An Etymological Dictionary of Astronomy and Astrophysics

A → carbon star of the → asymptotic giant branch type located in the constellation of → Antlia. Its other designations are HIP 51821 (→ Hipparcos Star Catalogue), HD 91793, and HR 4153. U Antliae is an → irregular variable that changes its → visual magnitude between 5.27 and 6.04. About 850 → light-years from Earth, it shines with a → luminosity approximately 8,000 times that of the → solar luminosity and has a → surface temperature 2,800 K. It is surrounded by a complex structure of dust shells.
Around 2,700 years ago, U Antliae went through a short period of rapid → mass loss. During this period of only a few hundred years, the material making up the shell seen in the → Atacama Large Millimeter Array (ALMA) data was ejected at high speed (Kerschbaum et al., 2017, A&A 605, A116, arXiv:1708.02915).

See also: U, according to → variable star designation; → Antlia.

The prototypical and first discovered → cataclysmic variable. It is a → dwarf nova system with a relatively long → orbital period of 4 hr 17 minutes. U Gem undergoes → outbursts on time-scales between 30 and 250 days and shows → grazing occultations.
The → accretion disk is partially eclipsed, but the → white dwarf remains visible during mideclipse. The mass of the white dwarf → primary is estimated to be 1.24 Msun, about two times higher than the mass of the → secondary; the → angle of inclination is 69°.7 (P. J. Groot 2001, ApJ 551, L89).

See also: → variable star designation; → Gemini.

Existing or being everywhere, especially at the same time; omnipresent.

Etymology (EN): From L. ubiquitarius, from ubique “everywhere,” from ubi “where” + que “any, also, ever.”

Etymology (PE): From visp-, → omni-, + bâšandé, bâš, bâšâ “existing, being,” from budan “to be,” → condition.

The state or capacity of being everywhere, especially at the same time; omnipresence.

Etymology (EN): From M.Fr. ubiquité, from L. ubique “everywhere,” → ubiquitous; → -ity.

Etymology (PE): Noun from visp-bâšandé, visp-bâš, visp-bâšâyi, → ubiquitous.

A → photometric system which consists of measuring an object’s → apparent magnitude through three broad-band filters: the ultraviolet (U) at 3650 Å, the blue (B) at 4400 Å, and the visual (V) in the green-yellow spectral region at 5500 Å. The filter bandwidths are 680, 980, and 890 Å respectively. The system is defined so that for A0 stars B - V = U - B = 0. The system was devised by Harold Johnson (1921-1980) and William Morgan (1906-1994) at Yerkes Observatory. It was extended to the R
and I bands centered at 7000 and 9000 Å respectively and later to other infrared bands.

See also: U, B, and V referring to “ultraviolet,” “blue,” and “visible” respectively; → system.

Short for → unidentified flying object.

See also: → unidentified flying object.

The act or occasion of catching sight of a UFO.

Etymology (EN): → 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.

Etymology (PE): 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”).

A term that describes the collective efforts of those who study → unidentified flying object (UFO) reports.

See also: Ufo, from → unidentified flying object; → -logy.

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.

See also: → UHECR; → puzzle.

Last; furthest or farthest; conclusive in a process or series; the highest or most significant.

Etymology (EN): 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.”

Etymology (PE): Ultom, from ul “up, upward,” ulêh “upward, above,” → ultra-, + -tom supelative suffix, → extreme.
Farjâmin “belonging to the end; concluding,” from farjâm “end; conclusion,” from Mid.Pers. farzâm “end; conclusion,” farzâftan “to finish, to be perfect;” from Proto-Iranian *fra-gam- “to send; to finish” (cf. O.Pers. prāgama- “to go forth”), from *gam- “to go; to come;” cf. Av. gam- “to come; to go,” jamaiti “goes;” O.Pers. gam- “to come; to go;” Mod./Mid.Pers. gâm “step, pace,” âmadan “to come;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE root *gwem- “to go, come.”

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.

See also: → ultimate; → Universe.

A prefix occurring originally in loanwords from L., with the basic meaning “on the far side of, beyond, extremely.”

Etymology (EN): 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.”

Etymology (PE): 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).

A galaxy of low stellar density, defined to have low central → surface brightness (> 24 mag arcsec^-2) and an → effective radius (R_e) of over 1.5 kpc. The question of whether UDGs represent a separate class of galaxies is still under debate. Currently, known UDGs that have been discovered in clusters, in groups, and in the field can have R_e as large as 5 kpc which is comparable to that of giant Milky Way like galaxies. This fact has been used to suggest that UDGs are “failed” giants. As R_e captures (at most) the central parts of giant galaxies, whether this radius can be used to fairly compare the sizes of UDGs to the more massive galaxies is questionable (see, e.g., Chamba et al., 2020, A&A 633, L3).

