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Stellar dynamics is the branch of
astrophysicsAstrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
which describes in a statistical way the collective motions of
starA star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s subject to their mutual gravity. The long range of gravity and the slow "relaxation" of stellar systems prevent the use of the methods of
statistical physicsStatistical physics is the branch of physics that uses methods of probability theory and statistics, and particularly the mathematical tools for dealing with large populations and approximations, in solving physical problems. It can describe a wide variety of fields with an inherently stochastic...
. The motion of stars in a
galaxyA galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
or in a
globular clusterA globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
are principally determined by the average distribution of the other, distant stars, and little influenced by the nearest stars.
The "relaxation" of stars is the process deflecting the individual trajectories of stars from the one they would have if the distribution of matter was perfectly smooth. The "2body relaxation" is induced by the individual starstar interactions, while the "violent relaxation" is induced by a large collective variation of the stellar system shape.