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

A measure of the surface temperature of a star derived from the total emitted energy, assuming that the star is a → blackbody emitter (→ Stefan-Boltzmann law, → Planck's radiation law). See also → brightness temperature; → color temperature.

→ effective; → temperature.

The reappearance of a celestial body after an eclipse, an occultation, or a transit; same as emersion. → ingress.

From L. egressus, from egredi "to go out," from → ex- "out" + -gredi, comb. form of gradi "to walk, go, step;" from PIE *ghredh- (cf. Lith. gridiju "to go, wander," O.C.S. gredo "to come").

Osgâm "going out," from os- "out," → ex-, + gâm "step, pace," Mid.Pers. gâm, O.Pers. gam- "to come; to go," Av. gam- "to come; to go," jamaiti "goes," Mod.Pers. âmadan "to come," Skt. gamati "goes," Gk. bainein "to go, walk, step," L. venire "to come," Tocharian A käm- "to come," O.H.G. queman "to come," E. come; PIE root *gwem- "to go, come."

A characteristic parameter occurring in the → Einstein model of → specific heats.

→ Einstein; → temperature.

The laws of physics are the same in all → inertial reference frames and are invariant under the → Lorentz transformation. The → speed of light is a → physical constant, i.e. it is the same for all observers in uniform motion. Einsteinian relativity is prompted by the → Newton-Maxwell incompatibility. See also: → Galilean relativity, → Newtonian relativity.

→ Einstein; → relativity.

The → rate at which → electric charge → flows past a given point through a → conductor, measured in → amperes.

→ electric; → current.

A process whereby an → unstable atom becomes stable. In this process, an → electron in an atom's inner shell is drawn into the → nucleus where it combines with a → proton, forming a → neutron and a → neutrino. The neutrino escapes from the atom's nucleus. The result is an element change, because the atom loses a proton. For example, an atom of → carbon (with 6 protons) becomes an atom of → boron (with 5 protons). Electron capture is also called K-capture since the captured electron usually comes from the atom's K-shell. See also → neutronization.

→ electron; → capture.

1) The temperature of electrons in an interstellar ionized nebula (e.g. in → H II regions and → planetary nebulae) as determined by characteristic → emission lines (optical → forbidden lines or → radio recombination lines).
2) In the → solar wind, the temperature derived from the mean → thermal agitation of the electrons. More specifically, electric field receivers on board space probes carry out the spectroscopy of the → thermal noise due to the potential fluctuations produced by the thermal agitation of the electrons, yielding the electron temperature in certain conditions (N. Meyer-Vernet, 2007, Basics of the Solar Wind, Cambridge Univ. Press). See also → proton temperature.

→ electron; → temperature.

Any emission mark in the spectrum of a celestial object resulting from line formation or of unknown origin.

→ emission; → feature.

The product of the square of the electron density times the linear size of the emitting region (in parsecs).

→ emission; → measure.

1) A political unit having an extensive territory or comprising a number of territories or nations and ruled by a single supreme authority.
2) The territory included in such a unit (TheFreeDictionary).

M.E., from O.Fr. empire "rule, authority, kingdom," from L. imperium "command, dominion, sovereignty," from → imperare "to command."

Parmâtgân, from parmât, present stem of parmâtidan, → imperare, + -gân suffix denoting relation and place appearinfg in the name of lands and localities (Šâpurgân, Gorgân, Golpâyegân, etc.).

1) Remain in existence; last.
2) Suffer (something painful or difficult) patiently (OxfordDictionaries.com).

From L. indurare "make hard," from in- + durare "to harden," from durus "hard," → duration.

Pâyidan, → last (v.);
Tâb âvardan b"endure," from tâb "toleration, patience," related to tavân, tavânestan "to be able," → conserve, + âvardan "to bring, to cause, produce," → production.

In the → epicyclic theory of Galactic rotation, the frequency at which a star in the → Galactic disk describes an ellipse around its mean circular orbit. The epicyclic frequency relates to the → Oort's constants. In the solar neighborhood the epicyclic frequency is about 32 km s^-1 kpc^-1.

→ epicyclic; → frequency.

→ reionization epoch.

→ epoch; → reionization.

The great circle of the celestial sphere through the celestial poles and equinoxes; the hour circle of the vernal equinox. → colure.

→ equinoctial; → colure.

The region between the → event horizon and the → stationary limit of a rotating → Kerr black hole. It is possible for a particle falling inside the ergosphere to break into two parts, one of which will fall into the black hole and the other will come out.

→ erg + → sphere.

→ spheres of Eudoxus.

→ spheres of Eudoxus.

If N(x,y) is a → function of two → independent variables, then dN = (∂N/∂x)dx + (∂N/∂y)dy is the exact differential.

→ exact; → differential.

A → differential equation composed of → continuous  → differentiable functions for which certain conditions are fulfilled. The equation M(x,y)dx + N(x,y)dy = 0 is called exact if M(x,y) and N(x,y) are continuous differentiable functions for which the following relationship is fulfilled: ∂M/∂y = ∂N/∂x, and ∂M/∂y and ∂N/∂x are continuous in some region.

→ exact; → differential; → equation.

Of a gas or plasma, the temperature deduced from the → populations of atomic → excited states, as expressed by the Boltzmann formula: N_u/N_l = (g_u/g_l) exp (-ΔE/kT_ex), where N_u and N_l are the upper level and lower level populations respectively, g_u and g_l the statistical weights, ΔE = hν the energy difference between the states, k is → Boltzmann's constant, and h→ Planck's constant. The higher the energy of the occupied states, the higher the excitation temperature.

→ excitation; → temperature.

A process in which an astronomical body loses mass mainly in the direction of its → equatorial regions, as opposed to → accretion.

From L. → ex- + crescere "to grow," → accretion.