Fr.: théorème de Larmor
If a system of → charged particles, all having the same ratio of charge to mass (q/m), acted on by their mutual forces, and by a central force toward a common center, is subject in addition to a weak uniform magnetic field (B), its possible motions will be the same as the motions it could perform without the magnetic field, superposed upon a slow → precession of the entire system about the center of force with angular velocity ω = -(q/2mc)B.
Fr.: relation de Larson
An → empirical relationship between the internal → velocity dispersion of → molecular clouds and their size. The velocity dispersions are derived from molecular → linewidths, in particular those of → carbon monoxide. It was first established on star forming regions and found to be: σ (km s-1) = 1.10 L (pc)0.38, where σ is the velocity dispersion and L the size. The relation holds for 0.1 ≤ L ≤ 100 pc. More recent set of cloud data yield: σ (km s-1) = L (pc)0.5. This relation indicates that larger molecular clouds have larger internal velocity dispersions. It is usually interpreted as evidence for → turbulence in molecular clouds. Possible sources of interstellar turbulence include the following processes operating at various scales: galactic-scale (→ differential rotation, → infall of extragalactic gas on the galaxy), intermediate-scale (expansion of → supernova remnants, → shocks, → stellar winds from → massive stars), and smaller-scale processes (→ outflows from → young stellar objects).
First derived by Richard B. Larson, American astrophysicist working at Yale University (Larson, 1981, MNRAS 194, 809). See Falgarone et al. (2009, A&A 507, 355) for a recent study; → relation.
Fr.: solution de Larson-Penston
The analytical solution to the → hydrodynamic equations describing the → collapse of an → isothermal sphere. The Larson-Penston solution is → self-similar for a purely dynamical isothermal collapse with spherical symmetry. It corresponds to the collapse prior to the formation of a → protostar, and thus is suitable for the study of → pre-stellar cores. The Larson-Penston solution was extended by Shu (1977) to obtain a whole family of solutions for this problem.
Named after R. B. Larson (1969, MNRAS 145, 271) and M. V. Penston (1969, MNRAS 144, 425), who simultaneously, but independently, did this study.
1) Of, pertaining to, or located in the larynx.
Fr.: son laryngé
A consonant generated in the → larynx with the → vocal cords partly closed and partly vibrating. It is hypothesized that the → Proto-Indo-European language contained some laryngeal consonants (denoted by H).
From M.Fr. larynx, from M.L. from Gk. larynx (genitive laryngos) "the upper windpipe," probably from laimos "throat," influenced by pharynx "throat, windpipe."
Hanjaré, from Ar. Hanjarah.
1) A device that generates an intense directional beam of
→ monochromatic and
→ coherent light by exciting atoms to a higher
energy level and causing them to radiate their energy in phase.
The high degree of collimation arises from the fact that excited atoms are
are situated in a cavity bounded by two parallel front and back mirrors.
A first photon stimulates
an atom which emits a second photon, and so on thanks to the mirrors.
The resulting photons are all identical. They have the same energy which gives them
the same color and a unique direction. The first
working laser, a pulsed ruby device, was developed by T. Maiman in 1959.
See also → gas laser,
→ stimulated emission; → maser.
Acronym for light amplification by stimulated emission of radiation, on pattern of → maser.
laser cooling technique
tašnik-e sardeš-e leyzeri
Fr.: technique de refroidissement par laser
A technique that uses a suitable arrangement of → laser beams and magnetic fields to capture → cesium (133Cs) atoms from a thermal vapor and slow the motion of the atoms, cooling them to just a few micro-kelvins above the → absolute zero. The technique allows trapping some 107 cesium atoms in a cloud a few millimeters in diameter in a few tenths of a second. At a temperature of 2 μK, the average thermal velocity of the cesium atoms is of the order of 1 cm s-1, so they stay together for a relatively long time. The laser cooling technique is the key tool which enabled the operation of an → atomic fountain clock.
Fr.: interféromètre laser
An optical instrument using laser → beams to form → interference pattern. There are two types of laser interferometers: → homodyne and → heterodyne. A homodyne interferometer, like → Michelson interferometer, uses a single-frequency laser source. A → heterodyne interferometer uses a laser source with two close frequencies.
Laser Interferometer Gravitational-Wave Observatory (LIGO)
nepâhešgâh-e mowjhâ-ye gerâneši bâ andarzaneš-sanji-ye leyzeri
Fr.: Observatoire d'ondes gravitationnelles par interférométrie laser
A facility dedicated to the detection and measurement of cosmic → gravitational waves. It consists of two widely separated installations, or detectors, within the United States, operated in unison as a single observatory. One installation is located in Hanford (Washington) and the other in Livingston (Louisiana), 3,000 km apart. Funded by the National Science Foundation (NSF), LIGO was designed and constructed by a team of scientists from the California Institute of Technology, the Massachusetts Institute of Technology, and by industrial contractors. Construction of the facilities was completed in 1999. Initial operation of the detectors began in 2001. Each LIGO detector beams laser light down arms 4 km long, which are arranged in the shape of an "L." If a gravitational wave passes through the detector system, the distance traveled by the laser beam changes by a minuscule amount -- less than one-thousandth of the size of an atomic nucleus (10-18 m). Still, LIGO should be able to pick this difference up. LIGO directly detected gravitational waves for the first time from a binary → black hole merger (GW150914) on September 14, 2015 (Abbott et al., 2016, Phys. Rev. Lett. 116, 061102). The Nobel Prize in physics 2017 was awarded to three physicists (Rainer Weiss, Barry C. Barish, and Kip S. Thorne) for decisive contributions to the LIGO detector and the observation of gravitational waves. LIGO had a prominent role in the detection of → GW170817, the first event with an → electromagnetic counterpart.
