An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 417
first approximation
  نزدین ِ نخست   
nazdin-e naxost

Fr.: première approximation   

1) Generally, an expression to indicate that a comment or result is only approximate.
2) Math.: In calculus, limiting a differential equation to its first derivative, for example: ex≅ 1 + x. Also called linear approximation.

first; → approximation.

first collapse
  رمبش ِ نخست   
rombeš-e naxost

Fr.: premier effondrement   

An early phase in the process of star formation which begins when the mass of a → molecular cloud → clump exceeds the → Jeans mass. The collapse is initially → optically thin to the thermal emission from → dust grains, and the compressional heating rate is much smaller than the cooling rate by the → thermal radiation. The collapse proceeds → isothermally. The isothermal condition is broken when the central density reaches about 10-13 g cm-3 and a small region at the center of the cloud starts to become → opaque. The heat generated by the collapse in this region is no longer freely radiated away, and the compression becomes approximately → adiabatic. The central temperature and pressure then begin to rise rapidly, soon becoming sufficient to decelerate and stop the collapse at the center. There then arises a small central core, called the → first core, in which the material has stopped collapsing and is approaching → hydrostatic equilibrium. Outside this core, the material is still nearly isothermal and continues to fall inward almost in → free fall. Consequently a shock front arises at the boundary of the core, where the infalling material is suddenly stopped. The initial mass and radius of the core are about 1031 g and 6 x 1013 cm, respectively, and the central density and temperature are about 2 x 10-10 g cm-3 and 170 K, respectively. As the collapse proceeds, the core grows in mass due to the infall of the surrounding material; at the same time, however, the core radius decreases because of radiative energy losses from the outer layers of the core. The process leads to the → second collapse (R. B. Larson, 1969, MNRAS 145, 271).

first; → collapse.

first contact
  پرماس ِ نخست   
parmâs-e naxost

Fr.: premier contact   

1) The beginning of a → solar eclipse when the eastern part of the lunar limb touches the western limb of the Sun, marking the beginning of an eclipse.
2) For a → lunar eclipse, the moment when the eastern limb of the Moon is the first to enter the Earth's shadow.

first; → contact.

first core
  مغزه‌ی ِ نخست   
maqze-ye naxost

Fr.: premier cœur   

A first object in → hydrostatic equilibrium predicted to form during early dynamical contraction of a → molecular cloud → clump in the course of the → first collapse.

first; → core.

first degree equation
  هموگش ِ درجه‌ی ِ یکم   
hamugeš-e daraje-ye yekom

Fr.: équiation du premier degré   

A equation in which the highest → exponent of the → variable is 1. Same as → linear equation.

first; → degree; → equation.

first dredge-up
  برونکشید ِ نخست   
borunkašid-e naxost

Fr.: premier dragage   

The → dredge-up occurring after core hydrogen burning as the core contracts before helium burning ignites (on the ascending giant branch). The hydrogen envelope becomes convective and this convective zone penetrates deep into the core dredging up material that has been processed by the central nuclear reactions. As a result the abundances of helium and nitrogen are boosted.

first; → dredge-up.

first law of thermodynamics
  قانون ِ نخست ِ گرماتوانیک   
qânun-e naxost-e garâtavânik

Fr.: première loi de la thermodynamique   

The total energy of a → closed system is constant. This means that energy can be changed from one form to another, or transferred from one system to another, but it cannot be created or destroyed. A mathematical formulation of the first law is: δQ = δU + δW, where δQ is the heat transferred to the system, δU the change in internal energy (resulting in a rise or fall of temperature), and δW is the work done by the system.

first; → law; → thermodynamics.

first light
  نخستین فروغ، ~ نور   
naxostin foruq, ~ nur

Fr.: première lumière   

The first astronomical observation done with a major newly built telescope.

first; → light.

First Point of Aries
  نقطه‌ی ِ آغاز ِ برّه   
noqte-ye âqâz-e barré (#)

Fr.: premier point de Bélier   

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → vernal equinox several thousands years ago.

first; → point; → Aries.