See also: Term proposed by van Dokkum et al. (2015), arXiv: 1410.8141v2; → ultra-; → diffuse; → galaxy.

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.

See also: → ultra- + → high-energy cosmic ray.

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.

See also: → ultra-; → high; → energy; → neutrino.

Extremely compact with respect to a comparison value. → ultracompact H II region, → ultracompact dwarf galaxy.

See also: → ultra-; → compact.

A type of very bright compact → stellar system (-14 ≤ M_V≥ -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).

See also: → ultracompact; → dwarf; → galaxy.

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.

See also: → ultra-; → compact; → H II region.

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.

See also: → ultra-; → cool; → dwarf.

The quality of an object whose → luminosity exceeds a certain value.

See also: → ultra-; → luminous.

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.

See also: → ultraluminous; → infrared; → galaxy.

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).

See also: → ultraluminous; → X-ray source.

Describing a system or situation for which the → Lorentz factor, γ, is much larger than 1. See also → subrelativistic.

See also: → ultra- + → relativistic

A gas composed of ultrarelativistic particles.

See also: → ultrarelativistic; → gas.

A → Cepheid star of → spectral type A-F with regular pulsation period of 1-3 hours and with small variations in amplitude. This group is also known as δ Scuti stars.

See also: → ultra-; → short; → period; → Cepheid.

The branch of physics dealing with elastic waves of frequencies above 20 kHz to 10¹⁰ kHz propagated in solids, liquids, and gases.

See also: → ultra-; sonic from L. sonus→ sound + → -ics.

Sound with a frequency lying above the audition frequency range, usually taken to be about 20 kHz. → sound wave.

See also: → ultra- + → sound.

The part of the electromagnetic radiation beyond the violet end of the visible spectrum with wavelengths approximately in the range 50 Å to 4,000 Å. → extreme ultraviolet; → far ultraviolet.

See also: → ultra-; → violet.

The study of astronomical objects in the ultraviolet portions of the electromagnetic spectrum, in the waveband 3000 Å to about 10 Å. At these wavelengths, the atmosphere prevents ultraviolet radiation from reaching the Earth surface. Therefore ground-based observatories cannot observe in the ultraviolet. Only with the advent of space-based telescopes has this area of astronomy become available for research.

See also: → ultraviolet; → astronomy.

A → paradox encountered in the classical theory of → thermal radiation (→ Rayleigh-Jeans law), whereby a → blackbody should radiate an infinite amount of energy at infinitely short wavelengths, in contradiction with what is observed. The problem was solved by Max Planck in 1900, who suggested that, rather than being continuous, the energy comes in discrete parcels called → quanta. The avoidance of the ultraviolet catastrophe was one of the first great achievements of → quantum mechanics.

See also: This problem was first raised by Lord Rayleigh (1842-1919), whereas the term ultraviolet catastrophe was first used by Paul Ehrenfest (1880-1933); → ultraviolet; → catastrophe.

Ultraviolet emission from an object in excess of that expected for a reference.
For example, → subdwarf stars show ultraviolet excess with respect to that expected from a star with → solar metallicity
at a given → effective temperature. In this case, UV excess results from smaller → line blanketing in
→ population II stars.

See also: → ultraviolet; → excess.

A star, such as O types or hot central stars of planetary nebulae, which radiates essentially in the ultraviolet part of the electromagnetic spectrum.

See also: → ultraviolet; → star.

1. Optics: A dark area, especially the blackest part of a shadow from which all light is cut off. → penumbra.
   

2. Sun: The central darkest part of a → sunspot, which is also the coolest part (~ 4,200 K).

Etymology (EN): From L. umbra “shade, shadow.”

Etymology (PE): Sâyé “shadow,” from Mid.Pers. sâyak “shadow;” Av. a-saya- “throwing no shadow;” Skt. chāya- “shadow;” Gk. skia “shade;” Rus. sijat’ “to shine;”
M.H.G. schinen, O.H.G. skinan, Ger. Schein “glow, shine;” PIE base *skai- “bright.”

The thirteenth of Uranus’s known satellites and the third largest. It has a diameter of 1170 km and orbits Uranus every 4.144 days at a mean distance of about 265,980 km. Umbriel is very dark, with an albedo of 0.18. It has a heavily cratered surface. Umbriel was discovered by William Lassell (1799-1880) in 1851.

