
Star system named DI Cha. While only two stars are apparent, it is actually a quadruple system containing two sets of binary stars.[11]

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.[12] 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[5][13] Keplerian orbits around the system's center of mass,[14] unlike the unstable trapezia systems or the even more complex dynamics of the large number of stars in star clusters and galaxies.
Triple star systems

Orbits of the HR 6819 hierarchical triple star system: an inner binary with one star (orbit in blue) and a black hole (orbit in red), encircled by another star in a wider orbit (also in blue).

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.[15][16] 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.[17] HR 6819 is an example of a physical hierarchical triple system, which has an outer star orbiting an inner physical binary composed of a star and a stellar black hole.[18] Although recently the notion that HR 6819 is a triple system is challenged.[19] 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.
Higher multiplicities
Mobile diagrams:

multiplex
simplex, binary system
simplex, triple system, hierarchy 1
simplex, quadruple system, hierarchy 2
simplex, quadruple system, hierarchy 3
simplex, quintuple system, hierarchy 4.

Hierarchical multiple star systems with more than three stars can produce a number of more complicated arrangements. These arrangements can be organized by what Evans (1968) called mobile diagrams, which look similar to ornamental mobiles hung from the ceiling. Examples of hierarchical systems are given in the figure to the right (Mobile diagrams). 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. Because, as we have already seen 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.[20]

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 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.[21]
The maximum hierarchy occurring in A. A. Tokovinin's Multiple Star Catalogue, as of 1999, is 4.[22] For example, the stars Gliese 644A and Gliese 644B form what appears to be a close visual binary star; because 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).;[23]

Higher hierarchies are also possible.[16][24] Most of these higher hierarchies either are stable or suffer from internal perturbations.[25][26][27] Others consider complex multiple stars will in time theoretically disintegrate into less complex multiple stars, like more common observed triples or quadruples are possible.[28][29]
Trapezia

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 galactic high-velocity stars.[30][31] They are named after the multiple star system known as the Trapezium Cluster in the heart of the Orion Nebula.[30] Such systems are not rare, and commonly appear close to or within bright nebulae. These stars have no standard hierarchical arrangements, but compete for stable orbits. This relationship is called interplay.[32] 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.[32] Example of such events may explain the runaway stars that might have been ejected during a collision of two binary star groups or a multiple system. This event is credited with ejecting AE Aurigae, Mu Columbae and 53 Arietis at above 200 km·s−1 and has been traced to the Trapezium cluster in the Orion Nebula some two million years ago.[33][34]
Designations and nomenclature
Multiple star designations

The components of multiple stars can be specified by appending the suffixes A, B, C, etc., to the system's designation. Suffixes such as AB may be used to denote the pair consisting of A and B. The sequence of letters B, C, etc. may be assigned in order of separation from the component A.[35][36] Components discovered close to an already known component may be assigned suffixes such as Aa, Ba, and so forth.[36]
Nomenclature in the Multiple Star Catalogue
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.[37]
Future multiple star system nomenclature

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 and variable star designations for eclipsing binary stars), and, worse, component letters may be assigned differently by different authors, so that, for example, one person's A can be another's C.[38] Discussion starting in 1999 resulted in four proposed schemes to address this problem:[38]

KoMa, a hierarchical scheme using upper- and lower-case letters and Arabic and Roman numerals;
The Urban/Corbin Designation Method, a hierarchical numeric scheme similar to the Dewey Decimal Classification system;[39]
The Sequential Designation Method, a non-hierarchical scheme in which components and subsystems are assigned numbers in order of discovery;[40] and
WMC, the Washington Multiplicity Catalog, a hierarchical scheme in which the suffixes used in the Washington 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.[41][42]

During the 24th General Assembly of the International 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.[38] A sample of a catalog using the WMC scheme, covering half an hour of right ascension, was later prepared.[43] 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.[44]

The sample WMC is hierarchically organized; the hierarchy used is based on observed orbital periods or separations. Since it contains many visual double 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.[38]
Examples
Binary
Sirius A (center), with its white dwarf companion, Sirius B (lower left) taken by the Hubble Space Telescope.

