An Etymological Dictionary of Astronomy and Astrophysics
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The three basic laws of → dynamics which were first formulated by Isaac Newton in his classical work "Mathematical Principles of Natural Philosophy" published in 1687. → Newton's first law of motion; → Newton's second law of motion; → Newton's third law of motion.

→ law; → dynamics.

The assumption that all distribution functions characterizing the material and its interaction with the radiation field at a point in the star are given by → thermodynamic equilibrium relations at local values of the temperature and density.

→ local; → thermodynamic; → equilibrium.

Of or relating to → magnetohydrodynamics.

→ magneto- + → hydrodynamic.

The dynamics of an ionized plasma in the non-relativistic, collisional case. In this description, charge oscillations and high frequency electromagnetic waves are neglected. It is an important field of astrophysics since plasma is one of the commonest forms of matter in the Universe, occurring in stars, planetary magnetospheres, and interplanetary and interstellar space.

From → magneto- + → hydrodynamics.

A modification of the Newton's law of gravitation below a critical acceleration of about 1.2 x 10^-8 cm s^-2, where the gravitational force scales as 1/r instead of 1/r². Originally put forward to describe the rotation curves of galaxies with no need to assume any dark matter, MOND is now tested at larger cosmological scales (Milgrom, M. 1983, ApJ, 270, 365).

→ modify; → Newtonian dynamics.

A → magnetohydrodynamics approach dealing with → plasmas which is an improvement with respect to → ideal magnetohydrodynamics. Non-ideal magnetohydrodynamics allows for a drift between particles, redistributing the → magnetic flux and acting on both the → angular momentum and magnetic flux conservation issues.

→ non-→ ideal; → magnetohydrodynamics.

A physical condition in which the assumption of the → local thermodynamic equilibrium does not hold.

→ non-; → local thermodynamic equilibrium.

Same as → chaos.

→ nonlinear; → dynamics.

A → magnetohydrodynamics treatment in which the → plasma consists only of one particle species and moves with the bulk speed. The thermal motion of the particles is neglected and thus there is no motion of particles relative to each other.

→ one; → fluid; → magnetohydrodynamics.

The → quantum field theory that deals with the → strong interaction and the structure of elementary particles in the framework of → quantum theory. The cohesive attraction between the → quarks, that constitute → hadrons, involves the participation of three particles. Each of these particles is assigned a different → color "charge." The existence of these "charges" requires a multiplicity of different messenger particles to communicate the interaction and glue the quarks together. These messengers are called → gluons and there are eight different types.

→ quantum; → chromodynamics

The → quantum field theory that describes the properties of → electromagnetic radiation and its interaction with electrically charged matter in the framework of → quantum theory.

→ quantum; → electrodynamics.

A → dynamical variable which does not have an → imaginary number part.

→ real; → dynamical; → variable.

1) Heat cannot be transferred from a colder to a hotter body without some other effect, i.e. without → work being done. Expressed in terms of → entropy: the entropy of an → isolated system tends toward a maximum and its available energy tends toward a minimum.
2) In language of → statistical physics, an isolated physical system will tend toward an equilibrium → macrostate with as large a total → entropy as possible, because then the number of → microstates is the largest. See also → Kelvin's postulate, → Clausius's postulate.

→ second; → law; → thermodynamics.

A numerical method for modeling → compressible hydrodynamic flows, which uses particles to simulate a continuous fluid flow. Because the system of hydrodynamical basic equations can be analytically solved only for few exceptional cases, the SPH method provides a numerical algorithm to solve systems of coupled → partial differential equations for continuous field quantities. The main advantage of the method is that it does not require a computational grid to calculate spatial → derivatives and that it is a Lagrangian method, which automatically focuses attention on fluid elements. The equations of motion and continuity are expressed in terms of ordinary differential equations where the body forces become classical forces between particles. This method was first independently developed by Lucy (1977, AJ 82, 1013) and Gingold & Monaghan (1977, MNRAS 181, 375).

Smoothed Particle Hydrodynamics, first used by Gingold & Monaghan (1977); → smooth; → particle; → hydrodynamics.

Same as → statistical mechanics.

→ statistical; → thermodynamics.

The field of astrophysics that describes systems of many → point mass particles whose mutual gravitational interactions determine their orbits. Theses systems include → star clusters, → globular clusters, and galaxies (→ galaxy) consisting of about 10²-10³, 10⁴-10⁶, and up to about 10¹² members respectively. Stellar dynamics deals with systems in which each member contributes importantly to the overall gravitational field and is usually concerned with the statistical properties of many orbits. It can be compared to the → kinetic theory of gases developed in the late 19th century. In contrast, → celestial mechanics deals with systems where the gravitational force of a massive planet or star determines the orbits of its satellites.

→ stellar; → dynamics.

A uniform atomic time scale for apparent geocentric ephemerides defined by a 1979 IAU resolution, which replaced Ephemeris Time. TDT is independent of the variable rotation of the Earth, and the length of the tropical year is defined in days of 86,400 seconds of international atomic time. In 1991 it was replaced by Terrestrial Time.

→ terrestrial; → dynamical; → time.

Of or pertaining to → thermodynamics.

→ thermo-; → dynamic.

The condition of a → thermodynamic system in which the available → energy is distributed uniformly among all the possible forms of energy. Furthermore, all → thermodynamic process es must be exactly balanced by their reverse processes. For example, inside a star there will be as many → ionizations of helium per second as there are → recombinations of free electrons and helium ions. Se also → local thermodynamic equilibrium (LTE).

→ thermodynamic; → equilibrium.

The loci of various changes between two → states through which a → thermodynamic system passes during a → thermodynamic process.

→ thermodynamic; → path.