Editing Star system (section)

====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>