Editing Star system (section)

==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 />