Editing Star system

{{Short description|Small number of stars that orbit each other}}
{{About|astronomical objects|the Hollywood star system|Star system (filmmaking)}}
{{Redirect|Multiple stars|apparent doubles|Double star}}
{{Distinguish|Planetary system|Solar System}}
{{For|exosolar systems|List of multiplanetary systems}}
{{Use dmy dates|date=March 2021}}

{{Multiple image
| perrow = 1/2
| total_width = 300px
| image1 = Algol triple star system imaged with the CHARA interferometer.jpg
| caption1 = The [[Algol|Algol three-star system]] imaged in the [[near-infrared]] by the [[CHARA interferometer]] with 0.5 [[Milliarcsecond|mas]] resolution in 2009. The shape of Algol C is an artifact.
| image2 = Algol AB movie imaged with the CHARA interferometer - labeled.gif
| caption2 = Algol&nbsp;A is being regularly eclipsed by the dimmer Algol&nbsp;B every 2.87 days. (Imaged in the [[List of astronomical interferometers at visible and infrared wavelengths#Current performance of ground-based interferometers|H-band]] by the CHARA interferometer. Sudden jumps in the animation are artifacts.)
| image3 = HD188753 orbit.jpg
| caption3 = Artist's impression of the orbits of [[HD 188753]], a triple star system.
}}

A '''star system''' or '''stellar system''' is a small number of [[star]]s that orbit each other,<ref>{{cite book |title=Modern Dictionary of Astronomy and Space Technology |editor=A.S. Bhatia |publisher=New Delhi: Deep & Deep Publications |year=2005 |isbn=81-7629-741-0 }}</ref> bound by [[gravity|gravitational attraction]]. It may sometimes be used to refer to a single star. <ref>{{cite web | url=https://www.astro.gsu.edu/RECONS/TOP100.posted.htm | title=The 100 Nearest Star Systems |archive-url=https://web.archive.org/web/20250429213406/http://www.astro.gsu.edu/RECONS/TOP100.posted.htm |archive-date=29 April 2025}}</ref> 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 system]]s, which include planets and similar bodies (such as [[comet]]s).

==Terminology<span class="anchor" id="triple star"></span><span class="anchor" id="Triple star"></span>==

A star system of two stars is known as a ''[[binary star]]'', ''binary star system'' or ''physical [[double star]]''. 

Systems with four or more components are less likely to occur.<ref name=toko /> Multiple-star systems are called ''triple'', ''ternary'', or ''trinary'' 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; and ''octuple'' or ''octenary'' with eight stars. 

These systems are smaller than [[open star cluster]]s, which have more complex dynamics and typically have from 100 to 1,000&nbsp;stars.<ref name="Galactic Dynamics 1987">{{cite book |last1=Binney |first1=James |last2=Tremaine |first2=Scott |year=1987 |title=Galactic Dynamics |page=247 |publisher=Princeton University Press |isbn=0-691-08445-9 |url=https://books.google.com/books?id=01yNf7mipb0C}}</ref>

==Optical doubles and multiples==

Binary and multiple star systems are also known as a ''physical'' multiple stars, to distinguish them from ''optical'' multiple stars, which merely look close together when viewed from Earth. ''Multiple stars'' may refer to either optical or physical,<ref>{{cite book |author=John R. Percy |year=2007 |title=Understanding Variable Stars |page=16 |publisher=Cambridge University Press |isbn=978-1-139-46328-7 |url=https://books.google.com/books?id=GQzCDQI3YP4C}}</ref><ref name=toko /><ref>{{cite web |website=Hipparcos |title=Double and multiple stars |publisher=[[European Space Agency]] |url=http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=Double_stars |access-date=31 October 2007}}</ref><ref>{{cite web |title=Binary and multiple stars |website=messier.seds.org |url=http://messier.seds.org/bina.html |access-date=26 May 2007}}</ref> but optical multiples do not form a star ''system''.

Triple stars that are ''not'' all gravitationally bound (and thus do not form a triple star system) might comprise a physical binary and an ''optical'' companion (such as [[Beta Cephei]]) or, in rare cases, a purely ''optical'' triple star (such as [[Gamma Serpentis]]).