Sirius, a binary consisting of a main-sequence type A star and a white dwarf
Procyon, which is similar to Sirius
Mira, a variable consisting of a red giant and a white dwarf
Delta Cephei, a Cepheid variable
Epsilon Aurigae, an eclipsing binary
Spica

Trinary

Alpha Centauri is a triple star composed of a main binary yellow dwarf pair (Alpha Centauri A and Alpha Centauri B), and an outlying red dwarf, Proxima Centauri. Together, A and B form a physical binary star, designated as Alpha Centauri AB, α Cen AB, or RHD 1 AB, where the AB denotes this is a binary system.[45] The moderately eccentric orbit of the binary can make the components be as close as 11 AU 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.[46]
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 the Hubble Space Telescope in 2006.
Gliese 667 is a triple star system with two K-type main sequence stars and a red dwarf. The red dwarf, C, hosts between two and seven planets, of which one, Cc, alongside the unconfirmed Cf and Ce, are potentially habitable.
HD 188753 is a triple star system located approximately 149 light-years away from Earth in the constellation Cygnus. The system is composed of HD 188753A, a yellow dwarf; HD 188753B, an orange dwarf; and HD 188753C, a red dwarf. B and C orbit each other every 156 days, and, as a group, orbit A every 25.7 years.[47]
Fomalhaut (α PsA, α Piscis Austrini) is a triple star system in the constellation Piscis Austrinus. It was discovered to be a triple system in 2013, when the K type flare star TW Piscis Austrini and the red dwarf LP 876-10 were all confirmed to share proper motion through space. The primary has a massive dust disk similar to that of the early Solar 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 a red giant and two main-sequence stars. The orbits of the stars are oriented in such a way that all three stars eclipse each other.

Quaternary
HD 98800 is a quadruple star system located in the TW Hydrae association.

Capella, a pair of giant stars orbited by a pair of red dwarfs, around 42 light years away from the Solar System. It has an apparent magnitude of around −0.47, making Capella one of the brightest stars in the night sky.
4 Centauri[48]
Mizar is often said to have been the first binary star discovered when it was observed in 1650 by Giovanni Battista Riccioli[49], p. 1[50] but it was probably observed earlier, by Benedetto Castelli and Galileo.[citation needed] Later, spectroscopy of its components Mizar A and B revealed that they are both binary stars themselves.[51]
HD 98800
The Kepler-64 system has the planet PH1 (discovered in 2012 by the Planet Hunters group, a part of the Zooniverse) orbiting two of the four stars, making it to be the first known planet to be in a quadruple star system.[52]
KOI-2626 is the first quadruple star system with an Earth-sized planet.[53]
Xi Tauri (ξ Tau, ξ Tauri), located about 222 light years away, is a spectroscopic and eclipsing quadruple star consisting of three blue-white B-type main-sequence stars, along with an F-type star. Two of the stars are in a close orbit 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.[54]

Quintenary

Beta Capricorni
Delta Orionis
HD 155448[55]
KIC 4150611[56]
1SWASP J093010.78+533859.5[57]

Sextenary

Beta Tucanae[58]
Castor[59]
HD 139691[60]
If Alcor is considered part of the Mizar system, the system can be considered a sextuple.

Septenary

Nu Scorpii[61]
AR Cassiopeiae[62]

See also

iconStar portal

Binary (and multiple) stars in fiction
Binary star
Double star
Exoplanet
Planetary system
Solar System

References

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Planet Hunters
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Lohr, M. E.; et al. (2015). "The doubly eclipsing quintuple low-mass star system 1SWASP J093010.78+533859.5". Astronomy and Astrophysics. 578: A103.arXiv:1504.07065. Bibcode:2015A&A...578A.103L. doi:10.1051/0004-6361/201525973. S2CID 44548756.
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ADS 9731: A new sextuple system, A. A. Tokovinin, N. I. Shatskii, and A. K. Magnitskii, Astronomy Letters, 24, No. 6 (November 1998), pp. 795–801.
Nu Scorpii Archived 10 April 2020 at the Wayback Machine, entry in the Multiple Star Catalog.

