An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A system in which there is no dissipation of energy so that the total energy remains unchanged with time.

→ conservative→ system.

Opposite of a → free system. See also → constraint.

Constrained, p.p. of → constrain; → system.

Math: A system for locating each point in space by a set of numbers.
Astro: Values in a reference system used to relate the position of a body on the celestial sphere. Four main coordinate systems are utilized in astronomy: the equatorial, horizontal, ecliptic, and galactic coordinates systems.

→ coordinate; → system.

A set of two or more mechanical vibrating systems connected so that they interact with one another.

→ coupled; → system;

One of seven possible basic crystal types that is defined by the relations between the axis lengths and angles of its unit cell. Crystal systems can produce an infinite → lattice by successive translations in three-dimensional space so that each lattice point has an identical environment. The seven crystal systems are: → cubic, → orthorhombic, tetragonal, trigonal, hexagonal, monoclinic, and triclinic.

→ crystal; → system.

A → crystal system whose three axes have equal lengths and all corners are 90°.

→ cubic; → crystal; → system.

A widely used scheme for classifying galaxies which is an extension to the → Hubble sequence Hubble sequence. De Vaucouleurs introduced a more elaborate classification system for spiral galaxies (→ spiral galaxy) based on three morphological characteristics:
1) Bars. Galaxies are divided on the basis of the presence or absence of a nuclear → stellar bar. SA denotes spiral galaxies without bars, complementing Hubble's use of SB for → barred spirals. De Vaucouleurs also defined an intermediate class, SAB, containing weakly barred spirals. → Lenticular galaxies are also classified as unbarred (SA0) or barred (SB0), with the notation S0 reserved for those galaxies for which it is impossible to tell if a bar is present or not.
2) Rings. Galaxies possessing ring-like structures are denoted (r), those without rings (s), and transition galaxies (rs).
3) Spiral arms. These are divided into the following subclasses: Sd (SBd), that is diffuse, broken arms made up of individual stellar clusters and nebulae; very faint central → bulge; Sm (SBm): irregular in appearance; no bulge component; and Im, that is highly irregular galaxy.
Galaxies in the classes Sm and Im are termed the → Magellanic spirals and irregulars, respectively. The → Large Magellanic Cloud is of type SBm, while the → Small Magellanic Cloud is an irregular (Im).

→ de Vaucouleurs radius; → system.

A system of numerals for representing real numbers that uses the → base 10. It includes the digits from 0 through 9.

→ decimal; → system.

same as → decimal number system.

→ decimal; → number; → system.

A binary system whose components are not in contact and in which no significant mass exchange is occurring.

P.p. of detach, from O.Fr. destachier (Fr. détacher), from des- "apart," + -tachier (as in atachier "to attach"); → system.

Râžmân→ system; jodâ "separate, distinct," Mid.Pers. jutâk (yutâk), jut (yut).

A system characterized by → differential rotation. In such a system the → angular velocity decreases as the distance from the rotation center increases.

→ differential; → rotating; → system.

A → dynamical system which undergoes energy → dissipation. Such a system gives rise to → irreversible processes, associated with a time-asymmetric evolution of observable quantities.

→ dissipative; → system.

A system of evolved → binary stars in which both → components have ejected their envelopes and evolve toward → white dwarf stage. So far a half dozen double-degenerate → binary systems are known, for example Henize 2-248 (M. Santander-Garcia et al., 2015, 518, 5).

→ double; → degenerate; → binary; → system.

A system composed of one or more entities in which one state develops into another state over the course of time.

→ dynamical; → system.

A period in the evolution of the → solar system when the planets and satellites were in the process of formation.

→ early; → solar; → system.

A physical system composed on the → Earth and the → Moon in which both objects directly influence each other. The total energy in the Earth-Moon system is conserved. The most notable influence that the two objects have on each other is → tides.
See also: → tidal braking, → tidal bulge, → tidal capture, → tidal coupling, → tidal disruption, → tidal force, → tidal friction, → tidal heating, → tidal locking, → tidal radius, → tidal stretching.

→ Earth; → Moon; → system.

Coordinate system with the ecliptic as the fundamental plane.

→ ecliptic; → system.

Any geographic area that includes all of the organisms and nonliving parts of their physical environment.

→ ecology; → system.

A set of celestial coordinates based on the celestial equator as the primary great circle. The coordinates are → declination and → right ascension.

→ equator; → system.

An astronomical → coordinate system for indicating the positions of → celestial objects on the → celestial sphere. The system consists of two components, → right ascension and → declination. Right ascension is the angle between the → vernal equinox and the point where the → hour circle intersects the → celestial equator. The right ascension is always measured eastward from the vernal equinox, in the units of hours, minutes, and seconds. Declination is the angle between the celestial equator and the position of the star measured along the star's hour circle. It is measured in degrees, minutes, and seconds north or south of the celestial equator. By definition, the vernal equinox is located at right ascension 0h and declination 0°. Equatorial coordinates change with time due to the → precession of the Earth's → rotation axis.

→ equatorial; → coordinate; → system.