1) Being in a state of action; not quiescent.
M.Fr. actif, from L. activus, from actus, p.p. of agere, → act.
Žirâ, adj. from stem žir, → act + suffix -â.
active galactic nucleus (AGN)
haste-ye kahkašân-e žirâ
Fr.: noyau actif de galaxie
A central region of an → active galaxy, which is a → light-year or less in diameter and has an abnormally high luminosity. The nucleus emits high energy radiation (→ gamma rays, → X-rays, → ultraviolet) and shows → variability over various time-scales, sometimes very short (hours to weeks). Emission line spectra reveal high velocity motions up to 104 km s-1. AGNs are divided into two main types. Type I refers to an AGN whose nucleus is visible (the spectra has both narrow and broad emission lines), while in type II AGN, the broad line region (BLR) is obscured and the lines are very narrow. This may be due either to the viewing angle or some intrinsic difference in structure. See also → broad-line region, → narrow-line region, → quasar.
Fr.: galaxie active
A galaxy that produces huge amounts of energy at its center, which cannot be attributed to normal processes from stars, interstellar medium, and their interactions. There are several types of active galaxies: → Seyfert galaxies, → quasars, and → blazars. All of these objects show brightness variations, some as short as 3 hours. These fluctuations indicate a relatively very small size for the central object, because an object cannot vary in brightness faster than light can travel across it. For example, an object that is one → light-year in diameter cannot vary significantly in brightness over a period of less than one year.
Fr.: optique active
A technique for improving the → resolving power of a telescope by controlling the shape of the main mirror at a relatively slow rate. The → image quality is optimized automatically through constant adjustments by in-built corrective → actuators operating at fairly low temporal frequency ~0.05 Hz or less. → adaptive optics.
Fr.: protubérance active
Fr.: région active
Fr.: soleil actif
The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum.
Having the quality of attracting.
Verbal adj. from → attract.
Fr.: force attractive
1) Acting one upon or with the other.
Fr.: optiquement actif
Relating to → optical activity.
Possessing, or pertaining to, → radioactivity.
sen yâbi-ye partow-žirâ
Fr.: datation radioactive
Determining the age of an object from the → radioactive decay of its constituting material. The technique consists of comparing the → abundance ratio of a → radioactive isotope to its → decay product. This will yield the number of half-lives that have occurred since the sample was formed. More specifically, if an object is made up of 50 % decay product then it has gone through 1 → half-life. 75% decay product equals 2 half-lives, 87.5% decay product equals 3 half-lives, 93.76% decay product equals 4 half-lives, and so on. For example, the decay product of → uranium-238 (238U) is → lead-206 (206Pb). The half-life of 238U is 4.5 billion years. Hence, if the sample has gone through two half-lives, it is 9 billion years old. See also: → radiocarbon dating.
Fr.: désintégration radioactive
Spontaneous emission by a nucleus of photons or particles.
Fr.: isotope radioactif
A → nuclide that is radioactive.
Fr.: nucléide radioactif
Fr.: déchets radioactifs
The radioactive by-products from the operation of a nuclear reactor or from the reprocessing of depleted nuclear fuel. Also known as nuclear waste.
Tending to react. Pertaining to or characterized by reaction.
From → react + -ive a suffix of adjectives expressing tendency, disposition, function, connection, etc.
Fr.: indice de réfraction
Of any optical medium, the ratio of the → speed of light in vacuum (c) to that in the medium (v): n = c/v. The refractive index for vacuum, by definition, is 1. The refractive index of air is 1.00029 at standard temperature (25 °C) and pressure (1 atm). The refractive index of a medium depends on the wavelength of refracted wave. With light waves, n increases as the wavelength decreases. → Snell's law can be used to derive n. Same as → index of refraction.