See also: Umbriel named for an evil spirit in Alexander Pope’s (1688-1744) poem The Rape of the Lock. Moreover, Umbriel derives from L. → umbra “shadow,” which fits the satellite’s appearance.

A prefix meaning “not.”

Etymology (EN): M.E., O.E. un- (cf. O.Fris., O.H.G., Ger. un-, Goth. un-, Du. on-), cognate with Pers. nâ-, as below.

Etymology (PE): Nâ- “not,” variants na “no, not,” ma- “not” (prohitive); Mid.Pers. nê, ma “no, not;” O.Pers. naiy, nai “not;” Av. nôit, naē “not;” cf. Skt. ná “not;” L. ne-, in-, un-; Gk. ni; Lith. nè; O.C.S. ne “not;” E. un-, as above; PIE *ne-.

The eye unassisted by an optical instrument, except for eyeglasses. Same as → naked eye.

Etymology (EN): → un- + aided, from aid, M.E. ayde, from
O.Fr. aidier, from L. adjutare, frequentative of adjuvare “to give help to,” from → ad- “to” + juvare “to help;” → eye.

Etymology (PE): Cašm, → eye; biyâvar “without help,” from bi- “without” + yâvar, variant yâr “helper; companion” (Mid.Pers. hayyâr “helper,” hayyârêh “help, aid, assistance,” Proto-Iranian *adyāva-bara-, cf. Av. aidū- “helpful, useful”); berehné, → naked.

Impossible to avoid.

See also: → avoid; → -ance.

Not biased or prejudiced.
Statistics: With an expected value that is equal to the parameter being estimated.

Etymology (EN): → un- + → biased.

Etymology (PE): Bi- “without” + varak, → bias.

A sample statistics when the mean of the sampling distribution of that statistic can be shown to be equal to the parameter being estimated.

See also: → unbiased; → estimator.

A sample in which every individual element in the population has
an equal chance of being selected.

See also: → unbiased; → sample.

A stellar atmosphere model which ignores metals and their physical effects. → line-blanketed model; → line blanketing.

See also: → un-; → blanketed model.

1. Not bound or tied up.
   

   2. Not held by a → chemical bond, → gravity, or other physical force (OxfordDictionaries.com).
      See: → unbound charge, → unbound cluster, → unbound system.

See also: → un-; → bound.

Same as → free charge.

See also: → unbound; → charge.

A cluster of objects, such as stars or galaxies, in which the members are not tied together gravitationally. In such a cluster the members scatter through space following different directions. → bound cluster.

See also: → unbound; → cluster.

An association of bodies which are not gravitationally tied together.

See also: → unbound; → system.

A class of gravitationally loose stellar conglomerate with a notable apparent shape making it different from typical → bound  → star clusters. The UYSS class
includes a large range of objects, which extend at various size scales and at various degrees of self-binding; from small (semi-)compact → unbound systems named → stellar associations, to huge extended superstructures of → massive stars that make up whole parts of Galactic → spiral arms, known as → stellar complexes (Gouliermis, D. A., 2018, PASP 130:072001; arXiv:1806.11541).

See also: → unbound; → young; → stellar; → system.

The function y = f(x) in a given range of the argument x if there is no number M such that for all values of x in the range under consideration the inequality | f(x) | ≤ M will be fulfilled. → bounded function.

See also: → unbound; → function.

The state of being uncertain; unpredictability; indeterminacy. → uncertainty principle.

See also: → un- + → certainty.

A quantum mechanical principle due to Werner Heisenberg which states that the position and momentum of a particle cannot be determined simultaneously with any arbitrary accuracy. These quantities can be determined only with accuracies limited by the relation Δx.Δp ≥ (1/2)ħ, where Δx is the error in the determination of the position and Δp is the error in the momentum. A similar relation holds for the energy of a particle and the time, ΔE.Δt ≥ (1/2)ħ. Same as → Heisenberg uncertainty principle.

See also: → uncertainty; → principle.

A temporary designation for a → B[e] star that cannot be placed in any of the four known classes.

See also: → classification; → B[e] star.

Difficult to see, hear, understand, or to be sure about.

See also: → un-; → clear.

Not limited by conditions; absolute.

See also: → un-; → conditional.

Not awake and aware; not responding to one’s environment.

See also: → non-; → conscious.

The → state of being → unconscious.

See also: → unconscious; → -ness.

A dune on a planetary surface, used in plural form undae.

Etymology (EN): From L. unda “wave,” cognate with → water.

Etymology (PE): Šen-tal, from šen “sand” + tal “hill, heap.”

A prefixal use of under, as to indicate place or situation below or beneath. Contrary to → over-.