First Point of Libra
  نقطه‌ی ِ آغاز ِ ترازو   
noqte-ye âqâz-e tarâzu (#)

Fr.: permier point de Balance   

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → autumnal equinox several thousands years ago. Because of → precession, this equinoctial point no longer lies in Libra but in neighboring Virgo.

first quarter
  چارک ِ نخست   
cârak-e naxost

Fr.: premier quartier   

A → lunar phase that occurs in the middle of the interval between the → new Moon and the → full Moon, when half of the Moon's disk is illuminated. At first quarter, the Moon is situated at 90° east of the Sun.

first; → quarter.

first star
  نخستین ستاره   
naxostin setâré

Fr.: première étoile   

A member of the → Population III → massive stars that formed some 500 Myr after the → Big Bang. First stars had a simple chemical composition consisting only of H, He, and traces of 7Li and were very short-lived. They are the most likely sources of → reionization of the Universe, which put an end to the → Cosmic Dark Age.

first; → star.

first-order differential equation
  هموگش ِ دگرسانه‌ای ِ رایه‌ی ِ نخست   
hamugeš-e degarsâne-yi-ye râye-ye naxost

Fr.: équation différentielle du premier ordre   

A → differential equation containing only the first → derivative. For example, dy/dx = 3x and 2y(dy/dx) + 3x = 5.

first; → order; → differential; → equation.

first-order logic (FOL)
  گوییک ِ فراسنی ِ رایه‌ی ِ نخست، ~ فراسنهای ِ ~ ~   
guyik-e farâsani-ye râye-ye naxost, ~ farâsanhâ-ye ~ ~

Fr.: logique des prédicats du premier ordre   

A system of → formal logic that is an extension of → propositional logic. It is also known as → first-order predicate calculus and → predicate logic. FOL eliminates deficiencies of propositional logic by: representing → objects (their properties, relations and statements about them), introducing → variables, and introducing → quantifiers.

first; → order; → predicate; → logic.

first-order partial derivative
  واخنه‌ی ِ پاری ِ رایه‌ی ِ نخست   
vâxane-ye pâri-ye râye-ye naxost

Fr.: dérivée partielle du premier degré   

For a → function with several → variables, the derivative, done only once, with respect to one of the variables. For example ∂u/∂x for u = u(x,y).

first; → order; → partial; → derivative.

first-order predicate calculus
  افماریک ِ فراسنی ِ رایه‌ی ِ نخست   
afmârik-e farâsani-ye râye-ye naxost

Fr.: cacul des prédicats du premier ordre   

Same as → first-order logic.

first; → order; → predicate; → calculus.

first-order spectrum
  بیناب ِ رایه‌ی ِ نخست   
binâb-e râye-ye naxost

Fr.: spectre du premier ordre   

An optical spectrum, produced by a diffraction grating, in which the difference in path length of light from adjacent slits is one wavelength.

first; → order; → spectrum.

šekâftani (#)

Fr.: fissile   

Any material that is capable of undergoing → nuclear fission by → thermal neutrons. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239. Although sometimes used as a synonym for → fissionable material, this term has acquired a more restricted meaning.

From L. fissilis, from fiss(us), + -illis a suffix of adjectives expressing capability, susceptibility, liability, aptitude, etc.

Šekâftani, from šekâft, → fission + -i a suffix expressing capability, aptitude, etc.

šekâft (#)

Fr.: fission   

1) The act or process of splitting or breaking into parts.
2) Splitting of the nucleus of an atom into two or more fragments of comparable size, usually as the result of the impact of a neutron on the nucleus. Same as → nuclear fission.

Fission, from L. fissionem "a breaking up, cleaving," from root of findere "to split."

Šekâft, stem of šekâftan "to split, break, tear," akin to kaftan, kâftan "to split; to dig," Parthian Mid.Pers. q'f- "to split;" Sogdian "to split;" Chorasmian kf- "to split, be split;" Proto-Iranian *kap-, *kaf- "to split."

fission products
  فر‌آورده‌های ِ شکافت   
farâvardehâ-ye šekâft (#)

Fr.: produits de fission   

Nuclides generated by the fission of higher mass elements or by subsequent radioactive decay of nuclides directly generated by fission.

fission; → product.

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