An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 703
Mestel theory
  نگره‌ی ِ مستل   
negare-ye Mestel

Fr.: théorie de Mestel   

The first quantitative model showing that the energy of → white dwarfs is the leftover heat from the star's past nuclear fusion that leaks slowly into space. In this analytic model constructed by Mestel (1952), a white dwarf consists of two layers. The inner layer, which contains most of the mass, is assumed to be → isothermal because of efficient thermal conductivity by the → degenerate electrons. Moreover, it is supposed that the electrons do not contribute significantly to the → heat capacity. The heat capacity comes entirely from the ions, which are assumed to behave as a classical → ideal gas. The thin non-degenerate outer layer forms an insulating blanket and controls the rate at which the energy from the ion reservoir is leaked out into space. The specific rate is controlled by the radiative opacity at the boundary between these two layers, and is assumed to obey → Kramers' opacity law. The Mestel theory shows that the cooling rate of a white dwarf is proportional to its temperature (hotter white dwarfs cool faster), and gives a relationship between the luminosity (L) of the white dwarf and the cooling time: t ∝ L-5/7. More recent models take into account some or all of the following processes neglected in the Mestel theory: neutrino cooling (important for L > 10-1.5 Lsun), latent heat of crystallization release (important for L < 10-4 Lsun), nuclear energy generation via proton-proton burning (important when MH ≥ 10-4 M*), and gravitational energy release from surface layers. The Mestel theory is a very good approximation of more recent calculations. For a review of the Mestel theory see Van Horn (1971, IAU Symp. 42, 97; W. J. Luyten, Editor), Wood (1990, J. Roy. Astro. Soc. Canada 84, 150), and Kepler and Brdaley (1995, Baltic Astron. 4, 166).

Named after Leon Mestel (1927-), British astrophysicist, who put forward this theory in 1952 (MNRAS, 112, 583); → theory.

Meszaros effect
  اسکر ِ مسزاروس   
oskar-e Meszaros

Fr.: effet de Meszaros   

Reduced growth or stagnation undergone by → cold dark matter perturbations during the period when the → early Universe was → radiation-dominated. The photons cannot collapse, and by their pressure prevent the matter to do so, when radiation dominates. Matter pertubation (collapse) remains frozen until the density equality between radiation and matter.

Péter Mészáros, 1974, A&A 37, 225; → effect.

matâ- (#)

Fr.: méta-   

A prefix appearing in loanwords from Gk., with the meanings 1) "after, behind;" 2) "changed, altered;" 3) "higher, beyond." → metagalaxy; metaphysics; → metastable.

From Gk. meta (prepositin) "in the midst of, among, with, after," originally me-ta (Mycenaean Greek), from PIE *me- "in the middle" (cf. Goth. miþ, O.E. mið "with, together with, among," E. with).

Matâ-, from Av. matay-, mati- "protrusion of mountain range," framanyente "to be protruding, jutting;" from PIE base *men- "to stand out, to project;" cf. L. mons (genitive montis) "mountain," minere "to project, jut, threaten" (other related terms: mouth, prominent, amount, etc.).


Fr.: métadonnées   

Specifically defined data elements that describe how and when a particular set of data was collected, and how it is formatted. Metadata is used to organize, manipulate, and work with data when it is not necessary or desired to actually deal with the data itself. The reason is that the metadata is usually far smaller and easier to work with than the data that it represents.

meta-; → data.


Fr.: métagalaxie   

An obsolete term which once denoted the entire system of galaxies including the Milky Way.

meta-; → galaxy.

felez (#)

Fr.: métal   

1) Chemistry: An → element in which the highest occupied energy band (→ conduction band) is only partially filled with electrons.
2) Astrophysics: Conventionally, any element heavier than → helium. The term "metal," as used for this concept, is in fact inappropriate. Same as → heavy element. See also → metallicity.

From O.Fr. metal, from L. metallum "metal, mine, quarry, what is got by mining," from Gk. metallon "metal, ore," originally "mine, quarry, pit," probably from metalleuein "to mine, to quarry," of unknown origin, but related somehow to metallan "to seek after."