== Abundance ==

[[Binary stars#Research findings|Research on binary and multiple stars]] estimates they make up about a third of the star systems in the [[Milky Way]] galaxy, with two-thirds of stars being single.<ref>[https://pweb.cfa.harvard.edu/news/most-milky-way-stars-are-single Most Milky Way Stars Are Single], Harvard-Smithsonian Center for Astrophysics.</ref>

Binary stars are the most common non-single stars. With multiple star systems, the number of known systems decreases exponentially with multiplicity.<ref name="tok1">{{cite journal |last1=Tokovinin |first1=A. |year=2001 |title=Statistics of multiple stars: Some clues to formation mechanisms |journal=The Formation of Binary Stars |volume=200 |page=84 |bibcode=2001IAUS..200...84T}}</ref> For example, in the 1999 revision of Tokovinin's catalog<ref name=toko>{{cite journal |last1=Tokovinin |first1=A.A. |year=1997 |title=MSC - a catalogue of physical multiple stars |journal=Astronomy and Astrophysics Supplement Series |volume=124 |page=75 |doi=10.1051/aas:1997181 |doi-access=free |bibcode=1997A&AS..124...75T}}<br />&nbsp; &nbsp; online versions at <br />{{cite web |url=http://cdsweb.u-strasbg.fr/viz-bin/VizieR?-source=J/A%2bAS/124/75 |title=online version at VizieR |archive-url=https://web.archive.org/web/20070311101531/http://cdsweb.u-strasbg.fr/viz-bin/VizieR?-source=J%2FA%2BAS%2F124%2F75 |archive-date=11 March 2007}}<br />&nbsp; &nbsp; and at <br />{{cite web |url=http://www.ctio.noao.edu/~atokovin/stars/ |title=Multiple star catalog |website=ctio.noao.edu |editor=A. Tokovin}}</ref> of physical multiple stars, 551&nbsp;out of the 728&nbsp;systems described are triple. However, because of suspected [[selection effect]]s, the ability to interpret these statistics is very limited.<ref>{{cite journal |last1=Tokovinin |first1=A. |year=2004 |title=Statistics of multiple stars |journal=Revista Mexicana de Astronomía y Astrofísica, Serie de Conferencias |volume=21 |page=7 |bibcode=2004RMxAC..21....7T}}</ref>

== Detection ==

There are various [[Binary star#Methods of observation|methods to detect]] star systems and distinguish them from optical binaries multiples. These include:
* Make observations six months apart and look for differences caused by [[stellar parallax|parallaxes]]. (Not feasible for distant stars.)
* Directly observe the stars orbiting each other or an apparently empty space (such as a dim star or [[neutron star]]). (Not feasible for distant stars or those with long orbital periods.)
* Observe a varying [[Doppler shift]].
* Observe fluctuations in brightness that result from [[eclipse]]s. (Relies on the Earth being in the [[orbital plane]].)
* Observe fluctuations in brightness that result from stars reflecting each others' light or gravitationally deforming each other.

== Orbital characteristics ==

In systems that satisfy the assumptions of the [[two-body problem]] – including having negligible [[Tidal force|tidal]] effects, perturbations (from the gravity of other bodies), and [[Mass transfer#Astrophysics|transfer of mass]] between stars – the two stars will trace out a stable [[elliptical orbit]] around the [[barycenter]] of the system. Examples of binary systems are [[Sirius]], [[Procyon]] and [[Cygnus X-1]], the last of which probably consists of a star and a [[black 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 a [[two-body problem]].
* Trapezia have unstable, strongly interacting orbits and are modelled as an [[n-body problem]], exhibiting [[chaos theory|chaotic]] behavior.<ref>{{cite book |first=Peter J.T. |last=Leonard |year=2001 |article=Multiple stellar systems: Types and stability |editor-first=P. |editor-last=Murdin |title=Encyclopedia of Astronomy and Astrophysics |edition=online |url=http://eaa.iop.org/ |archive-url=https://archive.today/20120709020850/http://eaa.iop.org/ |archive-date=2012-07-09 |publisher=Institute of Physics}} Nature Publishing Group published the original print edition.</ref> They can have 2, 3, or 4&nbsp;stars.

=== Hierarchical systems ===
[[File:Smoke ring for a halo.jpg|thumb|Star system named [[DI Cha]]. While only two stars are apparent, it is actually a quadruple system containing two sets of binary stars.<ref>{{cite web|title=Smoke ring for a halo|url=http://www.spacetelescope.org/images/potw1543a/|access-date=26 October 2015}}</ref>]]

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.<ref name=evans1968/> 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<ref name=toko /><ref name=heintz1>{{cite book | last=Heintz | first=W. D. | date=1978 | title=Double Stars | publisher=[[D. Reidel]] Publishing Company, Dordrecht | isbn=90-277-0885-1 | pages=[https://archive.org/details/DoubleStars/page/1 1] | url=https://archive.org/details/DoubleStars/page/1 }}</ref> [[Kepler's laws of planetary motion|Keplerian]] orbits around the system's center of mass.<ref>[http://www.ctio.noao.edu/~atokovin/papers/dynamics.pdf Dynamics of multiple stars: observations] {{webarchive|url=https://web.archive.org/web/20060919155603/http://www.ctio.noao.edu/~atokovin/papers/dynamics.pdf |date=19 September 2006 }}, A. Tokovinin, in "Massive Stars in Interacting Binaries", 16–20 August 2004, Quebec (ASP Conf. Ser., in print).</ref>

