Fr.: OH 231.8+4.2
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.
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.
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.
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.
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.
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,
→ 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.
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.
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.
Fr.: perte ohmique
Same as → Ohmic dissipation.
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.
kohan (#), pir (#)
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,
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.
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.
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)
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.
Fr.: effet ω
Omega (ω), Gk. letter of alphabet; → effect.
Fr.: nébuleuse Omega
An → H II region located in the rich star fields of the → Sagittarius area of the Milky Way. Its distance from the Earth is between 5,000 and 6,000 → light-years and spans some 15 light-years in diameter. Its other designations are: Swan Nebula, Messier 17, and NGC 6618.
Omega, Gk. alphabet letter; → nebula.