Laser Interferometer Space Antenna (LISA)
ânten-e fezâyi-e andarzanešsanj-e leyzeri
Fr.: Observatoire d'ondes gravitationnelles par interférométrie laser
A collaborative project between → NASA and → ESA to develop and operate a space-based gravitational wave detector sensitive at frequencies between 0.03 mHz and 0.1 Hz. LISA detects gravitational-wave induced strains in → space-time by measuring changes of the separation between fiducial masses in three spacecraft 5 million km apart. Ultimately, NASA and ESA decided in 2011 not to proceed with the mission. LISA was not the highest ranked mission in the 2010 Decadal Survey and funding constraints prevented NASA from proceeding with multiple large missions (http://lisa.nasa.gov). → LISA pathfinder.
Last, from O.E. latost (adj.) and lætest (adv.), superlative of læt (adj.) and late (adv.); cognate with O.Fris. lest, Du. laatst, O.H.G. laggost, Ger. letzt.
Vâpasin, from vâ-, as intensive prefix, → de-, + pasin, from pas "after, afterward, behind; finally, at last" (Mid.Pers. pas "behind, before, after;" O.Pers. pasā "after;" Av. pasca "behind (of space); then, afterward (of time);" cf. Skt. pazca "behind, after, later;" L. post "behind, in the rear; after, afterward;" O.C.S. po "behind, after;" Lith. pas "at, by;" PIE base *pos-, *posko-) + -in superlative suffix.
Fr.: dernier contact
Same as → fourth contact at an eclipse.
Fr.: dernier quartier
One of the phases of the Moon that appears when it is 90 degrees west of the Sun. Approximately one week after a full moon, when half of the Moon's disk is illuminated by the Sun. → first quarter.
Fr.: dernière diffusion
The epoch in the early evolution of the Universe when matter and photons decoupled. Once atoms formed, light and matter stopped constantly interacting with one another, and photons were able to travel freely. As a result, the Universe became transparent. Light from this period is observed today as the → cosmic microwave background radiation. Same as → decoupling era and → recombination era.
last scattering surface
ruye-ye vâpasin parâkaneš
Fr.: surface de dernière diffusion
The set of locations in space corresponding to the → last scattering epoch in the early Universe. It is a spherical surface around the present-day observer from which the → cosmic microwave background radiation appears to emanate.
Fr.: dernier né
Last in order of birth; youngest.
1) dir (#); 2) dirân
Fr.: tardif; tard; avancé
1) Happening or arriving after an expected or arranged time;
not on time, beyond usual time.
M.E., from O.E. læt "slow, late;" cf. Ger. lass "slothful;" O.N. latr, Goth. lats "slow, lazy;" L. lassus "tired, faint;" Gk. ledein "to be weary."
Dir "late; tardily, slowly; a long while; old, antique,"
from Mid.Pers. dêr, variants dagr, drâz "long"
(Mod.Pers. derâz "long," variant Laki, Kurdi derež);
O.Pers. darga- "long;"
Av. darəga-, darəγa- "long,"
cf. Skt. dirghá- "long (in space and time);"
L. longus "long;" Gk. dolikhos "elongated;"
O.H.G., Ger. lang; Goth. laggs "long;"
PIE base *dlonghos- "long."
Late Heavy Bombardment (LHB)
bombârân-e sangin-e dirân
Fr.: Grand Bombardement Tardif
A cataclysmic event in the history of the → solar system, estimated to have occurred 3.9 billion years ago (about 600 million years after the formation of the → terrestrial planets) during which → asteroid and → comet impacts with Earth were some 20,000 times more frequent than today. It is estimated that during this period the terrestrial planets were bombarded with an object 1 km in size every 20 years. This hypothetical event lasted 50 to 150 million years. Several explanations have been put forward, among which the occurrence of an instability in the outer solar system which caused → orbital migration of small bodies from the → Kuiper belt inward.
→ late, with respect to the formation time of the planets; → heavy; bombardment, noun from bombard, from Fr. bombarder, from bombarde "mortar, catapult" from bombe, from It. bomba, probably from L. bombus "a booming sound," from Gk. bombos "deep and hollow sound."
late helium flash
deraxš-e heliom-e dirân
Fr.: flash de helium tardif
A → helium flash event that occurs during the → post-AGB phase. Some of the central stars of planetary nebulae (→ CSPN) experience a final → thermal pulse after having achieved a → white dwarf configuration and begun their descent along a → white dwarf cooling track of nearly constant radius. During such a pulse, most of the hydrogen remaining in the star at pulse onset is incorporated into the helium-burning convective shell and completely burned. Following the pulse, the star swells briefly to → red giant dimensions (Iben et al. 1983; ApJ 264, 605).