An Etymological Dictionary of Astronomy and Astrophysics
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The first sighting of the → New Moon after its → conjunction with the Sun. Although the date and time of each New Moon can be computed exactly, the visibility of the lunar → crescent as a function of the → Moon's age depends upon many factors and cannot be predicted with certainty. The sighting within one day of New Moon is usually difficult. The crescent at this time is quite thin, has a low surface brightness, and can easily be lost in the → twilight. Generally, the lunar crescent will become visible to suitably-located, experienced observers with good sky conditions about one day after New Moon. However, the time that the crescent actually becomes visible varies from one month to another. The visibility depends on sky conditions and the location, experience, and preparation of the observer. Ignoring atmospheric conditions, the size and brightness of the lunar crescent depend on the → elongation which in turn depends on several factors: 1) The Moon's elongation at New Moon (the elongation of the Moon at New Moon is not necessarily 0). 2) The speed of the Moon in its elliptical orbit. 3) The distance of the Moon, and 4) The observer's location (parallax). The combined effect of the first three factors gives geocentric elongation of the Moon from the Sun at an age of one day which can vary between about 10 and 15 degrees. This large range of possible elongations in the one-day-old Moon is critical (US Naval Observatory).

→ crescent; → moon; → visibility.

1) Cosmology: The average density of matter in the Universe that would be needed to eventually halt the → cosmic expansion. In a spatially → flat Universe, the critical density is expressed by ρ_c = (3c²/8πG)H_t², where c is the → speed of light, G is the → gravitational constant, and H_t the → Hubble parameter. The critical density is currently 9.3 × 10^-30g cm^-3, about 6 hydrogen atoms per cubic meter (for H₀ = 70 km s^-1 Mpc^-1).
2) In → gravitational lensing, the minimum density that would be needed by an intervening object to bend light rays. It is expressed by: Σ = (c²/4πG)(d_os/d_old_ls), where c is the speed of light, G is the gravitational constant, d_os, d_ol, and d_ls represent angular diameter distances between the observer and the source, the observer and the lens, and the lens and the source respectively. It has units of mass per unit solid angle.
3) Radiative processes: The density at which the collisional → de-excitation rate equals the → radiative transition rate. The critical density for level j is given by: n_c = Σ_{i < j} A_ji = Σ_{i ≠ j} q_ji, where A_ji is the → Einstein coefficient of → spontaneous emission and q_ji is the rate for collisional de-excitation of → energy level j, summed over all possible processes. This expression often simplifies to the ratio of two numbers, since in many cases there is a single important path for emission and a dominant collisional de-excitation process. In the low density limit the → emissivity is proportional to the product N_e (electron density) x N_i (ion density), whereas for densities exceeding the critical density, the emissivity is proportional to N_i. Thus, line emission in a nebula occurs most efficiently near the critical density.

→ critical; → density.

The → metallicity of a → star-forming  → molecular cloud when → cooling → rates by → metals dominate the → gravitational  → heating during → protostellar collapse. The minimum → Jeans mass achieved by gravitational → fragmentation depends on the presence/absence of → coolants in the cloud. Since cooling rate in metal lines is more efficient than in primordial molecular lines (H₂ and HD), metals favor fragmentation in gas and formation of → low-mass stars.

→ critical; → metallicity.

1) Velocity of → fluid through a pipe at which the motion changes from → laminar to → turbulent flow.
2) Same as → break-up velocity.

→ critical; → velocity.

1) A state of molecular structure in some resins attributed to the existence of solid crystals with a definite geometric form.
2) The percentage of a polymer sample that has formed crystals (J. W. Gooch, Encyclopedic Dictionary of Polymers).

→ crystalline; → -ity.

The electric current per unit of cross-sectional area perpendicular to the direction of current flow. It is a vector quantity and represented by symbol J. Electric current density is usually expressed in amperes per square meter.

→ current; → density

Statics: The → energy that must be expended to permanently disturb a specific state of → equilibrium of a body.

→ degree; → stability.

1) A god or goddess.
2) Divine character or nature, especially that of the Supreme Being; divinity (Dictionary.com).

M.E. deite, from O.Fr., from L.L. deitat- (nominative deitas), from L. dei- (combining form of deus "god") + -tat- "-ty," formed after L. divinitas "divinity."

Izad "god;" from Mid.Pers. yazêt "god; angel," izišn "performance of the religious rites, worship," yašt "worship, religious ceremony," yaštan "to venerate, to perform the religious ceremony;" O.Pers. yad- "to worship;" Av. yaz- "to sacrifice, worship, venerate," yazata- "deities," yasna- "religious rite" (Mod.Pers. jašn "feast"); Proto-Ir. *iaz- "to sacrifice, worship, venerate."

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.

A performance criterion for an electronic detector, reciprocal of the corresponding → noise-equivalent power (D = 1/NEP).

From detective, relating to → detect + → -ity.

Of a mathematical function, the quality of being → differentiable.

Noun from → differentiable.

1) The ability to permit or undergo diffusion.
2) → magnetic diffusivity.

→ diffusive; → -ity.