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Star system

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).

This article is about astronomical objects. For the Hollywood star system, see Star system (filmmaking). "Multiple star" redirects here. For apparent doubles, see double star. Not to be confused with Exosolar system or Planetary system.

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 elliptical 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 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 , 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.[16]

Castor

The maximum hierarchy occurring in A. A. Tokovinin's Multiple Star Catalogue, as of 1999, is 4. 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).;[17]

[3]

Trapezia[edit]

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.[24][25] They are named after the multiple star system known as the Trapezium Cluster in the heart of the Orion Nebula.[24] 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.[26] 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.[26] This dynamic 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.[27][28]

Designations and nomenclature[edit]

Multiple star designations[edit]

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.[29][30] Components discovered close to an already known component may be assigned suffixes such as Aa, Ba, and so forth.[30]

a binary consisting of a main-sequence type A star and a white dwarf

Sirius

which is similar to Sirius

Procyon

a variable consisting of a red giant and a white dwarf

Mira

a Cepheid variable

Delta Cephei

an eclipsing binary

Almaaz

Spica

Exoplanet

Solar System

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

Archived 3 April 2019 at the Wayback Machine

General news on triple star systems, TSN, 2008 April 22

Archived 15 December 2008 at the Wayback Machine is located at the U.S. Naval Observatory

The Double Star Library

Naming New Extrasolar Planets