For example, stable trinary systems consist of two stars in a close [[binary star|binary system]], with a third orbiting this pair at a distance much larger than that of the binary orbit.<ref>{{cite book | last=Heintz | first=W. D. | date=1978 | title=Double Stars | publisher=D. Reidel Publishing Company, Dordrecht | isbn=90-277-0885-1 | pages=[https://archive.org/details/DoubleStars/page/66 66–67] | url=https://archive.org/details/DoubleStars/page/66 }}</ref><ref name=evans1968>{{cite journal | last=Evans | first=David S. | date=1968 | title=Stars of Higher Multiplicity | journal=Quarterly Journal of the Royal Astronomical Society | volume=9 | bibcode=1968QJRAS...9..388E | pages=388–400}}</ref> 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.<ref>{{cite journal |bibcode=1994MNRAS.267..161K |title=A note on the stability of hierarchical triple stars with initially circular orbits |last1=Kiseleva |first1=G. |last2=Eggleton |first2=P. P. |last3=Anosova |first3=J. P. |journal=Monthly Notices of the Royal Astronomical Society |year=1994 |volume=267 |page=161 |doi=10.1093/mnras/267.1.161 |doi-access=free }}</ref> [[EZ Aquarii]] is an example of a physical hierarchical triple system, which has an outer star orbiting an inner binary composed of two more [[red dwarf]] stars.

====Mobile diagrams====
[[File:Mobile-diagrams.png|thumb|''Mobile diagrams'': {{Ordered list |list_style_type=lower-alpha |item_style=margin-left:-12px; |multiplex |simplex, binary system |simplex, triple system, hierarchy 2 |simplex, quadruple system, hierarchy 2 |simplex, quadruple system, hierarchy 3 |simplex, quintuple system, hierarchy 4.}}]]

Hierarchical arrangements can be organized by what Evans (1968) called ''mobile 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 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 multiplexes 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''.<ref name=evans1968/>

* 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 (star)|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.<ref>{{cite book | last=Heintz | first=W. D. | date=1978 | page=[https://archive.org/details/DoubleStars/page/72 72] | title=Double Stars | publisher=D. Reidel Publishing Company, Dordrecht | isbn=90-277-0885-1 | url=https://archive.org/details/DoubleStars/page/72 }}</ref>
* The maximum hierarchy occurring in A. A. Tokovinin's Multiple Star Catalogue, as of 1999, is 4.<ref name=toko/> For example, the stars [[V1054 Ophiuchi|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).<ref>{{cite journal | last1 = Mazeh | first1 = Tzevi | display-authors = etal   | date = 2001 | title = Studies of multiple stellar systems – IV. The triple-lined spectroscopic system Gliese 644 | journal = Monthly Notices of the Royal Astronomical Society | volume = 325 | issue = 1 | pages = 343–357 | doi = 10.1046/j.1365-8711.2001.04419.x | doi-access = free |arxiv = astro-ph/0102451 |bibcode = 2001MNRAS.325..343M | s2cid = 16472347 }}; see §7–8 for a discussion of the quintuple system.</ref>

Higher hierarchies are also possible.<ref name=evans1968/><ref>{{cite book | last=Heintz | first=W. D. | date=1978 | pages=[https://archive.org/details/DoubleStars/page/65 65–66] | title=Double Stars | publisher=D. Reidel Publishing Company, Dordrecht | isbn=90-277-0885-1 | url=https://archive.org/details/DoubleStars/page/65 }}</ref> Most of these higher hierarchies either are stable or suffer from internal [[Perturbation (astronomy)|perturbations]].<ref>{{cite journal | title = Encounter Phenomena in Triple Stars | last1 = Harrington | first1 = R.S. | date = 1970 | bibcode=1970AJ.....75.1140H | journal = Astronomical Journal | volume = 75 | pages = 114–118 |doi = 10.1086/111067 }}</ref><ref>{{cite journal | title = Multiple stars: Anathemas or friends? | last1 = Fekel | first1 = Francis C | date = 1987 | bibcode = 1987VA.....30...69F | journal = Vistas in Astronomy | volume = 30 | issue = 1 | pages = 69–76 | doi=10.1016/0083-6656(87)90021-3}}</ref><ref>{{cite journal | title = Multiple stars with low hierarchy: stable or unstable? | last1 = Zhuchkov | first1 = R. Ya. | last2 = Orlov | first2 = V. V. | last3 = Rubinov | first3 = A. V. | date = 2006 | bibcode=2006POBeo..80..155Z | journal = Publications of the Astronomical Observatory of Belgrade | volume = 80 | pages = 155–160 }}</ref> Others consider complex multiple stars will in time theoretically disintegrate into less complex multiple stars, like more common observed triples or quadruples.<ref>{{cite journal | title = Dynamical Evolution of Multiple Stars: Influence of the Initial Parameters of the System | last1 = Rubinov | first1 = A. V. | date = 2004 | bibcode=2004ARep...48...45R | journal = Astronomy Reports | volume = 48 | issue = 1 | pages = 155–160 |doi = 10.1134/1.1641122 | s2cid = 119705425 }}</ref><ref>{{cite journal | title = Multiple Star Formation from N-Body System Decay | last1 = Harrington | first1 = R. S. | date = 1977 | bibcode=1977RMxAA...3..209H | journal = Rev. Mex. Astron. Astrofís. | volume = 3 | page = 209 }}</ref>

