The layer of the atmosphere located between the → stratosphere and the → ionosphere, where temperature drops rapidly with increasing height. It extends between 17 to 80 kilometers above the Earth's surface.
1) General: A piece of information (written, spoken, or by signals).
M.E., from O.Fr. message "message, news, embassy," from M.L. missaticum, from L. missus "a sending away, dispatching," from mittere "to send," → mission.
Fr.: catalogue de Messier
A catalog of more than 100 nebulous-appearing astronomical objects, initially established to avoid confusion with comets. These objects are now well known to be among the brightest and most striking gaseous nebulae, star clusters, and galaxies. The designations of the catalog are still used in identification; e.g. M1 is the Crab Nebula (in Taurus).
In honor of the French astronomer Charles Messier (1730-1817), who compiled the list between 1760 and 1784 in order to avoid confusion with comets; → catalog.
Fr.: objet de Messier
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.
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.
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.).
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.
An obsolete term which once denoted the entire system of galaxies including the Milky Way.
1) Chemistry: An → element in which the highest
occupied energy band (→ conduction band) is only partially
filled with electrons.
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.
Fr.: déficience en métaux
The quality of being metal deficient, e.g. → metal-deficient galaxy.
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") + felez→ metal + -i suffix denoting state.
Fr.: galaxie pauvre en métaux
Fr.: galaxie pauvre en métaux
Same as → metal-deficient galaxy.
Fr.: environnement riche en métaux
Any language that is used to describe a language. See also → object language.
Of, relating to, or consisting of metal.
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.
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.
metallicity distribution function (MDF)
karyâ-ye vâbâžeš-e felezigi
Fr.: fonction de distribution de métallicité
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.