Felez "metal," loanword from Ar. filizz.

metal deficiency

Fr.: déficience en métaux   

The quality of being metal deficient, e.g. → metal-deficient galaxy.

metal; deficiency from L. deficientem (nominative deficiens), pr.p. of deficere "to desert, fail," from → de- "down, away" + facere "to do, perform" + -ency a noun suffix, equivalent to -ence.

Kamfelezi, from kam "little, few; deficient, wanting; scarce" (Mid.Pers. kam "little, small, few," O.Pers./Av. kamna- "small, few," related to keh "small, little, slender" (related to kâstan, kâhidan "to decrease, lessen, diminish," from Mid.Pers. kâhitan, kâstan, kâhênitan "to decrease, diminish, lessen;" Av. kasu- "small, little;" Proto-Iranian *kas- "to be small, diminish, lessen") + felezmetal + -i suffix denoting state.

metal-deficient galaxy
  کهکشان ِ کم‌فلز   
kahkešân-e kamfelez

Fr.: galaxie pauvre en métaux   

A galaxy whose → metallicity is smaller than that of the → Milky Way galaxy.

Adj. from → metal deficiency; → galaxy.

metal-poor galaxy
  کهکشان ِ کم‌فلز   
kahkešân-e kamfelez

Fr.: galaxie pauvre en métaux   

Same as → metal-deficient galaxy.

metal; → poor; → galaxy.

metal-rich environment
  پرگیر ِ پرفلز   
pargir-e porfelez

Fr.: environnement riche en métaux   

An environment (→ galaxy, → nebula) whose → metallicity is larger than that of the → Milky Way galaxy.

metal; → rich; → environment.


Fr.: métalangue   

Any language that is used to describe a language. See also → object language.

meta-; → language.

felezi (#)

Fr.: métallique   

Of, relating to, or consisting of metal.

metal; → -ic.

metallic hydrogen
  هیدروژن ِ فلزی   
hidrožen-e felezi

Fr.: hydrogène métallique   

A kind of → degenerate matter resulting from hydrogen gas when it is sufficiently compressed to undergo a phase change to liquid or solid state. Metallic hydrogen is thought to be present in compressed astronomical objects, such as the interiors of the solar system planets Jupiter and Saturn. Above the core of these planets (at a temperature of 10,000 degrees and a pressure of 3 million bars) the electrons are squeezed out of the hydrogen atoms and the fluid starts to conduct like a metal.

metallic; → hydrogen.


Fr.: métallicité   

In a star, nebula, or galaxy, the proportion of the material that is made up of → metals, that is elements heavier than → helium. It is generally denoted by Z. The term "metallicity" is a misnomer used in astrophysics.
1) In practice, the metallicity of stars is usually expressed by the number ratio of → iron atoms to → hydrogen atoms per unit volume, with respect to the solar values: [Fe/H] = log10(NFe/NH)star - log10(NFe/NH)Sun, where NFe and NH are the numbers of iron and hydrogen atoms per unit volume. In fact it is taken to be equal to the iron → abundance with respect to the solar value. The solar logarithmic iron abundance is 7.50 ± 0.04 (Asplund et al. 2009, ARAA 47, 481), with respect to that of hydrogen which, by convention, is 12.00. Stellar metallicity is often expressed in mass fraction. See also → solar metallicity.
2) Nebular metallicity is often defined as the relative abundance of → oxygen: (NO/NH)neb/(NO/NH)Sun, where NO and NH represent the numbers of oxygen and hydrogen atoms per unit volume.

