An extremely rare radioactive chemical element; symbol Fr. Atomic number 87; atomic weight of most stable isotope 223; melting point about 27°C; boiling point about 677°C. Its most stable isotope (half-life about 22 minutes) occurs naturally, to a very limited extent, in uranium minerals. More than 30 other isotopes of francium are known; some are prepared by bombarding thorium with protons, deuterons, or alpha particles.
From France, where the French physicist Marguerite Perey (1919-1975) discovered it in 1939 in the alpha particle decay of actinium.
Fr.: raie de Fraunhofer
One of many absorption lines and bands in the spectrum of the Sun. The most prominent features are labeled with capital letters A to K, starting at the red end. The A and B bands are now known to be caused by absorption in Earth's atmosphere, while the rest are due to absorption in the Sun's → photosphere. C and F are now better known as H-alpha and H-beta (→ Balmer series); the → D lines are of sodium, the → H and K lines of calcium, and the G band of neutral iron and the interstellar → CH (methylidine) molecule.
Named after Joseph von Fraunhofer (1787-1826), German optician and physicist, who discovered these lines in 1814; → line.
Fr.: réfracteur de Fraunhofer
The first modern refracting telescope which had an outstanding quality. It was built in 1824 by Fraunhofer for the Russian Imperial Observatory in Dorpat, now Tartu in Estonia. It had a 23-cm → achromatic lens and a German-type → equatorial mounting driven by a clockwork. Wilhelm Struve (1793-1864) used the refractor to observe many → visual binaries, and attempted to measure the distances of stars through their visual → parallaxes. He also obtaibned accurate values for the diameters of the → Galilean satellites of → Jupiter.
Named after Joseph von Fraunhofer (1787-1826), German optician and physicist; → refractor.
Not physically bound by something.
From M.E. fre, O.E. freo "free, exempt from," also "noble, joyful;" cf. Ger. frei, Du. vrij; ultimately from PIE *prijos "dear, beloved;" cf. Av. frāy- "to satisfy, propitiate," friθa- "beloved; dear," friθô.tara- "more beloved," Mod.Pers. fari "happy, fortunate, blessed; pleasing, good," Skt. priyá- "beloved, wished for;" Gk. praos "mild, gentle."
Âzâd "free," from Mid.Pers. âzât "free, noble;" Av. āzāta- "high-born, noble," from zan- "to bear, give birth to a child, be born," infinitive zazāite, zāta- "born," āsna- "innate, natural;" cf. Skt. janati "begets, bears;" Gk. gignesthai "to become, happen," genes "born;" L. gignere "to beget;" PIE base *gen- "to give birth, beget."
javv-e âzâd, havâsepehr-e ~
Fr.: atmosphère libre
That part of the atmosphere where the effects of the ground on the → turbulence conditions are negligible.
jesm-e âzâd (#)
Fr.: corps libre
Fr.: charge libre
An electric charge which is not held by another charge, in contrast to a → bound charge.
elektron-e âzâd (#)
Fr.: électron libre
free expansion phase
fâz-e sopâneš-e âzâd
Fr.: phase d'expansion libre
The first phase of → supernova remnant (SNR) evolution in which the surrounding → interstellar medium (ISM) has no influence on the expansion of the → shock wave, and the pressure of the interstellar gas is negligible. The shock wave created by the → supernova explosion moves outward into the interstellar gas at highly → supersonic speed. Assuming that most of the → supernova energy ESN is transformed into → kinetic energy of the ejected gas, the ejection velocity ve can be estimated from ESN by using ESN = (1/2) Meve2, which leads to ve = (2ESN / Me)(1/2), where Me is the ejected mass. The schematic structure of the SNR at this phase can be described as follows: behind the strong → shock front which moves outward into the ISM, compressed interstellar gas accumulates forming a → shell of interstellar gas. This shell of swept-up material in front of shock does not represent a significant increase in the mass of the system. After some time the accumulated mass equals the ejected mass of stellar material, and it will start to affect the expansion of the SNR. By definition, this is the end of the free expansion phase, and the corresponding radius of the SNR, called → sweep-up radius, RSW, is defined by Me = (4π/3) RSW3ρ0, that is RSW = (3Me / 4πρ0)(1/3), where ρ0 is the initial density of the ISM. This radius is reached at the sweep-up time tSW = RSW/ve. The free expansion phase lasts some 100-200 years until the mass of the material swept up by the shock wave exceeds the mass of the ejected material. Then the following → snowplow phase starts.
oft-e âzâd (#)
Fr.: chute libre
Fr.: écoulement libre
A fluid flow which develops when density differences within the fluid are the only driving forces. See also → forced flow.
Fr.: occurrence libre
Fr.: oscillation libre
Oscillation of any system in stable equilibrium under the influence of internal forces only, or of a constant force originating outside the system, or of both.
Fr.: radical libre
A chemical radical that can exist independently from atoms or group of atoms.
Fr.: système libre
Fr.: émission libre-liée
zamân-e oft-e âzâd
Fr.: temps de chute libre
The characteristic time it would take a body to collapse under its own → gravitational attraction, if no other forces existed to oppose the collapse. It is given by: tff = (3π/32 ρ0 G)1/2, where ρ0 denotes the initial density and G the → gravitational constant. Free-fall time is independent of the starting radius. Also known as → dynamical time scale.
Fr.: objets flottants
A population of → substellar objects which are not bound to stars; they are detected in young star clusters. Their masses, estimated from their fluxes, is several Jupiter masses, lower than those of → brown dwarfs. Their formation is not yet explained. Among the envisaged possibilities: 1) These objects form like stars, from protostellar core collapse and subsequent accretion; 2) they form as low-mass members of small groups, and are ejected from the group; 3) they form like planets within circumstellar disks of higher-mass objects, but are ejected either due to internal dynamics or external interactions.
Barâxt, → object; šenâvar "that swims, floats," from šenâ "swimming;" Mid.Pers. šnâz "swim," šnâzidan "to swim;" Av. snā- "to wash, swim;" cf. Skt. snā- "to bathe, to wash;" L. nare, natare "to swim" (Fr. nage, nager, natation; Sp.nadar, natacion).
gosil-e âzâd-âzâd (#)
Fr.: emission libre-libre
The state of being free or at liberty.
From M.E. fredom, from O.E. freodom, from freo "free; noble, joyful," → free.
Âzâdi "freedom," noun from âzâd, → free.