An Etymological Dictionary of Astronomy and Astrophysics
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Density of the interstellar matter lying between an object and the Earth in a cylinder with a unity base.

→ column; → density.

Made up of distinct components. The components may retain part of their identities. → compound, → combination.

From O.Fr. composite, from L. compositus, p.p. of componere "to put together," → compose.

Hamnehâdé, p.p. of hamnehâdan→ compose.

A whole number which is the product of whole numbers other than itself and 1. The opposite of prime number. → compound number.

→ composite; → number.

A stellar population consisting of more than one → simple population. Also called complex population.

→ composite; → population.

A stellar spectrum displaying lines characterising two stars of different types.

→ composite; → spectrum.

1) General: The act or manner of composing; the result or product of composing.
2) Chemistry: The proportion of the elements of which a substance is made up.
3) Mechanics: The determination of a force whose effect is the same as that of two or more given forces acting simultaneously.

Hamnehesh, from ham-, → com-, + neheš verbal noun from nehâdan "to put, place," → compose.

A statement formed from simple statements by the use of words such as "and," "or," "not," "implies," or their corresponding symbols.

→ compound; → proposition.

A compound → proposition in which one → clause asserts something as true provided that the other clause is true. A conditional statement consists of two parts, a hypothesis in the "if" clause and a conclusion in the "then"clause. For instance, "If it rains, then they cancel school." It rains is the hypothesis. "They cancel school" is the conclusion. The clause following if is traditionally called the → antecedent, whereas the clause following then is called the → consequent.

→ conditional; → proposition.

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 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

The process of controlling telescope operations during observation and obtaining data.

→ data; acquisition, from L. acquisitionem, from acquirere "get in addition," from → ad- "extra" + quærere "to search for, obtain."

Alfanješ, verbal noun of alfanjidan (variant alfaqdan) "to acquire, get," Bactrian αλφανζ "to acquire," Sogdian δβ'yz "to acquire, gain, get" (Cheung 2007); → data.

A solution to → Einstein's field equations of → general relativity which contains no ordinary matter (Ω_M = 0) or radiation (Ω_R = 0), is → Euclidean (k = 0), but has a → cosmological constant (Ω_Λ > 0). The Universe expands exponentially forever. This solution was the first model expanding of → expanding Universe. See also → empty Universe, → Milne Universe.

After the Dutch mathematician and physicist Willem de Sitter (1872-1934) who worked out the model in 1917; → universe.

The process or the state of breaking down a physical entity or an organic material.

From → de- + → composition.

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.