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RHB star setâre-ye RHB Fr.: étoile RHB Same as → red horizontal branch star. → red; → horizontal; → branch; → star. |
Rhea Reâ (#) Fr.: Rhéa The second largest satellite of Saturn, also known as Saturn V. It has a diameter of 1530 km and orbits Saturn at a mean distance of 527,040 km in 4.518 days. Rhea is composed primarily of water ice with rock making up less than 1/3 of its mass. It was discovered by Cassini in 1672. In Gk. mythology Rhea was the sister and wife of Cronus (Saturn) and the mother of Demeter, Hades (Pluto), Hera, Hestia, Poseidon (Neptune), and Zeus (Jupiter). |
rhenium reniom (#) Fr.: rhénium A metallic chemical element; symbol Re. Atomic number 75; atomic weight 186.207; melting point about 3,180°C; boiling point about 5,625°C. Rhenium is a very dense, high-melting, silver-white metal; specific gravity 21.02 at 20°C. Of the elements, only carbon and tungsten have higher melting points and only iridium, osmium, and platinum are more dense. It was discovered by x-ray spectroscopy in 1925 by the German chemists, Walter Noddack, Ida Tacke and Otto Berg. The name derives from L. rhenus for "the Rhine river in Germany". |
rheology ravâné šenâsi (#) Fr.: rhéologie A branch of physics dealing with the way matter flows and deforms. From rheo- "flow, current, stream," from Gk. rheos "stream," cognate with Pers. rud "river," → Eridanus, + → -logy. Ravâné, from ravân "flowing, running," pr.p. of raftan "to go, walk; to flow;" (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- "to go; to attack") + šenâsi, → -logy. |
rheonomous ravândâtik Fr.: rhéonome Relating to a constraint or system that contains time explicitly. For example, a pendulum with an extensible string of length l rheonomous, the condition of constraint is: x^{2} + y^{2} = l^{2}(t), where l(t) is the length of the string at time t. From Gk. rheo-, from rheos "a flowing, stream, current," → rheology, + -nomous, → -nomy. |
Rho Cassiopeiae ρ Kâsiopé Fr.: Rho Cassiopée A → yellow hypergiant in the constellation Cassiopeia, classified as F8-G2 Ia0pe. Rho Cas is about 11,650 light-years away from Earth, yet can still be seen by the naked eye, as it is 550,000 times as luminous as the Sun. It is in fact one of the most luminous stars in our Galaxy. ρ Cas is also one of the only seven currently known yellow hypergiants in the Milky Way. It is variable and fluctuates around magnitude 5 but dimmed to 6th magnitude on 1946. Its last eruption happened in 2000-2001, during which the star ejected about 10 percent of a solar mass, dimmed by more than a full magnitude, and changed its spectral type from late F to early M. During this outburst, the star surface cooled from 7,000 to 4,000 K. Rho, a Gk. letter of alphabet used in → Bayer designation; Cassiopeiae, genitive of → Cassiopeia. |
Rho Ophiuchi Cloud abr-e rho Mâr-afsâ Fr.: Nuage de rho Ophiuchi A complex region of molecular and dust clouds containing emission and reflection nebulae near the star ρ Oph in the constellation → Ophiuchus. It is one of the closest star forming regions, some 400 light-years distant. Recent studies using the latest X-ray and infrared observations reveal more than 300 young stellar objects within the large central cloud. Their median age is only 300,000 years. |
rhodium rodiom (#) Fr.: rhodium A metallic chemical element; symbol Rh. Atomic number 45; atomic weight 102.9055; melting point about 1,966°C; boiling point 3,727±100°C; specific gravity 12.41 at 20°C. Rhodium was discovered in 1803 by the English chemist and physicist William Hyde Wollaston during experiments on crude platinum ore. The name derives from Gk. rhodon "rose" because of the "rose color of dilute solutions of its salts." |
rhombic lowzik Fr.: rhombique Shaped like a rhombus. |
rhombus lowzi (#) Fr.: losange A quadrilateral having all sides equal and all angles oblique. L.L. rhombus, from Gk. rhombos "rhombus, spinning top," from rhembesthai "to spin, whirl." Lowzi, resembling a lowz "almond." |
rhythm ritm (#) Fr.: rythme 1) An ordered recurrent alternation of strong and weak elements in the
flow of sound and silence in speech; a particular example or form of rhythm. From L. rhythmus "movement in time," from Gk. rhythmos "measured flow or movement, rhythm; proportion, symmetry; arrangement," related to rhein "to flow," from PIE root *sreu- "to flow" Ritm, loan from Fr. |