AR Cassiopeiae Archived 10 April 2020 at the Wayback Machine, entry in the Multiple Star Catalog.

External links
Wikimedia Commons has media related to Multiple star systems.

NASA Astronomy Picture of the Day: Triple star system (11 September 2002)
NASA Astronomy Picture of the Day: Alpha Centauri system (23 March 2003)
Alpha Centauri, APOD, 2002 April 25
General news on triple star systems, TSN, 2008 April 22
The Double Star Library is located at the U.S. Naval Observatory
Naming New Extrasolar Planets

Individual specimens

NASA Astronomy Picture of the Day: Triple star system (11 September 2002)
NASA Astronomy Picture of the Day: Alpha Centauri system (23 March 2003)
Alpha Centauri, APOD, 2002 April 25

vte

Stellar systems
Bound

Galaxy Dwarf galaxy Star cluster
Globular cluster Dark globular cluster Open cluster Hypercompact stellar system Star system Binary star Planetary system

Unbound

Stellar stream Stellar association Moving group Runaway star Hypervelocity star

Visual grouping

Double star Multiple star Star cloud Asterism Constellation

Category Category:Star systems

vte

Stars
Formation

Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track

Evolution

Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]

Remnants

White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster

Hypothetical

Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar

Stellar nucleosynthesis

Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova

Structure

Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism

Properties

Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram

Star systems

Binary
Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system

Earth-centric
observations

Sun
Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard

Lists

Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy

Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

vte

Systems science
System
types

Anatomical Art Biological Complex Complex adaptive Conceptual Coupled human–environment Database Dynamical Ecological Economic Energy Formal Holarchic Information Legal Measurement Metric Multi-agent Nervous Nonlinear Operating Physical Planetary Political Sensory Social Star Writing

Concepts

Doubling time Leverage points Limiting factor Negative feedback Positive feedback

Theoretical
fields

Chaos theory Complex systems Control theory Cybernetics Earth system science Living systems Sociotechnical system Systemics Urban metabolism World-systems theory

Analysis Biology Dynamics Ecology Engineering Neuroscience Pharmacology Psychology Theory Thinking

Scientists

Alexander Bogdanov Russell L. Ackoff William Ross Ashby Ruzena Bajcsy Béla H. Bánáthy Gregory Bateson Anthony Stafford Beer Richard E. Bellman Ludwig von Bertalanffy Margaret Boden Kenneth E. Boulding Murray Bowen Kathleen Carley Mary Cartwright C. West Churchman Manfred Clynes George Dantzig Edsger W. Dijkstra Fred Emery Heinz von Foerster Stephanie Forrest Jay Wright Forrester Barbara Grosz Charles A. S. Hall Mike Jackson Lydia Kavraki James J. Kay Faina M. Kirillova George Klir Allenna Leonard Edward Norton Lorenz Niklas Luhmann Humberto Maturana Margaret Mead Donella Meadows Mihajlo D. Mesarovic James Grier Miller Radhika Nagpal Howard T. Odum Talcott Parsons Ilya Prigogine Qian Xuesen Anatol Rapoport John Seddon Peter Senge Claude Shannon Katia Sycara Eric Trist Francisco Varela Manuela M. Veloso Kevin Warwick Norbert Wiener Jennifer Wilby Anthony Wilden

Applications

Systems theory in anthropology Systems theory in archaeology Systems theory in political science

Organizations

List Principia Cybernetica

Physics Encyclopedia

World

Index

Hellenica World - Scientific Library

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