===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 [[runaway stars|high-velocity stars]].<ref name=heintztrapezia>{{cite book | last=Heintz | first=W. D. | date=1978 | title=Double Stars | publisher=D. Reidel Publishing Company, Dordrecht | isbn=90-277-0885-1 | pages=[https://archive.org/details/DoubleStars/page/67 67–68] | url=https://archive.org/details/DoubleStars/page/67 }}</ref><ref>{{cite journal |bibcode=2006RMxAC..25...13A |title=Runaway Stars, Trapezia, and Subtrapezia |last1=Allen |first1=C.|author1-link=Christine Allen (astronomer) |last2=Poveda |first2=A. |last3=Hernández-Alcántara |first3=A. |journal=Revista Mexicana de Astronomía y Astrofísica, Serie de Conferencias |year=2006 |volume=25 |page=13 }}</ref> They are named after the multiple star system known as the [[Trapezium Cluster]] in the heart of the [[Orion Nebula]].<ref name=heintztrapezia /> Such systems are not rare, and commonly appear close to or within bright [[nebula]]e. These stars have no standard hierarchical arrangements, but compete for stable orbits. This relationship is called ''interplay''.<ref name="Heintz 1978 68">{{cite book | last=Heintz | first=W. D. | date=1978 | page=[https://archive.org/details/DoubleStars/page/68 68] | title=Double Stars | publisher=D. Reidel Publishing Company, Dordrecht | isbn=90-277-0885-1 | url=https://archive.org/details/DoubleStars/page/68 }}</ref> 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.<ref name="Heintz 1978 68" /> This dynamic may explain the [[runaway star]]s 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&nbsp;km·s<sup>−1</sup> and has been traced to the [[Trapezium cluster]] in the [[Orion Nebula]] some two million years ago.<ref name="Blaauw">
{{cite journal
 |last=Blaauw | first=A.
 |author2=Morgan, W.W.
 |date=1954
 |title=The Space Motions of AE Aurigae and mu Columbae with Respect to the Orion Nebula
 |journal=Astrophysical Journal
 |volume=119
 |pages=625
 |doi=10.1086/145866
 |bibcode=1954ApJ...119..625B
|doi-access=free
 }}</ref><ref name="Hoogerwerf">
{{cite journal
 |last1=Hoogerwerf |first1=R.
 |last2=de Bruijne |first2=J.H.J.
 |last3=de Zeeuw |first3=P.T
 |date=2000
 |title=The origin of runaway stars
 |journal=Astrophysical Journal
 |volume=544
 |issue=2
 |pages=133–136
 |doi=10.1086/317315
 |bibcode=2000ApJ...544L.133H
 |arxiv = astro-ph/0007436
|s2cid=6725343
 }}</ref>

==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''.<ref name=Heintz19>{{cite book |last=Heintz |first=W. D. |date=1978 |page=[https://archive.org/details/DoubleStars/page/19 19] |title=Double Stars |publisher=D. Reidel Publishing Company |location=Dordrecht |isbn=90-277-0885-1 |url=https://archive.org/details/DoubleStars/page/19 }}</ref><ref name=fmt>[http://ad.usno.navy.mil/wds/wdsweb_format.txt Format, The Washington Double Star Catalog] {{webarchive|url=https://web.archive.org/web/20080412092018/http://ad.usno.navy.mil/wds/wdsweb_format.txt |date=12 April 2008 }}, Brian D. Mason, Gary L. Wycoff, and William I. Hartkopf, Astrometry Department, [[United States Naval Observatory]]. Accessed on line 20 August 2008.</ref> Components discovered close to an already known component may be assigned suffixes such as ''Aa'', ''Ba'', and so forth.<ref name=fmt />

===Nomenclature in the Multiple Star Catalogue===
[[File:Tokovinin-multiple-star-notation.png|thumb|upright|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.<ref name=toko/>