Ricci scalar marpeli-ye Ricci Fr.: scalaire de Ricci The simplest curvature invariant for a → Riemannian manifold. It is derived from the → Ricci tensor R_{μν} ≡ R^{α}_{μαν} by contracting indices. Taking the trace of the Ricci tensor gives the Ricci scalar: R ≡ R_{μν}g^{μnu;} = R^{μ}_{ν} = R^{αμ}_{αμ}. Also called → scalar curvature. → Ricci tensor; → scalar. |
Ricci tensor tânsor-e Ricci Fr.: tenseur de Ricci A → rank 2, → symmetric tensor R_{μν} that is a contraction of the → Riemann curvature tensor R^{λ}_{μνλ}. More specifically, R_{μν} ≡ Σ (λ) R^{λ}_{μνκ} = R^{λ}_{μνκ}. Closely related to the Ricci tensor is the → Einstein tensor, which plays an important role in the theory of → general relativity. Named after the Italian mathematician Gregorio Ricci-Curbastro (1853-1925); → tensor. |
rich por-, pordâr Fr.: riche Having large amounts of something specified. → metal-rich environment, → rich cluster; → enrich, → enrichment, → richness, → poor. M.E., from O.E. rice "wealthy, powerful" (cf. Du. rijk, Ger. reich "rich"), from PIE base *reg- "move in a straight line," hence, "to direct, rule" (cf. Mod.Pers./Mid.Pers. râst "right, straight;" O.Pers. rāsta- "straight, true," rās- "to be right, straight, true;" Av. rāz- "to direct, put in line, set," razan- "order;" Skt. raj- "to direct, stretch," rjuyant- "walking straight;" Gk. orektos "stretched out;" L. regere "to lead straight, guide, rule," p.p. rectus "right, straight;" Ger. recht; E. right). Por "full, much, very, too much" (Mid.Pers. purr "full;" O.Pers. paru- "much, many;" Av. parav-, pauru-, pouru-, from par- "to fill;" PIE base *pelu- "full," from *pel- "to be full;" cf. Skt. puru- "much, abundant;" Gk. polus "many," plethos "great number, multitude;" O.E. full); pordâr, literally "having much possession," from por + dâr "having, possessor," from dâštan "to have, to possess," → property. |
rich cluster xuše-ye pordâr Fr.: amas riche A → galaxy cluster with a particularly large number of galaxies. |
Richardson cascade peyšâr-e Richardson Fr.: cascade de Richarson Same as → energy cascade Named after L. F. Richardson (1922), Weather Prediction by Numerical Process (Cambridge Univ. Press); → cascade. |
Richardson criterion sanjidâr-e Richardson Fr.: critère de Richardson A condition for the onset of → instability in multilayer fluids which compares the balance between the restoring force of → buoyancy and the destabilizing effect of the → shear. Named after the British meteorologist Lewis Fry Richardson (1881-1953), who first arrived in 1920 to the dimensionless ratio now called → Richardson number. The first formal proof of the criterion, however, came four decades later for → incompressible flows (Miles, J. W. 1961, J. Fluid Mech., 10, 496; Howard, L. N., 1961, J. Fluid Mech., 10, 509). Its extension to → compressible flows was demonstrated subsequently (Chimonas 1970, J. Fluid Mech., 43, 833); → criterion. |
Richardson number adad-e Richardson Fr.: nombre de Richardson A dimensionless number which is used according to the → Richardson criterion to describe the condition for the → stability of a flow in the presence of vertical density stratification. If the → shear flow is characterized by linear variation of velocity and density, with velocities and densities ranging from U_{1} to U_{2} and ρ_{1} to ρ_{2} (ρ_{2}>ρ_{1}), respectively, over a depth H, then the Richardson number is expressed as: Ri = (ρ_{2} - ρ_{1}) gH / ρ_{0} (U_{1} - U_{2})^{2}. If Ri < 0.25, somewhere in the flow turbulence is likely to occur. For Ri > 0.25 the flow is stable. → Richardson criterion; → number. |
richness pordâri Fr.: richesse The property of being very abundant. |
richness class rade-ye pordâri Fr.: classe de richesse A classification of → galaxy clusters into six groups (0 to 5), as in the → Abell catalog. It depends on the number of galaxies in a given cluster that lie within a → magnitude range m_{3} to m_{3+2}, where m_{3} is the magnitude of the 3rd brightest member of the cluster. The first group contains 30-49 galaxies and the last group more than 299 galaxies. |
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