Star system
This article is about astronomy. For the Hollywood star system, see star system (film).A
star system or
stellar system is a
system comprised of a
star or group of stars, and, perhaps,
planetary systems of smaller bodies (such as
planets or
asteroids), in
gravitational association. The
solar system is, properly, the star system comprised of our
Sun and other bodies, such as the
Earth, in orbit around it. (The word
solar is taken from
Sol, the
Latin word for "Sun.")
A star system of two stars is known as a
physical double star,
binary star, or
binary star system. 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
center of mass of the systemindefinitely. See
Two-body problem.
Systems with more than two starsare also possible; for example, a
star cluster or
galaxy is a kind of star system. Because of the large size of these systems,their
dynamics are much more complicated than that of the binary star.However, it is also possible to have star systems with a small number (greater than two) of stars and simple orbital dynamics. These systems are called
multiple star systems, or
physical multiple stars.
Nomenclature
Multiple star systems or physical multiple stars are called
triple,
trinaryor
ternary if they contain three stars;
quadruple or
quaternary if theycontain four stars;
quintuple with five stars;
sextuple with six stars;
septuple with seven stars;and so on.
Dynamical theory
Theoretically, modelling a multiple star system is more difficult than modelling a binary star, as the
dynamical system involved, the
n-body problem,may exhibit
chaotic behavior.Many configurations of small groups of stars are found to be unstable, as eventually onestar will approach another closely and be accelerated so much that it will escape fromthe system.
[Multiple Stellar Systems: Types and Stability, Peter J. T. Leonard, in Encyclopedia of Astronomy and Astrophysics, P. Murdin, ed., online edition at the Institute of Physics, orig. ed. published by Nature Publishing Group, 2001.] This instability can be avoided if the system is what Evans
[Stars of Higher Multiplicity, David S. Evans, Quarterly Journal of the Royal Astronomical Society 9 (1968), 388–400.] hascalled
hierarchical. In a hierarchical system, the stars in the system can bedivided into two smaller groups, each of which traverses a larger orbit around thesystem'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 arehierarchical, and so on.
Triple star systems
Triple systems are by far the most common type of multiple system. For example, in the 1999 revision ofTokovinin's catalog
[MSC—a catalogue of physical multiple stars, A. A. Tokovinin, Astronomy and Astrophysics Supplement Series 124 (1997), 75–84; online versions at VizieR and the Multiple Star Catalog.] of physical multiple stars, 551 out of the 728 systems described are triple. In accordance with the hierarchical principle,triple star systems generally contain a close binary pair which has a more distant companion.
Higher multiplicities
Many systems with more than three stars are known to exist.
Nu Scorpii containsat least seven stars.
Some star systems are:
* Our own
solar system, with the
Sun at its centre (one star)
*
Sirius (two stars)
*
Alpha Centauri (three stars) [
debated - see Proxima Centauri]
*
4 Centauri (four stars)
*
Mizar (five stars)
*
Castor (six stars)
*
Nu Scorpii (seven stars)
*
Cygnus X-1 (one star and one
black hole)
*
Solar system*
Binary star*
Star cluster*
Triple star*
Planetary system*
Galaxy
