An Etymological Dictionary of Astronomy and Astrophysics
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Mechanics: The condition of a moving mechanical system when the accelerating force is balanced by an imaginary kinetic reaction according to → d'Alembert's principle. See also → static equilibrium; → thermodynamic equilibrium.

Dynamic, adj. from → dynamics; → equilibrium.

Of a physical system, a condition in which the parts of the system are in continuous motion, but they move in opposing directions at equal rates so that the system as a whole remains in equilibrium.

→ dynamical; → equilibrium.

A state of balance or rest between the forces operating on or within a physical system. → stable equilibrium; → unstable equilibrium; → dynamical equilibrium.

From L. æquilibrium, from æquus, → equal + libra "a balance, scale."

Tarâzmandi, noun of tarâzmand "in equilibrium," from tarâz "level; a level" + possession suffix -mand. The first component from tarâzu "balance, scales," Mid.Pers. tarâzên-, taraênidan "to weigh;" Proto-Iranian *tarāz-, from *tarā- "balance, scale" (cf. Skt. tulā- "scales, balance, weight," from tul- "to weigh, make equal in weight, equal," tolayati "weighs, balances;" L. tollere "to raise;" Gk. talanton "balance, weight," Atlas "the Bearer" of Heaven;" Lith. tiltas "bridge;" PIE base telə- "to lift, weigh") + Av. az- "to convey, conduct, drive," azaiti drives" (cf. Skt. aj- "to dive, sling," ájati "drives," ajirá- "agile, quick;" Gk. agein "to lead, guide, drive, carry off;" L. agere "to do, set in motion, drive," from PIE root *ag- "to drive, move," → act).

A concept whereby chemical → concentrations among geological or environmental media are at equilibrium, and therefore the partitioning of metals in those media can be predicted based on → partition ratios.

→ equilibrium; → partitioning.

The position of an oscillating body at which no net force acts on it.

→ equilibrium; → position.

A state in which a → thermodynamic system is in → thermodynamic equilibrium.

→ equilibrium; → state.

The condition in a celestial body when gravitational forces acting on each point are balanced by some outward pressure, such as radiation pressure or electron degeneracy pressure, so that no vertical motion results.

→ gravitational; → equilibrium.

The state of a star when all its internal forces are in equilibrium. The main forces are gas pressure, radiation pressure due to thermonuclear fusion that tends to disrupt the star, and the opposing gravity. → hydrostatic equilibrium.

→ hydrodynamic; → equilibrium.

1) The physical situation reached in a fluid when complete balance exists between the internal pressure at any point and the weight of the material above the point.
2) For a star, the balance between the inward → gravitational force and the outward gas pressure everywhere in the star. → hydrostatic equation. See also → hydrodynamic equilibrium.

→ hydrostatic; → equilibrium.

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.

1) The state of a → rigid body if, as viewed from an → inertial frame of rest: 1) the → linear acceleration of its → center of mass is zero, and 2) its → angular acceleration about any axis fixed in this reference frame is zero. The center of mass may be moving with constant velocity and the body may be rotating about a fixed axis with constant angular velocity.
2) In → thermodynamics, the state of a system in which → pressure is the same every where with no other forces acting on the system except a uniform external pressure.

→ mechanical; → equilibrium.

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

→ non-; → local thermodynamic equilibrium.

In → nucleosynthesis models, the condition in which all nuclear species are in equilibrium under exchange of → protons and → neutrons. Matter in nuclear statistical equilibrium is characterized by a large abundance of iron peak nuclei. In such equilibria abundance of each nuclide can be calculated from repeated application of → Saha equation.

→ nuclear; → statistical; → equilibrium.

The condition of temperature and pressure under which different phases (e.g. gas, liquid, and solid) of a substance coexist.

→ phase; → equilibrium.

The balance between radiative emission and radiative absorption in a specified system.

→ radiative; → equilibrium.

An equilibrium state of a system in which if a small perturbation away from equilibrium is applied, the system will return to its equilibrium state. An example is a pendulum hanging straight down. If it is pushed slightly, it will experience a force back toward the equilibrium position. It may oscillate around the equilibrium position for a while, but it will finally regain its equilibrium position. → unstable equilibrium.

→ stable; → equilibrium.

The state of a rigid body which is not moving at all. The conditions for static equilibrium are: 1) the sum of the external forces on the object must equal zero, and 2) the sum of the → torques must equal zero. See also → dynamic equilibrium and → mechanical equilibrium.

→ static; → equilibrium.

A state in which the average density of atoms per cubic centimeter in any atomic state does not change with time and in which, statistically, energy is equally divided among all degrees of freedom if classical concepts prevail.

→ statistical; → equilibrium.

In thermodynamics, the state of a system all parts of which have attained a uniform temperature and no net heat exchange is taking place between it and its surroundings. If two bodies are in thermal equilibrium, they have the same temperature. Thermal equilibrium is the central criterion of the → zeroth law of thermodynamics. See also → local thermodynamic equilibrium (LTE).

→ thermal; → equilibrium.

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.