An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The upper part of the Earth's → mantle which begins at the base of the → crust around 35 km and extends downward to about 410 km.

→ upper; → mantle.

Same as → upper mass limit.

→ upper; → mass; → cut-off.

The highest mass range admitted in a star formation model. The high mass end of the → initial mass function. The upper mass limit is a critical parameter in understanding → stellar populations, → star formation, and → massive star feedback in galaxies.

→ upper; → mass; → limit.

The movement of a celestial body across a celestial meridian's upper branch. Same as → upper culmination.

→ upper; → transit.

1) Toward or in the higher part of a stream; against the current. → downstream
2) Directed upstream; situated upstream.

→ up; → stream.

Farâz, → height; water, → water.

A → radioactive metallic → chemical element; symbol U. → Atomic number 92; → atomic weight 238.0289; → melting post 1,132 °C; → boiling point 3,818°C; → specific gravity 19.1 at 25 °C. Uranium has 14 known → isotopes of which ²³⁸U is the most abundant in nature. This isotope (→ half-life 4.5 billion years) is 138 times more abundant than ²³⁵U (half-life 710 million years). The metal was first isolated by the French chemist Eugène-Melchior Peligot in 1841. See also: → uranium oxide, → uranium conversion, → uranium dioxide, → uranium enrichment, → uranium hexafluoride, → uranium-233, → uranium-235, → uranium-238, → plutonium, → fissile isotope, → fertile isotope, → yellowcake.

From the name of the planet → Uranus. The German chemist Martin-Heinrich Klaproth discovered the element in 1789, following the German/English astronomer William Hershel's discovery of the planet in 1781.

A chemical process converting the → yellowcake to → uranium hexafluoride. The uranium hexafluoride is heated to become a gas and loaded into cylinders. When it cools, it condenses into a solid.

→ uranium; → conversion.

A black crystalline solid (UO_{2pitchblende, carnotite, and autunite and is used chiefly as a source of nuclear energy by fission of the radioisotope uranium-235. After the → uranium hexafluoride is enriched, a fuel fabricator converts it into uranium dioxide powder and presses the powder into fuel pellets.}

→ uranium; → dioxide.

The process by which the percentage of → fissile uranium in a sample is increased. Uranium obtained from mining contains several → isotopes of uranium in different compositions, such as U-238 (~99%), U-235 (~0.7%), and U-234 (~0.02%). Among them, U-235 is the only one that is fissile, i.e. can be used in a → nuclear reactor to produce heat (and consequently electricity) in a controlled manner. As such, the concentration of U-235 as a fuel used in a reactor needs to be increased, which is done using several techniques, such as → gaseous diffusion.

→ uranium; → enrichment.

A white solid compound (UF₆) of → uranium and → fluorine obtained by chemical treatment of → yellowcake, forming a vapor at temperatures above 56 °C. It contains both of the naturally occurring → isotopes of uranium U-235 and U-238. The isotopes are separated on the basis of differences in their diffusion properties. UF₆ is the process medium for all separation processes for → uranium enrichment. It is essential that fluoride be a pure element and therefore solely the mass differences of U-235 and U-238 determine the separation process.

→ uranium; → hexa-; → fluoride.

A chemical compound made up of → uranium and → oxygen. The most common forms of uranium oxide are U₃O₈ and UO₂. Both oxide forms are solids that have low solubility in water and are relatively stable over a wide range of environmental conditions. Triuranium octaoxide (U₃O₈) is the most stable form of uranium and is the form most commonly found in nature. → Uranium dioxide (UO₂) is the form in which uranium is most commonly used as a → nuclear reactor fuel. At ambient temperatures, UO₂ will gradually convert to U₃O₈.

→ uranium; → oxide.

A → fissile isotope bred by the → fertile isotope thorium-232. It is similar in weapons quality to → plutonium-239.

→ uranium.

The only naturally occurring → fissile isotope. Natural uranium has 0.7 percent of ²³⁵U; light water reactors use about 3 percent and weapons materials normally consist of 90 percent of this isotope.

→ uranium.

A → fertile isotope from which → plutonium-239 can be bred. It comprises 99.3 percent of natural uranium.

→ uranium.

