off-axis optical system
râžmân-e nurik-e ap-âsé
Fr.: système optique hors axe
Of computers, operating independently of, or disconnected from, an associated computer.
Fr.: observation hors source
An observation when the telescope is pointed away from the source in order to measure the sky background contribution.
1) A shift in the pointing position of a telescope with respect to a
Ap-, → off-; + neh present stem of nehâdan "to place, put; to set" Mid.Pers. nihâtan; Av. ni- "down; below; into," → ni-, + dā- "to put; to establish; to give," dadāiti "he gives;" cf. Skt. dadāti "he gives;" Gk. didomi "I give;" L. do "I give;" PIE base *do- "to give."
Fr.: guidage décalé
Guiding an astronomical exposure on a star, when the object of interest is nearby, but invisible.
Fr.: étoile Ofpe/WN9
A small class of evolved → massive stars showing spectral properties intermediate between those of → Of star and → WN Wolf-Rayet stars. Several of them have been found to possess non-spherical nitrogen-rich circumstellar nebulae. Ofpe/WN9 stars are considered to be transition objects between Of and W-R stars. This type of stars was first identified by Walborn (1982), who introduced the classification Ofpe/WN9, indicating that the stars could not be classified solely as Of stars, nor as WNL stars. Ofpe/WN9 stars have been found in the → Milky Way, the → Large Magellanic Cloud, → M31, and M33. Currently 10 Ofpe/WN9 stars are known in the LMC. Observational evidence suggests a close relationship between the class of → LBVs and the Ofpe/WN9 stars. A notable example is the prototype Ofpe/WN9 star R127 in the LMC that became an LBV on a time-scale of the order of a year. The possibility of such a relationship has been explored by Smith et al. (1994), who proposed that some LBVs show spectral morphologies that make them appear as an extension of the WN sequence toward later spectral types. Hence, they reclassified Ofpe/WN9 stars as WN10-11.
Ofpe, from → Of star; p for "peculiar;" e refers to the presence of other emission lines in addition to the Of ones, mainly H and He I, although also Si III. This peculiar class in the LMC was first described by Walborn (1977, ApJ 215, 53), where he called them "O Iafpe extr" based on the most similar Galactic objects known at that time. Subsequently Walborn (1983, ApJ 256, 452) and Bohannan & Walborn (1989, PASP 101, 520) suggested an extension of the WN sequence and/or transition between Of and WN. This nomenclature Ofpe/WN9 has been rather widely adopted. Later on, Smith et al. (1994) broke the Ofpe/WN9 and related Galactic types into WN10-11 subtypes to include this group of emission line stars.
Many times; frequently; in many cases.
M.E. oftin, from O.E. oft "often, frequently," akin to cognates: O.Frisian ofta, Danish ofte, O.H.G. ofto, Ger. oft, of unknown origin.
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."
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.