An Etymological Dictionary of Astronomy and Astrophysics
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The brightest star, of magnitude V = 1.25, in the constellation → Cygnus. It is a → supergiant of → spectral type A2 Ia.

Deneb "tail," from Ar. dhanab ad-dajajah (ذنب الدجاجه) "tail of the hen," referring to the legendary swan (Cygnus) in Gk. mythology.

Zanab, from Ar., as above.

The second brightest star, of magnitude V = 2.14, in the constellation → Leo. A → main sequence star of → spectral type A3 V.

Denebola, from Ar. dhanab al-asad (ذنب الاسد) "tail of the lion," referring to the lion in Gk. mythology.

Širdom "tail of the lion," from šir "lion" → Leo + dom "tail," → comet. Zanab-ol-asad from Ar., as above.

1) An assertion that something said, believed, alleged, etc., is false.
2) Refusal to believe a doctrine, theory, or the like.
3) Disbelief in the existence or reality of a thing (Dictionary.com).

→ deny; → -al.

1) To give a specific name to.
2) To express in a multiple of a unit of measurement.

→ de-; → nominate.

1) A name or designation.
2) The action of naming or classifying something. 3) One of the grades or degrees in a series of designations of quantity, value, measure, weight, etc. (Dictionary.com).

Verbal noun of → denominate.

The quantity y in a fraction x/y. The quantity x is the → numerator.

→ denominate; → -or.

1) To condemn or censure openly or publicly.
2) To make a formal accusation against, as to the police or in a court (Dictionary.com).

→ de-; → announce.

Having relatively high → density.
Math.: The quality of a subset A of a topological space X, indicating that any point in X can be well approximated by points in A.

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."

An opaque region of a → molecular cloud (A_V  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.

→ core mass function.

→ dense; → core; → mass; → function.

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 A_V > 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).

→ dense; → molecular; → cloud.

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.
See also:
→ charge density, → column density, → critical density, → current density, → density fluctuation, → density parameter, → density profile, → density wave, → density-bounded H II region, → density-wave theory, → electron density, → energy density, → flux density, → magnetic flux density, → maximum density of water, → neutral density filter, → nuclear density, → number density, → optical density, → period-mean density relation, → Planck density, → potential density, → power spectral density, → probability density function, → radio flux density, → relative density, → specific density, → spectral density, → surface density.

Noun form of → dense.

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.

→ density; → cusp.

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; → fluctuation.

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.

→ density; → element.

One of the four terms that describe an arranged version of the → Friedmann equations. They are all time dependent.
1) For matter: Ω_m = 8πGρ_m/(3H²), where G is the → gravitational constant, ρ_m is the mean matter density, and H the → Hubble parameter. The matter density parameter is also expressed as Ω_m = ρ_m/ρ_crit, where ρ_crit is the → critical density.
2) For radiation: Ω_r = 8πGρ_r/(3H²), where ρ_r is the radiation equivalent of matter density. This parameter is also expressed as Ω_r = ρ_r/ρ_crit.
3) For the → cosmological constant: Ω_Λ = Λc²/(3H²). Similarly, Ω_Λ = ρ_Λ/ρ_crit, where &rho_Λ = Λc²/(8πG) is sometimes referred to as the density of → dark energy.
4) For the → curvature of space-timeΩ_k = -kc²/(R²H²), where k is the → curvature constant and R the → cosmic scale factor.
Note that: Ω_m + Ω_r + Ω_Λ + Ω_k = 1, and Ω_total = Ω_m + Ω_r + Ω_Λ = 1 - Ω_k.

→ density; → parameter.

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.