===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, [[double star designation|discoverer designation]]s 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''.<ref name=post191>{{cite web
 |url=http://ad.usno.navy.mil/wds/wmc/wmc_post191.html
 |title=Addressing confusion in double star nomenclature: The Washington Multiplicity Catalog
 |author=William I. Hartkopf
 |author2=Brian D. Mason
 |name-list-style=amp
 |publisher=United States Naval Observatory
 |access-date=2008-09-12
 |archive-date=17 May 2011
 |archive-url=https://web.archive.org/web/20110517130354/http://ad.usno.navy.mil/wds/wmc/wmc_post191.html
 |url-status=dead
 }}</ref> Discussion starting in 1999 resulted in four proposed schemes to address this problem:<ref name=post191 />
* 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;<ref>
{{cite web
 |url=http://ad.usno.navy.mil/wds/wmc/uc.txt
 |archive-url=https://web.archive.org/web/20030424134423/http://ad.usno.navy.mil/wds/wmc/uc.txt
 |url-status=dead
 |archive-date=24 April 2003
 |title=Urban/Corbin Designation Method
 |publisher=United States Naval Observatory
 |access-date=2008-09-12
}}</ref>
* The Sequential Designation Method, a non-hierarchical scheme in which components and subsystems are assigned numbers in order of discovery;<ref>
{{cite web
 |url=http://ad.usno.navy.mil/wds/wmc/seq.txt
 |archive-url=https://web.archive.org/web/20030424134045/http://ad.usno.navy.mil/wds/wmc/seq.txt
 |url-status=dead
 |archive-date=24 April 2003
 |title=Sequential Designation Method
 |publisher=United States Naval Observatory
 |access-date=2008-09-12
}}</ref> 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.<ref>{{cite web
 |url=http://ad.usno.navy.mil/wds/wmc/toko_hwds.txt
 |title=On the designation of multiple stars
 |author=A. Tokovinin
 |date=18 April 2000
 |access-date=2008-09-12
 |archive-date=22 September 2007
 |archive-url=https://web.archive.org/web/20070922032710/http://ad.usno.navy.mil/wds/wmc/toko_hwds.txt
 |url-status=dead
 }}</ref><ref>{{cite web
 |url=http://ad.usno.navy.mil/wds/wmc/toko_exam.txt
 |title=Examples of multiple stellar systems discovery history to test new designation schemes
 |author=A. Tokovinin
 |date=17 April 2000
 |access-date=2008-09-12
 |archive-date=22 September 2007
 |archive-url=https://web.archive.org/web/20070922214652/http://ad.usno.navy.mil/wds/wmc/toko_exam.txt
 |url-status=dead
 }}</ref>

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.<ref name=post191 /> A sample of a catalog using the WMC scheme, covering half an hour of [[right ascension]], was later prepared.<ref>{{cite web
|url=http://ad.usno.navy.mil/wds/wmc/wmc110_intro.html
|title=Sample Washington Multiplicity Catalog
|author=William I. Hartkopf
|author2=Brian D. Mason
|name-list-style=amp
|publisher=United States Naval Observatory
|access-date=2008-09-12
|archive-date=21 July 2009
|archive-url=https://web.archive.org/web/20090721025435/http://ad.usno.navy.mil/wds/wmc/wmc110_intro.html
|url-status=dead
}}</ref> 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.<ref>{{cite journal |bibcode=2004Obs...124...94A |title=A new classification scheme for double and multiple stars |last1=Argyle |first1=R. W. |journal=The Observatory |year=2004 |volume=124 |page=94 }}
</ref>

The sample WMC is hierarchically organized; the hierarchy used is based on observed orbital periods or separations. Since it contains many visual [[double star]]s, 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.<ref name=post191 />

==Examples==

===Binary===
[[File:Sirius A and B Hubble photo.jpg|thumb|right|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|Almaaz]], an eclipsing binary
* [[Spica]]
{{further|Category:Binary stars}}