Etymology (EN): M.E., from O.E. (cf. O.H.G. untar, Ger. unter, Goth. undar), from PIE *ndhero- “lower;” cf. Av. aδara- (adj.), aδairi- (prep.) “below;” Mid.Pers. êr “below, down; low, under,” adar “low,” azêr “below, under” (Mod.Pers. zir “below, down”); Skt. ádhara- “lower;” O.E. under “under, among”).

Etymology (PE): Kam- “little, few; deficient, wanting; scarce”
(Mid.Pers. kam “little, small, few,” O.Pers./Av. kamna- “small, few”).

The abundance of a chemical element being lower than a reference value, in particular compared to that of the Sun.

Etymology (EN): → under-; → abundance.

The behavior of a damped system when the amount of damping is weak so that the system oscillates with the amplitude gradually decreasing to zero. → overdamping.

See also: → under-; → damping.

1. To estimate at too low a value, rate, or the like.
   
2. An estimate that is too low. → overestimate.

See also: → under-; → estimate

Insufficiently qualified for a particular job (OxfordDictionaries.com).

See also: → under-; → qualified.

The circumstance in which there are not enough pixels in a star’s image. The number of pixels that make up a star’s image is determined by the relationship between the telescope focal length, the physical size of the pixels, and the size of the star’s image. With an undersampled image it is not possible to obtain accurate estimates of the star’s image size or its position. Undersampling occurs when the atmospheric seeing conditions are exceptionally good and the pixel size not small enough.

See also: → under-; → sampling.

To perceive the meaning of.

Etymology (EN): M.E. understanden, understonden, O.E. understandan “comprehend, grasp the idea of,” probably literally “to stand in the midst of,” from → under- + standan “to stand,” cognate with Pers. istâdan, → standard.

Etymology (PE): Fahmidan, from Ar. fahm + infinitive suffix -idan.

The ability to understand something; the → power of → abstract  → thought.

See also: → understand; → -ing.

1. Not definitely or authoritatively decided or settled.
   

   2. Not known.

See also: → un-; → determined.

A type of meteorite in which the constituting materials (stone, glass, metal) are mixed together in a disorderly mass, in contrast to → differentiated meteorites.

See also: → un-; → differentiated meteorite.

A wave; the motion of waves.
A wavelike motion in any medium.

Etymology (EN): From L.L. undulatus “wavy, undulated,” from undula “wavelet,” diminutive of L. unda “wave.”

Etymology (PE): Mowješ, verbal noun of mowjidan “to undulate,” from mowj→ wave.

A prefix with the meaning of “one” occurring in loanwords from L., such as → uniform, → uniaxial, → universe, etc.

Etymology (EN): From uni- a combining form meaning “one,” from L. uni-, from unus, → one;

Etymology (PE): Yek-, from yek, → one.

Crystal with double refraction possessing only one → optic axis.

See also: → uni-; axial adj. from → axis; → crystal.

The attribute of someone or something whose identity is not established. → unidentified flying object; → unidentified line.

See also: → un-; → identify.

Any flying object or phenomenon that cannot be identified by the observer.

See also: → unidentified; fly; M.E. flien; O.E. fleogan (cf. O.H.G. fliogan, O.N. flügja, M.Du. vlieghen, Ger. fliegen); → object.

A no longer in general use name for → Aromatic Infrared Band.

See also: → unidentified; → infrared; → band.

A spectral line whose origin is not clearly established. → line identification.

See also: → unidentified; → line.

1. The process of unifying or uniting; union.
   

2. The state or condition of being unified (Dictionary.com).

See also: Verbal noun of → unify.

Without variations; identical, always the same in quality, degree, character, or manner.

Etymology (EN): → uni- + → form.

Etymology (PE): Yekdis, from yek, → uni-, + dis,
→ form; yeknavâxt, literally “with one rhythm,”
from yek, → one, + navâxt “rhythm,” from navâxtan, navâzidan “to play an instrument; to gratify,” navâ “music, song, melody” (Mid.Pers. nw’c “to treat kindly, honour,” niwag “music, melody;” Proto-Iranian *ni-uac-, from ni- “down; into,” → ni- (PIE),

• *uac- “to speak, treat kindly”).

The motion of an object around a fixed point at a constant angular speed, and at constant radius.

See also: → uniform; → circular; → motion.

A field that at a given instant has the same value at all points within a specified region of interest.

See also: → uniform; field.

A → magnetic field whose direction does not change and whose strength is constant at every point.

See also: → uniform; → magnetic; → field.

