Fr.: observation au méridien
The observation of a star when it crosses an observer's meridian.
Fr.: passage au méridien
The moment when a celestial object crosses an observer's meridian. Same as meridian transit.
Of, pertaining to, or resembling a meridian.
From nimruzân, → meridian, + -i adj. suffix.
Fr.: circulation méridien
The mass motion of material within a → rotating star generated by the star's departure from spherical symmetry. For a rotating star in which → centrifugal forces are not negligible, → radiative equilibrium and → hydrostatic equilibrium cannot be satisfied. In this condition energy transfer is accomplished by means of the physical motion of material. According to → von Zeipel theorem, the heating on an → equipotential surface is generally higher in the polar direction than in the equatorial direction, which drives a large scale circulation current rising at the pole and descending at the equator. As a consequence, → mixing of material takes place in the stellar interior; see also → Eddington-Sweet time scale. The meridional circulation plays an important role in the evolution of → massive stars. The circulation current was first suggested by Arthur S. Eddington in 1926 (The Internal Constitution of the Stars, Dover Pub. Inc., New York) and subsequently quantified by P. A. Sweet (1950, MNRAS 110, 548).
Fr.: courant méridien
Meteo.: A flow between the poles, or between the equator and the poles. A positive value indicates flow away from the equator; a negative value, flow toward the equator.
meridional magnetic field
meydân-e meqnâtisi-ye nimruzâni
Fr.: champ magnétiquue méridien
In the → solar dynamo model, a magnetic field that points from the north to south or south to north.
One of the stars in the Pleiades with a visual magnitude 4.17. It is a giant of spectral type B6 lying at a distance of about 1400 light-years.
Merope was one of the seven Pleiades, daughters of the Titan Atlas.
Fr.: nombre de Mersenne premier
A → prime number of the form 2p - 1, where p is a prime. As of February 2013, 48 Mersenne primes are known. The largest known prime number is 257,885,161 - 1. Each prime gives rise to an even → perfect number.
Marin Mersenne (1588-1648), French theologian, philosopher, mathematician and music theorist; → prime.
Mesarthim (γ Arietis)
A star of visual magnitude 4.8 lying 204 light-years away in the constellation → Aries. It is in fact a triple star system.
The origin of Mesarthim (or Mesartim) is a matter of controversy.
Some scholars have related it to the Ar. methartim (
Fr.: maille, maillage
1) One of the open spaces between the cords or ropes of a net.
M.E. mesche "open space in a net," apparently from O.E. max "net," earlier mæscre (cf. Dan. maske, Sw. maska, M.Du. maessce, Du. maas "mesh," O.H.G. masca, Ger. Masche "mesh").
Bâncé "aperture, opening, window" in (Kermânšâhi) Kurd., ultimately from Proto-Ir. *banaka-, from *baH- "to shine," cf. Av. bāmya- "light, bright;" Pers. bâm "morning, dawn; splendor, light," Ossetic bon "day," probably related to bezel "opening, aperture," in several dialects of the Fârs province (Lâr, Gerâš, Xonj, Fišvar), → morning.
Meso-, loan from Gk.
A nuclear particle with a mass intermediate between that of a proton and an electron, which is believed to be responsible for the strong nuclear force. In contrast to the case of baryons or leptons, meson number is not conserved: like photons, mesons can be created or destroyed in arbitrary numbers. Their charge can be positive, negative, or zero.
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.).