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

A physical process which allows a → molecular cloud to decouple from → interstellar magnetic field in order to undergo → gravitational collapse. A cloud of pure molecular gas would form stars very fast through collapse since neutral matter does not respond to the magnetic field. However, the magnetic field holds up a collapse because the ions present in the cloud collide with the neutrals and tie them to the field. The collapse can then only proceed if the magnetic field can be separated from the gas. In denser molecular cores the ionization degree decreases substantially and therefore neutrals and ions decouple.

→ ambipolar; → diffusion.

1) General: The act or result of amplifying, enlarging, or extending.
2) Physics: The process of increasing the magnitude of a variable quantity, especially the magnitude of voltage, power, or current, without altering any other quantity.

Verbal noun of → amplify.

1) Electronics: The extent to which an → analogue → amplifier boosts the strength of a → signal. Also called → gain.
2) In → gravitational lensing, the ratio of the lensed brightness to unlensed brightness. This factor depends on the mass of the → lensing object and the closeness of the alignment between observer, lens, and source (→ impact parameter).

→ amplification; → factor.

A function which can be represented by a convergent → power series.

→ analytic; → function.

The angle between the → incident ray of light entering an → optical system (such as a prism) and the → refracted ray that emerges from the system. Because of the different indices of refraction for the different wavelengths of visible light, the angle of deviation varies with wavelength.

→ angle; → deviation.

1) General: The angle between one plane and another, or the angle formed by a reference axis and a given line.
2) Binary systems: The angle between the plane of the orbit and the → plane of the sky.
3) Rotating stars: The angle between the → equatorial plane and the → plane of the sky.

→ angle; → inclination.

The angle between the light entering and exiting the prism when the light passing through the prism is parallel to the prism's base. Angle of minimum deviation (D) is used to measure the → index of refraction (n) of the prism glass, because: n = sin [(A + D)/2]/sin (A/2), where A is the → prism angle.

→ angle; → minimum; → deviation.

The angle between the reflected ray and the normal to the reflecting surface.

→ angle; → reflection.

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

→ angle; → refraction.

The rate of change of → angular velocity. It is equal to the → first derivative of the → angular velocity: α = dω/dt =d²θ/dt² = a_t/r, where θ is the angle rotated, a_t is the linear tangential acceleration, and r is the radius of circular path.

→ angular; → acceleration.

The rate of change of the angles of emergence θ of various wavelengths from a dispersing prism: dθ/dλ.

→ angular; → dispersion.

Of a telescope, the smallest angle betwwen two → point sources that produces distinct images. It depends on both the wavelength at which observations are made and on the diameter of the telescope. Same as → spatial resolution.

→ angular; → resolution.

Same as → angular distance.

→ angular; → separation.

1) An act or instance of animating or enlivening; the state or condition of being animated.
2) The technique of imaging successive drawings or positions of cartoons to create an illusion of movement when the film is shown as a sequence.

→ animate; → -tion.

A → negatively charged → ion or → radical. Anions in a liquid subjected to electric potential are attracted toward the → anode. See also → cation.

From Gk. anienai "to go up," from an-, from → ana- "up," + -i-, from ire, eo "to go, walk," cf. Pers. ây-, → assembly, + -on, as in → ion.

The process in which the entire → mass of → two colliding → particles, one of → matter and one of → antimatter, is → converted into → radiant energy in the form of → gamma rays. See also → annihilation operator.

L. annihilatus, p.p. of annihilare "to reduce to nothing," from ad- "to" + nihil "nothing," from ne- "not" + hilum "small thing, trifle"

Nâbudi, from nâ- "not" + bud "to be, exist," from budan "to be, exist" + -i noun forming suffix.

In → quantum field theory, the operator that lowers → eigenstates one → energy level, contrarily to the → creation operator.

→ annihilation; → operator.

The apparent, small displacement in position of a star during the year due to the → aberration of starlight. It depends on the → celestial latitude, and its maximum value is about 20''.50. See also → constant of aberration; → aberration orbit.

→ annual; → aberration.

→ annual motion.

→ annual; → apparent; → motion.

An irregularity in the Moon's orbit, which can amount to 11 degrees in a period of one year. It results from the Sun's disturbing effect on the motion of the Moon due to varying distance between them.

→ annual; → equation.