===Triple===
<!-- "Trinary star" redirects here. See [[MOS:HIDDENLINKADVICE]]. -->
* [[Alpha Centauri]] is a triple star composed of a main binary [[Yellow dwarf star|Yellow dwarf]] and an [[Orange dwarf star|Orange dwarf]] pair ([[Alpha Centauri A|Rigil Kentaurus]] and [[Alpha Centauri B|Toliman]]), and an outlying [[red dwarf]], [[Proxima Centauri]]. Together, Rigil Kentaurus and Toliman 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 star|binary system]].<ref name="wds2006">
{{cite journal
 | last1=Mason | first1=Brian D.
 | last2=Wycoff | first2=Gary L.
 | last3=Hartkopf | first3=William I.
 | last4=Douglass | first4=Geoffrey G.
 | last5=Worley | first5=Charles E.
 | title=The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog 
 | journal=The Astronomical Journal
 | publisher=U. S. Naval Observatory, Washington D.C.
 | volume=122 | issue=6 | pages=3466–3471
 | date=December 2001
 | doi=10.1086/323920
 | bibcode=2001AJ....122.3466M
 | doi-access=free
 }}</ref> The moderately eccentric [[orbit]] of the binary can make the components be as close as 11 [[astronomical unit|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.<ref>{{cite journal |bibcode=2017A&A...598L...7K |title=Proxima's orbit around α Centauri |last1=Kervella |first1=P. |last2=Thévenin |first2=F. |last3=Lovis |first3=C. |journal=Astronomy and Astrophysics |year=2017 |volume=598 |pages=L7 |doi=10.1051/0004-6361/201629930 |arxiv=1611.03495 |s2cid=50867264 }}</ref> 
* [[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-year]]s away from [[Earth]] in the [[constellation]] [[Cygnus (constellation)|Cygnus]]. The system is composed of HD 188753A, a [[Yellow dwarf star|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.<ref>[https://web.archive.org/web/20120103092449/http://tripsn.com/ Does triple star orbit directly affect orbit time], Jeremy Hien, Jon Shewarts, ''Astronomical News'' '''132''', No. 6 (November 2011)</ref>
* [[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.
{{further|Category:Triple star systems}}

===Quadruple===
[[File:HD 98800.jpg|thumb|[[HD 98800]] is a quadruple star system located in the [[TW Hydrae]] association.]]

* [[Capella]], a pair of [[giant star]]s orbited by a pair of [[red dwarf]]s, around 42 light years away from the Solar System. It has an [[apparent magnitude]] of around 0.08, making Capella one of the brightest stars in the night sky.
* [[4 Centauri]]<ref>[http://www.ctio.noao.edu/~atokovin/stars/index.php?cat=HD&number=120955 4 Centauri] {{webarchive|url=https://web.archive.org/web/20110615045153/http://www.ctio.noao.edu/~atokovin/stars/index.php?cat=HD&number=120955 |date=15 June 2011 }}, entry in the [http://www.ctio.noao.edu/~atokovin/stars/ Multiple Star Catalog].</ref>
* [[Mizar (star)|Mizar]] is often said to have been the first [[binary star]] discovered when it was observed in 1650 by [[Giovanni Battista Riccioli]]<ref>{{cite book|author=Robert Grant Aitken|title=The Binary Stars|url=https://books.google.com/books?id=gexmxAEACAAJ|year=2019|publisher=Creative Media Partners, LLC|isbn=978-0-530-46473-2}}</ref><sup>,&nbsp;p.&nbsp;1</sup><ref>[http://leo.astronomy.cz/mizar/riccioli.htm Vol. 1, part 1, p. 422, ''Almagestum Novum''] {{Webarchive|url=https://web.archive.org/web/20110810035504/http://leo.astronomy.cz/mizar/riccioli.htm |date=10 August 2011 }}, Giovanni Battista Riccioli, Bononiae: Ex typographia haeredis Victorij Benatij, 1651.</ref> but it was probably observed earlier, by [[Benedetto Castelli]] and [[Galileo]].{{Citation needed|date=January 2010}} Later, [[spectroscopy]] of its components Mizar A and B revealed that they are both binary stars themselves.<ref name="mizar">[http://leo.astronomy.cz/mizar/article.htm A New View of Mizar] {{Webarchive|url=https://web.archive.org/web/20080307111656/http://leo.astronomy.cz/mizar/article.htm |date=7 March 2008 }}, Leos Ondra, accessed on line 26 May 2007.</ref>
* [[HD 98800]]
* The [[PH1]] system has the planet [[PH1 b]] (discovered in 2012 by the [[Planet Hunters]] group, a part of the [[Zooniverse]]) orbiting two of the four stars, making it the first known planet to be in a quadruple star system.<ref name="PlanetHunters">{{Cite web |date=2012-10-15 |title=PH1 : A planet in a four-star system |url=https://blog.planethunters.org/2012/10/15/ph1-a-planet-in-a-four-star-system/ |access-date=2024-01-13 |website=Planet Hunters}}</ref>
*[[KOI-2626]] is the first quadruple star system with an Earth-sized planet.<ref>{{Cite web |last=Ciardi |first=David |title=KOI 2626: A Quadruple System with a Planet? |url=https://nexsci.caltech.edu/conferences/KeplerII/abstracts_posters/Ciardi.pdf |access-date=2024-01-13 |website=nexsci.caltech.edu}}</ref>
* [[Xi Tauri]] (ξ Tau, ξ Tauri), located about 222 [[light year]]s away, is a [[spectroscopic]] and eclipsing quadruple star consisting of three blue-white B-[[Stellar classification|type]] [[main-sequence star]]s, 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.<ref name="Nemravová2013">{{cite journal | title=An Unusual Quadruple System ξ Tauri | last1=Nemravová | first1=J. A. | last2=Harmanec | first2=P. | last3=Bencheikh | first3=J. | last4=Bolton | first4=C. T. | last5=Božićc | first5=H. | last6=Brož | first6=M. | last7=Engle | first7=S. | last8=Grunhut | first8=J. | last9=Guinan | first9=E. F. | last10=Hummel | first10=C. A. | last11=Korčáková | first11=D. | last12=Koubský | first12=P. | last13=Mayer | first13=P. | last14=Mourard | first14=D. | last15=Ribeiro | first15=J. | last16=Šlechta | first16=M. | last17=Vokrouhlický | first17=D. | last18=Votruba | first18=V. | last19=Wolf | first19=M. | last20=Zasche | first20=P. | display-authors=1 | journal=Central European Astrophysical Bulletin | volume=37 | issue=1 | pages=207–216 | year=2013 | bibcode=2013CEAB...37..207N }}</ref>

