From the chemical element → actinium.
A silver-white radioactive → chemical element; symbol Ac. The first member of the → actinide series of the → periodic table. → Atomic number 89; → atomic weight 227.0278; → melting point about 1,050°C; → boiling point 3,200°C ± 300°C; → specific gravity 10.07; → valence +3. It is found with uranium minerals in pitchblende. Its longest lived → isotope is 227Ac with a → half-life of 21.77 years.
From actin-, variant of actino-, from Gk. aktinos "ray, beam" + → -ium. The discovery of actinium is shared between two chemists who independently found the element. The earlier discovery was made by the French chemist André Debierne (1874-1949) in 1899 in pitchblende residues left after Pierre and Marie Curie had extracted → radium. The element was rediscovered in 1902 by the German chemist Friedrich Otto Giesel (1852-1927), who called it emanium.
Any instrument for measuring the intensity of radiation, especially that of the Sun, in its thermal, chemical, and luminous aspects.
Actinometer, from actino- combining form with the meaning "ray, beam," from Gk. aktis, aktin "ray," + → -meter.
žireš, koneš (#)
1) The process or state of acting or of being active.
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 kwer- "to do, to make."
action at a distance
žireš az dur
Fr.: action à distance
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.
Fr.: variable d'action
The time integral associated with the evolution of a physical system in the phase space.
1) To induce activity in a system that is static, as in neutron activation
Activate, verb from → active.
Žirândan, transitive verb from žir, → act.
1) The process of inducing or creating a state of → activity.
Fr.: énergie d'activation
Chemistry: The minimum amount of energy that is required to activate → atoms or → molecules to a condition in which they can undergo a → chemical reaction. Most reactions involving neutral molecules cannot take place at all until they have acquired the energy needed to stretch, bend, or otherwise distort one or more → bonds. In most cases, the activation energy is supplied by → thermal energy.
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.
1) The doctrine or practice of vigorous action or involvement as a
means of achieving political or other goals, sometimes by
demonstrations, protests, etc. (Dictionary.com). See also
An especially active, vigorous advocate of a cause, especially a political cause (Dictionary.com).
Agent noun from → activism.
For a radioactive substance, the average number of atoms disintegrating per unit time.
Activity, from → active + -ity.
Žirandegi, noun from žirandé, → active.
1) One who takes part; a participant.
Žirandé adj./noun from žir, → act + suffix -andé. Žirmand, from žir + mand.