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

Same as → Eddington parameter.

→ Eddington limit; → factor.

The theoretical upper limit of → luminosity at which the → radiation pressure of a light-emitting body would exceed the body's → gravitational attraction. A star emitting radiation at greater than the Eddington limit would break up. The Eddington luminosity for a non-rotating star is expressed as: L_Edd = 4πGMm_pcσ_T^-1, where G is the → gravitational constant, M the star mass, m_p the → proton mass, c the → speed of light, and σ_T the → Thomson cross section. It can also be written as L_Edd = 4πGMcκ^-1, where κ is the → opacity. In terms of solar mass, the Eddington limit can be expressed by: L_Edd = 1.26 × 10³⁸ (M/Msun) erg s^-1. See also → rotational Eddington limit.

Named after Arthur Stanley Eddington (1882-1944), prominent British astrophysicist; → limit.

Same as → Eddington limit.

→ Eddington limit; → luminosity.

A → dimensionless parameter indicating the degree to which a star is close to the → Eddington limit. It is expressed as Γ = L / L_Edd = κ L / (4πGMc), where L and M are the star luminosity and mass respectively, κ is the opacity, c the speed of light, and G the → gravitational constant. At the Eddington limit, Γ = 1, the star would become unbound. Because stellar luminosity generally scales with a high power of the stellar mass (L∝ M^3-4), → massive stars with M larger than 10 Msun generally have electron Eddington parameters of order Γ ≅ 0.1-1.

After Arthur Stanley Eddington (1882-1944), prominent British astrophysicist; → parameter.

A theoretical model in which the → cosmological constant plays a crucial role by allowing an initial phase that is identical to the Einstein static Universe. After an arbitrarily long time, the Universe begins to expand. The difficulty with this model is that the initiation of galaxy formation may actually cause a collapse rather than initiate an → expansion of the Universe.

→ Eddington limit; Lemaître in honor of Georges-Henri Lemaître (1894-1966), a Belgian Roman Catholic priest, who first proposed the Big Bang theory; → universe.

The time required for the redistribution of → angular momentum due to → meridional circulation. The Eddington-Sweet time for a uniformly → rotating star is expressed as: τ_ES = τ_KH . GM / (Ω² R³), where τ_KH is the → Kelvin-Helmholtz time scale, R, M, and L designate the radius, mass, and luminosity respectively, Ω the → angular velocity, and G the → gravitational constant. The Eddington-Sweet time scale can be approximated by τ_ES≅ τ_KH / χ, where χ is the ratio of the → centrifugal force to → gravity. For the Sun, χ ≅ 10^-5 resulting in an Eddington-Sweet time scale which is too long (10¹² years), i.e. unimportant. In contrast, for a rotating → massive star  χ is not so much less than 1. Hence the Eddington-Sweet circulation is very important in massive stars.

Named after the prominent British astrophysicist Arthur S. Eddington (1882-1944), who was the first to suggest these currents (in The Internal Constitution of the Stars, Dover Pub. Inc., New York, 1926) and P. A. Sweet who later quantified them (1950, MNRAS 110, 548); → time scale.

A macroscopic process that occurs in a → fluid because of the relative motions induced by the non-uniform → turbulent motions of the fluid. Also known as turbulent → diffusion. Eddy diffusion may occur in an atmosphere if it is unstable against turbulence. It dominates the atmosphere below the homopause. See also → molecular diffusion.

→ eddy; → diffusion.

A → spiral galaxy oriented edge-on to our view. → face-on galaxy.

→ edge; on, from O.E. on, variant of an "in, on, into" (cf. Du. aan; Ger. an; Goth. ana "on, upon"), from PIE base *ano "on" (cf. Av. ana "on;" Gk. ana "on, upon;" L. an-); → galaxy.

Kahkašân, → galaxy; pahlunemâ "showing the side," from pahlu, → side, + nemâ, from nemudan "to show, display," → display.

1) The act or process of editing.
2) A version of anything, printed or not, presented to the public.

Verbal noun of → edit.

The act or process of educating.
The knowledge or abilities gained through being educated.

Verbal noun of → educate.

The effective value of the → Eddington parameter in a non-homogeneous system (porous opacity).

→ effective; → Eddington limit; → parameter.

1) Math.: An → eigenvector for a linear → operator on a → vector space whose vectors are → functions. Also known as proper function.
2) Quantum mechanics: A → wave function corresponding to an → eigenvalue. Eigenfunctions represent the stationary → quantum states of a system.

From Ger. Eigenfunktion, from eigen- "characteristic, particular, own" (from P.Gmc. *aigana- "possessed, owned," Du. eigen, O.E. agen "one's own") + → function.

Viž-karyâ, from viž, contraction of vižé "particular, charcteristic" + karyâ, → function. Vižé, from Mid.Pers. apēcak "pure, sacred," from *apa-vēcak "set apart," from prefix apa- + vēcak, from vēxtan (Mod.Pers. bixtan) "to detach, separate, sift, remove," Av. vaēk- "to select, sort out, sift," pr. vaēca-, Skt. vic-, vinakti "to sift, winnow, separate; to inquire."

A notation convention in → tensor analysis whereby whenever there is an expression with a repeated → index, the summation is done over that index from 1 to 3 (or from 1 to n, where n is the space dimension). For example, the dot product of vectors a and b is usually written as: a.b = Σ (i = 1 to 3) a_i.b_i. In the Einstein notation this is simply written as a.b = a_i.b_i. This notation makes operations much easier. Same as Einstein summation convention.

→ Einstein; → notation.

A system of ten non-linear → partial differential equations in the theory of → general relativity which relate the curvature of → space-time with the distribution of matter-energy. They have the form: G_μν = -κ T_μν, where G_μν is the → Einstein tensor (a function of the → metric tensor), κ is a coupling constant called the → Einstein gravitational constant, and T_μν is the → energy-momentum tensor. The field equations mean that the curvature of space-time is due to the distribution of mass-energy in space. A more general form of the field equations proposed by Einstein is: G_μν + Λg_μν = - κT_μν, where Λ is the → cosmological constant.

Named after Albert Einstein (1879-1955); → field; → equation.

The coupling constant appearing in → Einstein's field equations, expressed by: κ = 8πG/c⁴, where G is the Newtonian → gravitational constant and c the → speed of light.

→ einstein; → gravitational; → constant.

In → general relativity, the → action that yields → Einstein's field equations. It is expressed by:
S_{EH = (1/2κ)∫d⁴x (-g)^1/2R + S^m,
where κ ≡ 8πG and S^m is the matter part of the action.}

→ Einstein; → Hilbert space; → action.

Act or instance of ejecting; the state of being ejected.

Verbal noun of → eject.

A collision between two particles which conserves the total kinetic energy and momentum of the system.

→ elastic; → collision.

A deformation of a → solid body in which the change (→ strain) in the relative position of points in the body disappears when the deforming stress is removed. See also → elastic limit.

→ elastic; → deformation.

A measure of a material's ability to conduct an electrical current. It is the reciprocal of the → resistivity. Conductivity is expressed by σ = ne²l/(2mv), where n is the number of electrons per cm³ volume of the → conductor, e is the → electron charge, l is the → mean free path, m is the → electron mass, and v is the arithmetic mean velocity of thermal motion of electrons at a given temperature.

→ electrical; → conductivity.