An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) A → profile representing the → density of a quantity.
2) A → profile representing the distribution of stars as a function of their number in a region.

→ density; → profile.

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.

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.
As the density waves rotate, they are overtaken by the individual stars and nebulae/molecular clouds that are rotating around the galaxy at a higher rate. The molecular clouds passing through the density wave are subjected to compression because it is a region of higher density. This triggers the formation of clusters of new stars, which continue to move through the density wave.
The short-lived stars die, most likely as supernovae, before they can leave the spiral density wave. But the longer-lived stars that are formed pass through the density wave and eventually emerge on its front side and continue on their way as a slowly dissipating cluster of stars. Density wave theory explains much of the spiral structure that we see, but there are some problems. First, computer simulations with density waves tend to produce very orderly "grand design" spirals with a well-defined, wrapped 2-arm structure. But there are many spiral galaxies that have a more complex structure than this (→ flocculent spiral galaxy). Second, density wave theory assumes the existence of spiral density waves and then explores the consequences.
See also: → stochastic self-propagating star formation.

→ density; → wave; → theory.

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.

→ density; → bounded; → region.

To state that (something declared or believed to be true) is not true (Dictionary.com).

M.E. denien, from O.Fr. denoiir "deny, repudiate, withhold," from L. denegare "to deny, reject, refuse," from → de- "away" + negare "refuse, say no," from Old L. nec "not," from PIE root *ne- "no, not."

Niyârdan, literally "to bring (say) no," from ni-, variant of na→ not, → non-, → un-, + ârdan contraction of âvardan "to bring; to cause or produce," → production.

1) General: Determined or conditioned by something else.
2) Math.: A variable whose value depends on the value assigned to another variable.
3) Statistics: An event which is not independent.

M.E. dependant, from M.Fr., pr.p. of dépendre, from L. dependere, from → de- + pendere "to hang, be suspended," PIE base *(s)pen(d)- "to pull, stretch."

Vâbasté, from vâ-→ de- + basté p.p. of bastan "to bind, shut," from Mid.Pers. bastan/vastan "to bind, shut," Av./O.Pers. band- "to bind, fetter," banda- "band, tie," cf. Skt. bandh- "to bind, tie, fasten," PIE *bhendh- "to bind" (Ger. binden, E. bind).

Math.: A variable whose value depends on the value assigned to another value. For example, in the equation y = 2x, the value of y depends on that of x. See also → independent variable.

→ dependent; → variable.

To make two signals out of phase. For example, to get one signal at its highest peak while the other signal is at its lowest peak; they will be 180 degrees out of phase.

→ de-; → phase.

Same as → out of phase.

Past participle of → dephase.

To decrease markedly the supply or abundance of; exhaust; to cause → depletion.

From L.L. depletionem "blood-letting," from L. deplere "to un-fill, to empty out," from → de- "off, away" + plere "to fill," from plenus "full," from PIE *ple- "to be full," PIE base *pelu- "full," cf. O.Pers. paru- "much, many," Av. parav-, pauru-, pouru-, par- "to fill," Mod.Pers. por "full," Skt. puru-, Gk. polus. por "full," Gk. polus, O.E. full "full," from P.Gmc. *fullaz, O.H.G. fol, Ger. voll, Goth. full.

Tisâyidan, from Tabari tisâ "empty, naked, bare" + -idan infinitive suffix. Variants of tisâ in dialects and literary Pers.: Saraxsi, Lâsgardi, Sangesari tusâ "empty, naked," Aftari tussâ "empty," literary Pers. tohi "empty," Mid.Pers. tuhig, Av. taoš- "to become empty," pres. tusa-, caus. taošaya-, tusən "they lose their posture," Skt. tuccha-, tucchya- "empty," L. tesqua, tesca "deserted place," Russian tošcij "hollow;" PIE base *teus- "to empty."

General: The act or process of depleting. The state of being depleted.
Astro.: The process or state of a material which contains less of a particular chemical element or isotope than the expected standard amount.

Noun from → deplete

The region of a semiconductor in which the density of mobile carriers is too low to neutralize the fixed charge density of donors and acceptors.

→ depletion; → layer.

The time it takes for a given → chemical species to be reduced below a significant level of → abundance in a → compound or → object.

→ depletion; → time.

Reducing or randomizing the polarization of an electromagnetic wave, by transmission through a non-homogeneous medium or a depolarizer.

from → de- + → polarization.

To remove or reduce the population of, as by destruction or expulsion (Dictionary.com).

→ de-; → populate.

To expel forcibly a foreigner from a country; banish.

→ de-; → export.

The action of deporting an alien from a country.

→ deport; → -tion.

1a) Something precipitated, delivered and left, thrown down, or accumulated, as by a natural process.
1b) Substance which settles down from a solution or a suspension, such as the natural sediment of wine in a bottle. See also → sediment.
2) To leave or form a layer of some substance (sand, sediment, etc.) as a gradual process in one place, or be left in this way.

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."

An act of attacking or plundering.

→ de-; → predation.