===Quintuple===
* [[Beta Capricorni|Dabih]]
* [[Mintaka]]<ref>{{cite web |author=Lee Mohon |date=Nov 12, 2015 |url=https://www.nasa.gov/universe/more-than-meets-the-eye-delta-orionis-in-orions-belt/ |title=More Than Meets the Eye: Delta Orionis in Orion’s Belt}}</ref>
* [[HD 155448]]<ref>{{cite journal | bibcode = 2011A&A...533A..54S | title=The young B-star quintuple system HD 155448 | journal=Astronomy and Astrophysics | volume=533 | year=2011 | author1=Schütz, O. |author2=Meeus, G. |author3=Carmona, A. |author4=Juhász, A. |author5=Sterzik, M. F.| pages=A54 |arxiv = 1108.1557 |doi = 10.1051/0004-6361/201016396 | s2cid=56143776 }}</ref>
* [[KIC 4150611]]<ref>{{cite journal|title=A Syzygy of KIC 4150611|author1=Gregg, T. A.|author2=Prsa, A.|author3=Welsh, W. F.|author4=Orosz, J. A.|author5=Fetherolf, T.|journal=American Astronomical Society|date=2013|volume=221|pages=142.12|bibcode=2013AAS...22114212G}}</ref>
* [[1SWASP J093010.78+533859.5]]<ref>{{cite journal|title=The doubly eclipsing quintuple low-mass star system 1SWASP J093010.78+533859.5|author1=Lohr, M. E.|display-authors=etal|journal=Astronomy & Astrophysics|volume=578|date=2015|pages=A103|bibcode=2015A&A...578A.103L|arxiv=1504.07065|doi = 10.1051/0004-6361/201525973 |s2cid=44548756}}</ref>

===Sextuple===
* [[Beta Tucanae]]<ref>{{Cite web |url=http://www.ctio.noao.edu/~atokovin/stars/index.php?ids=00316-6258 |title=Multiple Star Catalog (MSC) |access-date=23 December 2012 |archive-date=3 March 2016 |archive-url=https://web.archive.org/web/20160303233838/http://www.ctio.noao.edu/~atokovin/stars/index.php?ids=00316-6258 |url-status=dead }}</ref>
* [[Castor (star)|Castor]]<ref name="castor">{{cite journal |bibcode=2003A&A...402..719S |title=Castor a and Castor B resolved in a simultaneous Chandra and XMM-Newton observation |last1=Stelzer |first1=B. |last2=Burwitz |first2=V. |journal=Astronomy and Astrophysics |year=2003 |volume=402 |issue=2 |pages=719–728 |doi=10.1051/0004-6361:20030286 |arxiv=astro-ph/0302570 |s2cid=15268418 }}</ref>
* [[HD 139691]]<ref name="ads9731">{{cite journal |bibcode=1998AstL...24..795T |title=ADS 9731: A new sextuple system |last1=Tokovinin |first1=A. A. |last2=Shatskii |first2=N. I. |last3=Magnitskii |first3=A. K. |journal=Astronomy Letters |year=1998 |volume=24 |issue=6 |page=795 }}</ref>
* [[TYC 7037-89-1]]<ref>{{Cite web |last=Md |first=By Jeanette Kazmierczak NASA's Goddard Space Flight Center, Greenbelt |title=Discovery Alert: First Six-star System Where All Six Stars Undergo Eclipses |url=https://exoplanets.nasa.gov/news/1672/discovery-alert-first-six-star-system-where-all-six-stars-undergo-eclipses/ |access-date=2022-06-29 |website=Exoplanet Exploration: Planets Beyond our Solar System}}</ref>
* If [[Alcor (star)|Alcor]] is considered part of the [[Mizar (star)|Mizar]] system, the system can be considered a sextuple.

