Deneb (α Cygni)
Deneb "tail," from Ar. dhanab ad-dajajah (
Zanab, from Ar., as above.
Denebola (β Leonis)
Širdom, Zanab-ol-asad (#)
Denebola, from Ar. dhanab al-asad (
Fr.: dénégation, démenti
1) An assertion that something said, believed, alleged, etc., is false.
1) To give a specific name to.
Fr.: groupe, catégorie, unité; dénomination
1) A name or designation.
Verbal noun of → denominate.
The quantity y in a fraction x/y. The quantity x is the → numerator.
1) To condemn or censure openly or publicly.
Having relatively high → density.
From L. densus "thick, crowded," cognate with Gk. dasys "hairy, bushy, thick grown."
Cagâl "dense, thick," related to ceqer "stiff, hard, tough, firm" (dialectal Kermâni ceqel, Šândizi caqal), caqâlé "stiff, unripe fruit."
Fr.: coeur dense
An opaque region of a → molecular cloud (AV 10 mag) which is considered to be the progenitor of → star formation. Dense cores have temperatures of about 10 K and masses of roughly 1 to 10Msun each and in which the → molecular hydrogen density is roughly 104-105 cm-3 and size 0.1 pc. The → self-gravity of a dense core plays a central part in star formation. See also → hot molecular core.
dense core mass function
karyâ-ye jerm-e maqze-ye cagâl
Fr.: fonction de masse des cœurs denses
dense molecular cloud
abr-e molekuli-ye cagâl
Fr.: nuage moléculaire dense
A type of → interstellar medium cloud in which → carbon (C) becomes almost completely molecular due to relatively high → extinction. The chemistry is qualitatively different from that of → diffuse molecular clouds, as the → electron abundance is very low (→ cosmic-ray ionization being the dominant source) and the reactive C is replaced by the very stable → carbon monoxide (CO). This regime is found only in → sightlines with AV > 5-10 mag; not all such sightlines will contain dense cloud material and if dense cloud material is present it is likely to be surrounded by → translucent material. These clouds are typically → self-gravitating, and are most often observed by → infrared absorption and → millimeter wave emission methods. Their densities are typically at least 104 cm-3, and their → kinetic temperatures are typically on the order of 10-50 K in the quiescent regions. Most of the more than 140 currently known → interstellar molecules were found through observations of → microwave→ rotational transitions in such clouds, starting with the discovery of OH, followed by a host of other new detections such as CO, NH3, H2O, and H2CO (Snow & McCall, 2006, ARA&A 44, 367).
The amount of any quantity per unit volume. The mass density is the
mass per unit volume. The energy density is the energy per unit
volume; particle density is the number of particles per unit volume.
Noun form of → dense.
Fr.: cuspide de densité
A localized increase in number of → stellar black holes near a → supermassive black hole predicted by models of galactic → stellar dynamics (Bahcall, Wolf, 1976, ApJ, 209, 214). Same as → stellar cusp.
Fr.: fluctuations de densité
In the early Universe, localized enhancements in the density of either matter alone or matter and radiation. According to models, very small initial fluctuations (less than 1 percent) can lead to subsequent formation of galaxies.
density of an element
Fr.: densité d'élément
The number of units of mass of the → chemical element that are present in a certain volume of a medium. The density of an element depends on temperature and pressure. The element Osmium has the highest known density: 22.61 g/cc; in comparison gold is 19.32 g/cc and lead 11.35 g/cc.
Fr.: paramètre de densité
One of the four terms that describe an arranged version of the
→ Friedmann equations. They are all time dependent.
Fr.: profile de densité
mowj-e cagâli (#)
Fr.: onde de densité
A wave phenomenon in which the density fluctuations of a physical quantity propagates in a compressible medium. For example, the → spiral arms of a → galaxy are believed to be due to a density wave which results from the natural instability of the → galactic disk caused by its own gravitational force. A common example of a density wave concerns traffic flow. A slow-moving vehicle on a narrow two-lane road causes a high density of cars to pile up behind it. As it moves down the highway the "traffic density wave" moves slowly too. But the density wave of cars does not keep the same cars in it. Instead, the first cars leave the density wave when they pass the slow vehicle and continue on at a more normal speed and new ones are added as they approach the density wave from behind. Moreover, the speed with which the density wave moves is lower than the average speed of the traffic and that the density wave can persist well after its original cause is gone. See → density wave theory.
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.