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density wave theory
negare-ye mowj-e cagâli
Fr.: théorie des ondes de densité
One possible explanation for → spiral arms,
first put forward by B. Lindblad in about 1925 and developed later by
C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material
structures, but regions of somewhat enhanced density, created by
→ density waves. Density waves are perturbations amplified by
the self-gravity of
the → galactic disk. The perturbation results from natural
non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters.
Density waves rotate around the → galactic center and periodically
compress the disk material upon their passage. If the spiral arms were
rigid structures rotating like a pinwheel,
the → differential rotation
of the galaxy would wind up the arms completely in a relatively
short time (with respect to the age of the galaxy), → winding problem.
Inside the region defined by the → corotation radius,
density waves rotate more slowly than the galaxy's stars and gas; outside that
region they rotate faster.
density-bounded H II region
nâhiye-ye H II-ye cagâli karânmand
Fr.: bornée par la densité
An → H II region which lacks enough matter to absorb all → Lyman continuum photons of the → exciting star(s). In such an H II region a part of the ionizing photons escape into the → interstellar medium. See also → ionization-bounded H II region.
1) lerd; 2) lerd andâxtan, lerd oftâdan, lerdidan
Fr.: 1) dépôt, gisement, lie, tarte; 2) déposer, se déposer
1a) Something precipitated, delivered and left, thrown down, or accumulated,
as by a natural process.
From L. depositus, p.p. of deponere "to lay aside, put down," from → de- "away" + ponere "to put," → position.
Lerd "sediment, tartar of wine," probably a variant of dord "dregs, lees, sediment, tartar of wine."
The process by which water vapor changes directly to ice without first becoming a liquid. This is how snow forms in clouds, as well as frost and hoar frost on the ground. The opposite of deposition is → sublimation. → condensation.
M.E., from O.Fr. deposition, from L. deposition- "putting aside, testimony, burial," from deposit(us) "laid down," p.p. of deponere "to put down," from → de- + ponere "to put, place."
Vâneheš, from vâ-→ de- + neheš verbal noun of nehâdan "to put, place," Mid.Pers. nihâtan, O.Pers./Av. ni- "down; into," → ni-, + dā- "to put; to establish; to give," dadāiti "he gives," cf. Skt. dadâti "he gives," Gk. didomi "I give," L. do "I give;" PIE base *do- "to give."
Fr.: transition discrète
A transition between two quantum-mechanical energy levels. See also → discrete spectrum.
→ discrete; → transition.
The state or fact of being diverse; difference; unlikeness.
Fr.: viscosité dynamique
Same as → viscosity and → absolute viscosity.
Fr.: luminosité d'Eddington
Same as → Eddington limit.
→ Eddington limit; → luminosity.
Einstein-de Sitter effect
oskar-e Einstein-de Sitter
Fr.: effet Einstein-de Sitter
Same as → geodetic precession.
Einstein-de Sitter Universe
giti-ye Einstein-de Sitter
Fr.: Univers Einstein-de Sitter
The → Friedmann-Lemaitre model of → expanding Universe that only contains matter and in which space is → Euclidean (ΩM > 0, ΩR = 0, ΩΛ = 0, k = 0). The Universe will expand at a decreasing rate for ever.
→ Einstein; de Sitter, after the Dutch mathematician and physicist Willem de Sitter (1872-1934) who worked out the model in 1917; → Universe.
dartanuyi-e barqi (#)
Fr.: intensité électrique
The strength of an electric field at any point as measured by the force exerted upon a unit positive charge placed at that point.
cagâli-ye elektroni (#)
Fr.: densité électronique
The number of electrons per unit volume in an ionized medium, like an → H II region, as determined from → emission lines.
joft-e elektron-pozitron (#)
Fr.: paire électron-positron
The simultaneous formation of an → electron and a → positron in the → pair production process.
Fr.: transition électronique
The → transfer of an → electron from one → energy level to another.
→ electronic; → transition.
Fr.: densité d'énergie
The amount of energy in the form of radiation per unit volume, expressed in ergs cm-3. In particular, the energy density of blackbody radiation at temperature T is aT4, where the radiation constant a = 7.56 × 10-15 erg cm-3 (K)-4.
Fr.: transit au méridien des éphémérides
The passage of a celestial body or point across the → ephemeris meridian.
naheš-e tarâzmandi (#)
Fr.: position d'équilibre
The position of an oscillating body at which no net force acts on it.
→ equilibrium; → position.
Complete set of points in any given space group which are obtained by performing the symmetry operations of the space group on a single point (x, y, z).
→ equivalent; → position.
Fr.: transit d'exoplanète
The passage of an → exoplanet across the face its star.
→ exoplanetary; → transit.
1) The act of setting forth information or a viewpoint.
Verbal noun of → expose.
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