Star 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.

[7] For example, in the 1999 revision of Tokovinin's catalog[3] of physical multiple stars, 551 out of the 728 systems described are triple.

[11] Each level of the hierarchy can be treated as a two-body problem by considering close pairs as if they were a single star.

Triple stars that are not all gravitationally bound 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).

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.

[24][25] They are named after the multiple star system known as the Trapezium Cluster in the heart of the Orion Nebula.

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.

A. Tokovinin's Multiple Star Catalogue uses a system in which each subsystem in a mobile diagram is encoded by a sequence of digits.

[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 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.[31] Discussion starting in 1999 resulted in four proposed schemes to address this problem:[31] For a designation system, identifying the hierarchy within the system has the advantage that it makes identifying subsystems and computing their properties easier.

[31] A sample of a catalog using the WMC scheme, covering half an hour of right ascension, was later prepared.

[36] 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.

[37] The sample WMC is hierarchically organized; the hierarchy used is based on observed orbital periods or separations.

Subsequent levels would use alternating lower-case letters and numbers, but no examples of this were found in the sample.