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Short for → unidentified flying object.
Fr.: vision d'OVNI
The act or occasion of catching sight of a UFO.
→ UFO; sighting, from sight, M.E. from O.E. sihth; cf. Dan. sigte, Swed. sigt, Du. zicht, O.H.G. siht, Ger. Sicht, Gesicht, related to see.
Didâr "sight, vision," verbal noun from didan "to see" (Mid.Pers. ditan "to see, regard, catch sight of, contemplate, experience;" O.Pers. dī- "to see;" Av. dā(y)- "to see," didāti "sees;" cf. Skt. dhī- "to perceive, think, ponder; thought, reflection, meditation," dādhye; Gk. dedorka "have seen").
Fr.: ufologie, ovnilogie
A term that describes the collective efforts of those who study → unidentified flying object (UFO) reports.
cistân-e partowhâ-ye keyhâni-ye ultar-meh-kâruž
Fr.: énigme des rayons cosmiques de très haute énergie
The question of the origin and nature of the → ultra high energy cosmic rays. According to the → GZK cutoff, the UHECRs should be nearby. They are expected to be exceptional, therefore visible by some astrophysical counterpart. However, there is nothing visible (within a few tens of → Mpc) in the direction of all the UHECR detected up to now.
Last; furthest or farthest; conclusive in a process or series; the highest or most significant.
L.L. ultimatus, p.p. of ultimare "to be final, come to an end," from L. ultimus "last, final, farthest, extreme," superlative of *ulter "beyond."
Ultom, from ul "up, upward," ulêh "upward, above,"
→ ultra-, + -tom supelative suffix,
ultimate fate of the Universe
sarnevešt-e farjâmin-e giti
Fr.: sort utlime de l'Univers
The future evolution of the → Universe which is a subject of study in → cosmology. The ultimate fate of the Universe can be explored using → general relativity. And since there is more than one possible solution to the equations of general relativity, there is more than one possible ultimate fate of the Universe. Moreover, the fate will depend on three factors: the Universe's overall shape or → geometry, its → dark energy content, and the → equation of state parameter. See also: → oscillating Universe, → Big Rip, → Big Crunch, → Big Freeze, → heat death.
A prefix occurring originally in loanwords from L., with the basic meaning "on the far side of, beyond, extremely."
From L. ultra- from ultra (adverb and preposition) "beyond, on the further side," from *ulter, from uls "beyond;" + *-ter suffix of comparative adj.; PIE base *al- "besides, other, beyond."
Ultar-, from Mid.Pers. ul "up, upward," ulêh "upward, above" (Av. ərəδuua- "upright, risen; cf. Skt. ūrdhvá- "high, above, elevated; Gr. orthos "set upright, straight;" L. arduus "high, steep;" → ortho-) + -tar suffix forming comparative adjectives (Mid.Pers. -tar; Av. -tara- (masculine); PIE base *-tero).
ultra-high-energy cosmic ray (UHECR)
partowhâ-ye keyhâni-ye ultar-meh-kâruž
Fr.: rayons cosmiques de très haute énergie
A particle belonging to the most energetic population of → cosmic rays with an energy above ~ 1020 → electron-volts. The UHECRs constitute a real challenge for theoretical models, because their acceleration requires extreme conditions hardly fulfilled by known astrophysical objects. See also → UHECR puzzle, → Greisen-Zatsepin-Kuzmin cutoff.
Fr.: neutrino ultra haute énergie
A neutrino particle accelerated to energies above 1018 → electron-volts. They are produced by the interaction of → ultra-high-energy cosmic ray (UHECR)s with the → cosmic microwave background radiation. Also called → cosmogenic neutrinos. See also → Greisen-Zatsepin-Kuzmin limit.
ultracompact dwarf galaxy (UCD)
kahkešân-e kutule-ye ultar-hampak
Fr.: galaxie naine ultracompacte
A type of very bright compact → stellar system (-14 ≤ MV≥ -12) that is intermediate between → globular clusters (GCs) and → compact elliptical galaxies (cEs). With masses of M > 2 × 106 Msun and radii > 10 → parsecs (pc), UCDs are among the densest stellar systems in the Universe. Nevertheless, the nature and origin of these objects is still widely debated. Early interpretations suggested that UCDs could be the most massive GCs or possibly the → tidally stripped remnants of → dwarf galaxies. However, there is evidence that both formation mechanisms could contribute to the UCD population. → Supermassive black holes (SMBHs) have been confirmed in most UCDs with masses M > 107 Msun. The most massive UCD discovered to date, M59-UCD3 (M* ~ 2 × 108 Msun, radius ~ 25 pc), hosts a SMBH (Ahn et al., 2018, arxiv/1804.02399, and references therein).
ultracompact H II region
nâhiye-ye H II-ye ultar hampak
Fr.: région H II ultracompacte
A very young → H II region fully embedded in its natal molecular cloud. Ultracompact H II regions are distinguished from classical H II regions by their small sizes (diameter ≥ 0.1 pc), high densities (Ne ≥ 105 cm-3), and high emission measures (EM ≥ 107 pc cm-6). Their typical ionized gas content is about 10-2 → solar masses, in contrast to classical H II regions with a mass of about 105 solar masses. Due to very important extinction, ultracompact H II regions are not accessible to visible wavelengths.
Fr.: naine ultrafroide
A star-like objects with an → effective temperature of less than 2,700 K. Ultracool dwarfs constitute a heterogeneous group including stars of extremely low mass as well as → brown dwarfs, and represent about 15% of the population of astronomical objects near the Sun.
The quality of an object whose → luminosity exceeds a certain value.
ultraluminous infrared galaxy (ULIRG)
kahkešân-e forusorx-e ultar-tâbân
Fr.: galaxie ultralumineuse en infrarouge
A galaxy that emits more than 90% of its energy in the infrared (8-1000 µm) and whose infrared luminosity exceeds 1012 solar luminosities. → luminous infrared galaxy (LIRG). Quasars can also have such high or even higher bolometric luminosities. However LIRGs and ULIRGs emit the bulk of their energy in the infrared. Most of ULIRGs are found in merging and interacting galaxy systems. It is thought that their luminosity results from galactic collisions, which increase the rate of star formation.
ultraluminous X-ray source (ULX)
xan-e partow-e iks-e ultar-tâbân
Fr.: source ultralumineuse en rayons X
An X-ray source that is not in the nucleus of a galaxy, and is more luminous than 1039 ergs s-1, brighter than the → Eddington luminosity of a 10 → solar mass → black hole. In general, there is about one ULX per galaxy in galaxies which host ULXs. The Milky Way contains no such objects. ULXs are thought to be powered by → accretion onto a → compact object. Possible explanations include accretion onto → neutron stars with strong → magnetic fields, onto → stellar black holes (of up to 20 → solar masses) at or in excess of the classical Eddington limit, or onto → intermediate-mass black holes (103-105 solar masses). NGC 1313X-1, NGC 5408X-1, and NGC 6946X-1 are three ULXs with X-ray luminosities up to ~ 1040 erg s-1 (Ciro Pinto et al., 2016, Nature 533, N) 7601).
Fr.: gaz ultrarelativiste
A gas composed of ultrarelativistic particles.
Fr.: céphéide à très courte période
Fr.: physique des ultra-sons
The branch of physics dealing with elastic waves of frequencies above 20 kHz to 1010 kHz propagated in solids, liquids, and gases.