The seventh planet from the Sun and the third largest, discovered by William Herschel in 1781. It has a diameter about 51,100 km, four times that of the Earth, and a mass 14.5 times Earth's mass. Uranus orbits the Sun at a distance over 19 times the Earth-Sun distance. Although it takes about 84 years for Uranus to make a revolution, it completes a fast rotation in only 17 and a half hours. Unlike the other planets, its axis of rotation lies mostly in the plane of the Solar System. Uranus is internally less active than the other giant planets, which added to its larger distance from the Sun, makes it colder. It has a dense atmosphere made of mostly molecular hydrogen (83 percent) and helium (15 percent), with two percent methane and traces of acetylene and other hydrocarbons. The planet's greenish-blue color is due to light scattering as in Earth's sky and the absorption of red light by its small amount of atmospheric methane. Uranus has a ring system and 27 known satellites.

L. Uranus, from Gk. Ouranos "heaven." In Gk. mythology he was the the primeval sky god, and responsible for both the sunshine and the rain. He was the son and husband of Gaia, the goddess of the Earth and the father of Titans.

A cycle of nuclear reactions, primarily among the iron group of elements, accompanied by a high rate of neutrino formation. Neutrinos carry away energy quickly leading to the cooling of the stellar core and a dramatic drop in the pressure. Due to this process in pre-supernova stars, the core becomes unable to support the weight of the overlying layers. These layers collapse onto the core and subsequently rebound and escape in the supernova explosion.

Named for the de Urca Casino in Rio de Janeiro, by George Gamow and Mario Schönberg, who first studied this process. Gamow is reported to have pointed to the similarity between the disappearance of money in the crowded gambling rooms of the Casino and the disappearance of electron energy into invisible neutrinos; → process.

1a) To push or force along; impel with force or vigor.
1b) To drive with incitement to speed or effort.
1c) To impel, constrain, or move to some action.
2) An act of urging; impelling action, influence, or force; impulse (Dictionary.com).

From L. urgere "to press hard, push forward, drive, compel,," from PIE root *ureg- "to push, shove, drive;" cf. Lith. verziu "tie, fasten, squeeze," vargas "need, distress;" O.C.S. vragu "enemy;" Gothic wrikan "to persecute;" O.E. wrecan "avenge; drive, hunt, pursue;" E. wreak "to inflict, to cause;" Av. uruuaj- "to walk," see below.

Garžidan, from Proto-Ir. *uraj- "to go forth" (Cheung 2007; on the model of *urad- "to soften," Baluchi grast "to cook in oil or water"); cf. Av. uruuaj- "to go forth, walk;" cf. Kurd. guš, gušin (?) "to press, squeeze, shake;" Skt. vraj- "to stride, walk, proceed;" cognate with L. urgere, as above (Cheung 2007).

The quality or state of being urgent; imperativeness; insistence.

Noun from → urgent.

Compelling or requiring immediate action or attention; imperative; pressing (Dictionary.com).

Adj. from → urge.

The Great Bear. An extensive and prominent constellation in the region of the north celestial pole, at approximately 10h 40m right ascension, +56° declination. The seven brightest stars of Ursa Major are known as the → Big Dipper. The region contains the planetary → Owl Nebula and the spiral galaxies M81 and M82. Abbreviation: UMa, Genitive: Ursae Majoris.

Ursa,from L. ursus "bear," cognate with Pers. xers, as below; Major irregular comparative adj. of magnus "large, great," cognate with Pers. meh "great, large" (Mid.Pers. meh, mas; Av. maz-, masan-, mazant- "great, important," mazan- "greatness, majesty," mazišta- "greatest;" cf. Skt. mah-, mahant-; Gk. megas; PIE *meg- "great").

In Gk. mythology, Zeus falls in love with the nymph Callisto, and they have a son named Arcus. To hide Callisto from the wrath of his wife, Hera, Zeus changed her into a bear. Later on, when Arcus saw the bear Callisto, drew his bow and prepared to shoot her. To prevent Arcus from killing his mother, Zeus placed them together in the sky as the Big Bear and the Little Bear.

Xers "bear," dialectal Tabari aš; Mid.Pers. xirs, Av. arša- "bear;" cf. Skt. rksa- "bear;" Gk. arktos; L. ursus; PIE base *rtko- "bear;" bozorg "large, magnificient, great;" Mid.Pers. vazurg "great, big, high, lofty;" O.Pers. vazarka- "great;" Av. vazra- "club, mace" (Mod.Pers. gorz "mace"); cf. Skt. vájra- "(Indra's) thunderbolt," vaja- "strength, speed;" L. vigere "be lively, thrive," velox "fast, lively," vegere "to enliven," vigil "watchful, awake;" P.Gmc. *waken (Du. waken; O.H.G. wahhen; Ger. wachen "to be awake;" E. wake); PIE base *weg- "to be strong, be lively."