From metallic, from → metal + → -ity.

metallicity distribution function (MDF)
  کریای ِ واباژش ِ فلزیگی   
karyâ-ye vâbâžeš-e felezigi

Fr.: fonction de distribution de métallicité   

A plot representing the number of stars (or systems) per metallicity interval, usually expressed in [Fe/H] (abundance of → iron relative to → hydrogen).

metallicity; → distribution; → function.

metallicity gradient
  زینه‌ی ِ فلزیگی   
zine-ye felezigi

Fr.: gradient de métallicité   

The decrease in the → abundances of → heavy elements in a → disk galaxy as a function of distance from the center. Radial metallicity gradients are observed in many galaxies, including the → Milky Way and other galaxies of the → Local Group. In the case of the Milky Way, several objects can be used to determine the gradients: → H II regions, → B stars, → Cepheids, → open clusters, and → planetary nebulae. The main diagnostic elements are oxygen, sulphur, neon, and argon in photoionized nebulae, and iron and other elements in Cepheids, open clusters, and stars. Cepheids are probably the most accurate indicators of abundance gradients in the Milky Way. They are bright enough to be observed at large distances, so that accurate distances and spectroscopic abundances of several elements can be obtained. Average abundance gradients are generally between -0.03 → dex/kpc and -0.10 dex/kpc, with a a flattening out of the gradients at large galactocentric distances (≥ 10 kpc). The existence of these gradients offers the opportunity to test models of → chemical evolution of galaxies and stellar → nucleosynthesis.

metallicity; → gradient.

metastable state
  حالت ِ متاپایدار   
hâlat-e matâpâydâr

Fr.: état métastable   

An excited state in an atom, which is at the origin of the spectral lines called → forbidden lines. The time duration of the excited state being relatively long, under laboratory conditions the atom cannot pass directly to the ground state by emitting radiation. In the extremely rarefied interstellar medium, however, such highly improbable transitions do occur.

meta-; → stable; → state.

šahâb (#)

Fr.: météore   

A streak of light caused when a → meteoroid enters Earth's → atmosphere and becomes incandescent, mostly from → friction with the air at high speed. Meteors are also referred to as shooting stars. Very bright meteors are called → fireball or → bolide. Most of visible meteors arise from particles ranging in size from about that of a small pebble down to a grain of sand, and generally weigh less than 1-2 grams. The brilliant flash of light from a meteor is mainly caused by the → meteoroid's high level of → kinetic energy as it collides with the atmosphere at high speeds (11-72 km/s). The increase in the number of meteors visible toward the end of the night results from the fact that the Earth rotates about its axis in the same direction as it orbits the Sun. This means that the leading edge (morning side) of the Earth encounters more meteoroids than the trailing edge (evening side). In general, 2 to 3 times as many meteors can be seen in the hour or so just before morning twilight, than can be seen in the early evening. Moreover, the numbers of random, or sporadic, meteors vary from season to season, due to the tilt of the Earth on its axis and other factors. See also → meteor shower.

From M.Fr. meteore, from M.L. meteorum (nom. meteora), from Gk. ta meteora "the celestial phenomena," pl. of meteoron, literally "thing high up," neuter of meteoros (adj.) "high up," from → meta- "over, beyond" + -aoros "lifted, hovering in air," related to aeirein "to raise."

Šahâb, from Ar. Šihâb.

Meteor Crater
  لاوک ِ شهاب‌سنگ، کندال ِ ~، ~ آسمان‌سنگ   
lâvak-e šahâbsang, kandâl-e ~, ~ âsmânsang

Fr.: Meteor Crater   

A → meteorite impact crater located about 55 km east of Flagstaff, near Winslow in the northern Arizona desert of the United States. Meteor Crater is about 1,200 m in diameter and some 170 m deep. It is thought to have formed between 20,000 to 50,000 years ago, by the impact of a small → asteroid about 25 m in diameter. Same as → Barringer Crater.

meteor; → crater.

meteor echo
  پژواک ِ شهاب   
pažvâk-e šahâb

Fr.: écho de météore   

The reflection of → radio waves from transmitters located on the ground by a → meteor or by the corresponding trail left behind. When a meteor enters the Earth's upper atmosphere it excites the air molecules, producing a streak of light and leaving a trail of ionization behind it tens of kilometers long. This ionized trail occurs typically at a height of about 85 to 105 km, and may persist for less than 1 second up to several minutes.

meteor; → echo.

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