<< < -oi obj obs obs occ odd OH Ome Oor ope opp opt opt orb ord ori ort osc out ove oxi > >>
OH OH Fr.: OH 1) A Hydroxyl radical formed by abstraction of a hydrogen atom from water. From → hydro- + ox(y)- a combining form meaning "sharp, acute, pointed, acid," used in the formation of compound words, from Gk, oxys "sharp, keen, acid" + -yl a suffix used in the names of chemical radicals, from Fr. -yle, from Gk. hyle "matter, substance." |
OH 231.8+4.2 OH 231.8+4.2 Fr.: OH 231.8+4.2 |
OH line xatt-e OH Fr.: raie de OH Emission or absorption lines on an electromagnetic spectrum generated by hydroxyl, → OH molecules. At present, four principal lines are known in the radio domain at frequencies of 1612, 1665, 1667, and 1720 MHz, or wavelengths of approximately 18 centimeters. |
OH maser meyzer-e OH Fr.: maser OH A → maser phenomenon created by → OH molecules with characteristic → OH lines. OH masers are detected toward a variety of astronomical environments, including massive star formation regions and evolved late-type stars. |
OH source xan-e OH (hidroksil) Fr.: source OH An astronomical source emitting microwave radiation characteristic of the hydroxyl OH molecule, especially one showing a maser effect. OH sources are found in molecular clouds in interstellar medium and in the cool envelopes of evolved stars. OH, chemical compound hydroxyl; → source. |
OH/IR star setâre-ye OH/forusorx Fr.: étoile OH/IR An evolved Mira-type star which is associated with strong OH maser and strong infrared (IR) emission from the surrounding shell of warm gas and dust. |
ohm ohm (#) Fr.: ohm A unit of electrical resistance equal to that of a conductor in which a current of one ampere is produced by a potential of one volt across its terminals. Named after Georg Simon Ohm (1789-1854), the German physicist who discovered the law which bears his name. |
Ohm's law qânun-e Ohm (#) Fr.: loi d'Ohm 1) For a → conductor at rest, the
→ voltage across the
conductor is equal to the product of the current flowing through it and its
→ resistance. In other words, when such a conductor is
subjected to an electric field E,
the resulting
→ current density, J, is proportional to the
electric field E: J = σE, where σ
is the → conductivity, i.e. the reciprocal of
→ resistivity, ρ = 1/σ. |
Ohmic Ohmi (#) Fr.: ohmique Of or relating to a system which obeys Ohm's law. |
Ohmic decay time zamân-e tabâhi-ye Ohmi Fr.: temps de dissipation ohmique An upper bound on the time scale on which the magnetic field of a system would decay in the absence of any other agent. It is expressed as: τμ = R2 / μ, where R is the scale size of the system, η the magnetic diffusivity (η = 1 / μσ, where μ is the magnetic permeability and σ the electrical conductivity). For a star like the Sun, τμ ≅ 1010 years, so a fossil magnetic field could survive for the star's lifetime on the main sequence. For the Earth, τμ ≅ 104 years, so a → dynamo is required to explain the persistence of the geomagnetic field. |
Ohmic dissipation eftâl-e ohmi Fr.: dissipation ohmique 1) A loss of electric energy due to conversion into heat when a current
flows through a resistance. Same as Ohmic loss. → Ohmic; → dissipation. |
Ohmic loss dastraft-e Ohmi Fr.: perte ohmique Same as → Ohmic dissipation. |
Olbers' paradox pârâdaxš-e Olbers (#) Fr.: paradoxe d'Olbers The puzzle of why the night sky is not as uniformly bright as the surface of the Sun if, as used to be assumed, the Universe is infinitely large and filled uniformly with stars. It can be traced as far back as Johannes Kepler (1571-1630), was discussed by Edmond Halley (1656-1742) and Philippe Loys de Chéseaux (1718-1751), but was not popularized as a paradox until Heinrich Olbers took up the issue in the nineteenth century. This paradox has been resolved by the → Big Bang theory. In a Universe with a beginning, we can receive light only from that part of the Universe close enough so that light has had time to travel from there to here since the Big Bang. The night sky is dark because the galaxies are only about ten billion years old and have emitted only a limited amount of light, not because that light has been weakened by the expansion of the Universe (P. S. Wesson et al., 1987, ApJ 317, 601). Formulated in 1826 by Heinrich Wilhelm Olbers (1758-1840), German physician and amateur astronomer, who discovered the asteroids Pallas and Vesta as well as five comets; → paradox. |
old kohan (#), pir (#) Fr.: vieux Of an astronomical object, having existed as specified with relation to younger or newer objects of the same category; e.g. → old star. From M.E., from O.E. eald, ald; cf. Du. old, Ger. alt, Goth. altheis; akin to O.N. ala "to nourish." Kohan "old, ancient," kohné "worn;" Mid.Pers. kahwan "old, aged,
worn." |
old star setâre-ye kohan (#), kohan-setâré (#), setâre-ye pir (#) Fr.: vielle étoile A member of a population of stars that, according to stellar evolution theories, are almost as aged as the galaxy in which it resides. |
old stellar population porineš-e setâre-yi-ye kohan Fr.: population stellaire vielle A population of stars in a stellar system that have definitely left the → main sequence. → old; → stellar; → population. |
olivine olivin (#) Fr.: olivine A silicate mineral of magnesium (Mg2SiO4) and iron (Fe2SiO4,) found commonly in basalt and in carbonaceous chondrites. From Ger. Olivin, from olive, because of its olive-green to gray-green color, + -in equivalent to -ine a noun suffix used in chemical and mineralogical nomenclature. |
Olympus Mons kuh-e Olumpos Fr.: Olympus Mons The highest peak on Mars, and the largest volcano in the solar system. It rises to a height of 27 kilometres above the datum level selected on the basis of atmospheric pressure.This gigantic shield volcano, 700 kilometres across, is similar in nature to volcanoes on Earth but its volume is at least fifty times greater than its nearest terrestrial equivalent. From L. Mons, → mountain, + Olympus, from Gk. Olympos a mountain (2966 m) in north-east Greece, on the boundary between Thessaly and Macedonia, mythical abode of the greater Grecian gods. |
Omega Centauri (ω Cen) Omegâ Kentawros Fr.: Omega centauri The largest and most luminous → globular cluster associated with the Milky Way Galaxy. Omega Centauri is located about 18,300 → light-years away and contains several million old stars. The stars in its center are so crowded that they are believed to be only 0.1 light-year away from each other. It is about 12 billion years old. Omega Centauri was first listed in Ptolemy's catalog nearly two thousand years ago. In 1677 Edmond Halley reported it as a nebula, and in the 1830s John Herschel was the first to correctly identify it as a globular cluster. Also called NGC 5139. Omega, Gk. alphabet letter; Centauri, → Centaurus. |
omega effect oskar-e omega Fr.: effet ω In the → solar dynamo model, the process whereby the → meridional magnetic field is stretched into an → azimuthal magnetic field by → differential rotation. See also → alpha effect. Omega (ω), Gk. letter of alphabet; → effect. |
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