An Etymological Dictionary of Astronomy and Astrophysics
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1) The mental process in which an element or quality is separated from a total object. Also the result of this process.
2) Chemistry: → abstraction reaction.

Verbal noun from → abstract; → -tion.

Chemistry: A bimolecular chemical reaction that involves removal of an atom or ion from a molecule. For example, hydrogen abstraction from methane: CH₄ + Cl → CH₃ + HCl.

→ abstraction; → reaction.

1) The process or state of acting or of being active.
2) According to → Newton's third law of motion, an external force that is applied to a body and that is counteracted by an equal force in the opposite direction ( → reaction).
3) A quantity whose → dimension (ML²T^-1) coincides with that of → angular momentum, the → impulse of a force, or → energy x → time. The action plays an important part in → analytical mechanics, → quantum mechanics, and in a number of other fields of physics. Initially introduced in analytical mechanics, the concept of action has become a basic ingredient of modern physics, due to the role it has played in the generalization of → variational principle.
4) A scalar quantity computed as a function of the path followed by a system during its evolution between an initial instant t_i and a final instant t_f. It is defined by the → integral of the → Lagrangian between the two instants:
S = ∫L dt
In the framework of the → field theory, the action is expressed by the integral of the → Lagrangian density over the corresponding space-time volume:
S = ∫L_d d⁴x.
In classical physics, the path actually followed by the system is the one for which S is stationary (→ least action problem).
5) → quantum of action.
6) Math.: The action is a → functional, a mathematical relationship which takes an entire path and produces a single number.

Action, from O.Fr. action, from L. actionem, from agere "to do," → act.

Žireš, verbal noun from žir stem of žiridan "to act;" → act. Koneš, noun from kardan "to do, to make," Mid.Pers. kardan, O.Pers./Av. kar- "to do, make, build," Av. kərənaoiti "makes," cf. Skt. kr- "to do, to make," krnoti "makes," karma "act, deed;" PIE base k^wer- "to do, to make."

The instantaneous action of a body on another body independently of the distance separating them. The description of → gravity by → Newton's law and → electrostatics by → Coulomb's law are examples of action at a distance. According to Newton, → gravitation acts directly and instantaneously between two objects. For example, if the Sun should suddenly break apart, the Earth's orbit would be affected instantaneously. However, action at a distance violates the → principle of relativistic causality. According to → general relativity, gravitational effects travel at the → speed of light. For modern physics there is no instantaneous action at a distance.

→ action; → distance.

The time integral associated with the evolution of a physical system in the phase space.

→ action; → variable.

The angle between the direction in which a ray is refracted and the normal to the refracting surface.

→ angle; → refraction.

The → angular → displacement of a point on the → celestial sphere due to the Earth's → atmospheric refraction.

→ astronomical; → refraction.

The shift in apparent direction of a celestial object caused by the bending of light while passing through the Earth's atmosphere. Since the density of the atmosphere decreases with altitude, the starlight will bend more as it continues down through the atmosphere. As a result, a star will appear higher in the sky than its true direction.

→ atmospheric; → refraction.

The act or capability of attracting. A physical force (gravitational, electric, magnetic, etc.) exerted by material bodies.

Attraction, n. from → attract.

The ability of a → liquid to → flow in a → narrow space, such as a thin → tube, without the assistance of, and in opposition to, external forces like → gravity. Also called → capillarity. It occurs because of intermolecular → attractive forces between the liquid and solid surrounding surfaces. If the diameter of the tube is sufficiently small, then the combination of → surface tension (which is caused by → cohesion within the liquid) and → adhesion (between the liquid and the → container) acts to lift the liquid. The capillarity of the liquid is high when adhesion is greater than cohesion. For example, water in a thin glass tube has strong → adhesive forces due to the hydrogen bonds that form between the water molecules and the oxygen atoms in the glass wall (made of → silica, SiO₂). In contrast, mercury is characterized by stronger cohesion, and hence its capillarity is much lower.

→ capillary; → action.

A point toward which a force on a body is always directed.

→ center; → attraction.

A succession of → nuclear fissions when the neutrons released by previous fissions produce other nuclear fissions which themselves cause other reactions and the reactions goes on increasing exponentially.

→ chain; → reaction.

A → change or → transformation in which a → substance → decomposes, → combines with other → substances, or interchanges constituents with other substances.

→ chemical; → reaction.

A fraction written as a/b where a and b are → positive  → integers, as opposed to a → decimal fraction; for example, 5/7. Common fractions are sometimes also called → vulgar fractions.

→ common; → fraction.

1) The act of compacting or the state of being compacted.
2) Geology: The physical process whereby the volume of a fixed mass of sediment reduces either by the weight of overlying deposits or by the pressure of earth movements.

→ compact; → -tion.

A fraction in which the → numerator or → denominator, or both, contain fractions. For example (3/5)/(6/7). Also called → compound fraction.

→ complex; → fraction.

Same as → complex fraction.

→ compound; → fraction.

In mathematics, a fraction whose numerator is an integer and whose denominator is an integer plus a fraction whose numerator is an integer and whose denominator is an integer plus a fraction and so on.

→ continued; → fraction.

An act or instance of contracting; the quality or state of being contracted.
→ gravitational contraction, → Kelvin-Helmholtz contraction, → length contraction, → Lorentz contraction.

Verbal noun of → contract.

A spiral-shaped density enhancement formed around a star when fast stellar winds collide with slower material. This large-scale wind structure can extend from the stellar surface to possibly several tens of stellar radii. The CIRs can be produced by intensity irregularities at the stellar surface, such as dark and bright spots, magnetic loops and fields, or non-radial pulsations. The surface intensity variations alter the radiative wind acceleration locally, which creates streams of faster and slower wind material. CIRs are responsible for the → discrete absorption components seen in some ultraviolet → resonance lines of → hot stars (S. R. Cranmer & S. P. Owocki, 1996, ApJ 462, 469).

→ corotate; → interaction; → region.