===Septuple===
* [[Nu Scorpii|Jabbah]]<ref>[http://www.ctio.noao.edu/~atokovin/stars/stars.php?cat=HD&number=145501 Nu Scorpii] {{webarchive|url=https://web.archive.org/web/20200410223657/http://www.ctio.noao.edu/~atokovin/stars/stars.php?cat=HD&number=145501 |date=10 April 2020 }}, entry in the [http://www.ctio.noao.edu/~atokovin/stars/ Multiple Star Catalog].</ref>
* [[AR Cassiopeiae]]<ref>[http://www.ctio.noao.edu/~atokovin/stars/stars.php?cat=HIP&number=115990 AR Cassiopeiae] {{webarchive|url=https://web.archive.org/web/20200410225127/http://www.ctio.noao.edu/~atokovin/stars/stars.php?cat=HIP&number=115990 |date=10 April 2020 }}, entry in the [http://www.ctio.noao.edu/~atokovin/stars/ Multiple Star Catalog].</ref>
* [[V871 Centauri]]<ref>{{cite journal|doi=10.1051/0004-6361/202243723|title=Multiply eclipsing candidates from the TESS satellite |year=2022 |last1=Zasche |first1=P. |last2=Henzl |first2=Z. |last3=Mašek |first3=M. |journal=Astronomy & Astrophysics |volume=664 |pages=A96 |arxiv=2205.03934 |bibcode=2022A&A...664A..96Z |s2cid=248571745 }}</ref>

===Octuple===
* [[Gamma Cassiopeiae]]<ref name=Hutter>{{cite journal|doi=10.3847/1538-4365/ac23cb|title=Surveying the Bright Stars by Optical Interferometry. III. A Magnitude-limited Multiplicity Survey of Classical Be Stars|year=2021|last1=Hutter|first1=D. J.|last2=Tycner|first2=C.|last3=Zavala|first3=R. T.|last4=Benson|first4=J. A.|last5=Hummel|first5=C. A.|last6=Zirm|first6=H.|journal=The Astrophysical Journal Supplement Series|volume=257|issue=2|page=69|arxiv=2109.06839|bibcode=2021ApJS..257...69H|s2cid=237503492 |doi-access=free }}</ref>

===Nonuple===
* [[QZ Carinae]]<ref>{{cite journal|doi=10.1051/0004-6361/202142108|title=Towards a consistent model of the hot quadruple system HD 93206 = QZ Carinæ — I. Observations and their initial analyses |year=2022 |last1=Mayer |first1=P. |last2=Harmanec |first2=P. |last3=Zasche |first3=P. |last4=Brož |first4=M. |last5=Catalan-Hurtado |first5=R. |last6=Barlow |first6=B. N. |last7=Frondorf |first7=W. |last8=Wolf |first8=M. |last9=Drechsel |first9=H. |last10=Chini |first10=R. |last11=Nasseri |first11=A. |last12=Pigulski |first12=A. |last13=Labadie-Bartz |first13=J. |last14=Christie |first14=G. W. |last15=Walker |first15=W. S. G. |last16=Blackford |first16=M. |last17=Blane |first17=D. |last18=Henden |first18=A. A. |last19=Bohlsen |first19=T. |last20=Božić |first20=H. |last21=Jonák |first21=J. |journal=Astronomy & Astrophysics |volume=666 |pages=A23 |s2cid=248177961 |bibcode=2022A&A...666A..23M |arxiv=2204.07045 }}</ref>

==See also==
* [[Exoplanet]]
* [[Solar System]]

==Footnotes==
{{notelist}}

==References==
{{reflist|25em}}

==External links==
{{Commons category|Multiple star systems}}
* {{APOD |date=11 September 2002 |title=Triple star system}}
* {{APOD |date=23 March 2003 |title=Alpha Centauri system}}
* [http://apod.oa.uj.edu.pl/apod/ap020425.html Alpha Centauri, APOD, 2002 April 25]
* [http://www.triplestarnews.com General news on triple star systems, TSN, 2008 April 22] {{Webarchive|url=https://web.archive.org/web/20190403194118/http://www.triplestarnews.com/ |date=3 April 2019 }}
* [http://ad.usno.navy.mil/wds/dsl.html The Double Star Library] {{Webarchive|url=https://web.archive.org/web/20081215092336/http://ad.usno.navy.mil/wds/dsl.html |date=15 December 2008 }} is located at the U.S. Naval Observatory
* [http://www.space.com/searchforlife/090319-seti-planet-nomenclature.html Naming New Extrasolar Planets ]

===Individual specimens===
* {{APOD |date=11 September 2002 |title=Triple star system}}
* {{APOD |date=23 March 2003 |title=Alpha Centauri system}}
* [http://apod.oa.uj.edu.pl/apod/ap020425.html Alpha Centauri, APOD, 2002 April 25]

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