Motion at a constant → velocity. The state of rest is a special case of uniform motion. → accelerated motion; → inertial motion.

See also: → uniform; motion.

1. Of, characterized by, or conforming to → uniformity.
   

   2. Of or relating to → uniformitarianism.

See also: → uniformity + -arian.

The doctrine whereby geologic processes (→ erosion, → deposition, → compaction, and → uplift) observed at Earth’s surface now are the same that have shaped Earth’s landscape over long periods of time in the past.

The term uniformitarianism was first used in 1832 by William Whewell, to present an alternative explanation for the origin of the Earth. The prevailing view at that time was that the Earth was created through supernatural means and had been affected by a series of catastrophic events such as the biblical Flood. This theory is called → catastrophism.

The ideas behind uniformitarianism originated with the work of Scottish geologist James Hutton. In 1785, Hutton presented at the meetings of the Royal Society of Edinburgh that the Earth had a long history and that this history could be interpreted in terms of processes currently observed. For example, he suggested that deep soil profiles were formed by the weathering of bedrock over thousands of years. He also suggested that supernatural theories were not needed to explain the geologic history of the Earth (PhysicalGeography.net).

See also: → uniformitarian; → -ism.

The state or quality of being uniform.

See also: → uniform; → -ity.

To make or become a single unit or entity.
→ grand unified theory

Etymology (EN): M.Fr. unifier, from L.L. unificare “to make one,” from L. uni-, → one, + facere “to make” (cf. Fr. faire, Sp. hacer), from PIE base *dhe- “to put, to do” (cognate with Mod.Pers. dâdan “to give;” O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives; puts;” Skt. dadáti “puts, places;” Hitt. dai- “to place;” Gk. tithenai “to put, set, place;” Lith. deti “to put;” Czech diti, Pol. dziac’, Rus. det’ “to hide,” delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do”).

Etymology (PE): Yegânestan, yegânidan “to make one,” infinitive from yek,
→ one.

Difficult or impossible to believe.

See also: → un-; → imaginable.

General: The act of uniting two or more things.
A number of persons, organizations, states, etc., joined or associated together for some common purpose, e.g. → International Astronomical Union (IAU).
Math.: A set containing all the elements of two or more sets being combined.
It is represented by ∪.

Etymology (EN): M.E. from O.Fr. union from L.L. unionem (nominative unio) “oneness, unity,” from unus, → one, cognate with Pers. yek, as below.

Etymology (PE): Yekâyeš, from yek, → one,

• -ây- epenthetic vowel, + -eš verbal noun suffix, → -tion.

Existing as the only one or as the sole example.
Limited to a single outcome or result. → uniqueness theorem.

Etymology (EN): From Fr. unique, from L. unicus “single, sole,” from unus, → one.

Etymology (PE): Yektâ “unique,” from yek, → one,

• tâ “fold, plait, ply; piece, part” (Mid.Pers. tâg “piece, part”).

The state or condition of being → unique.

See also: Noun from → unique.

1. Physics: A → potential that satisfies both → Poisson’s equation and the → boundary conditions pertinent to a particular field is the only possible potential.
   

2. Math.: If two → continuous functions φ(t) and ψ(t) have one and the same → Laplace transform F(p), then these functions are identically equal.
   

3. Astro.: A → black hole can only be characterized by its → mass, → electric charge, and → angular momentum. See also → no hair theorem.

See also: → uniqueness; → theorem.

A quantity or dimension adopted as a standard of measurement.
Math.: The lowest positive natural number.
The first digit to the left of the decimal point in decimal notation, representing a whole number less than ten.
An element in a ring that has a multiplicative inverse.

Etymology (EN): Back formation from → unity.

Etymology (PE): Yekâ, yekân, from yek, → one.

A vector of length 1, also called a direction vector.

See also: → unit; → vector.

1. The condition of being → unitary.
   

   2. → quantum unitarity.

See also: → unitary; → -ity.

1. Of or pertaining to a unit or units; having the indivisible character of a unit.
   

2. Math.: An → operator  U satisfying the relations: U⁺ U = 1, U U⁺ = 1, where U⁺ is the → adjoint .

See also: → unit + -ary.

The set of n × n unitary matrices (→ unitary matrix).

See also: → unitary; → group.

A square matrix whose inverse equals its adjoint.

See also: → unitary; → matrix.

A linear operator whose inverse is its → adjoint. In addition to → Hermitian operators, unitary operators constitute a fundamentally important class of quantum-mechanical operators.

See also: → unitary; → operator.

A transformation whose reciprocal is equal to its Hermitian conjugate.

See also: → unitary; → transformation.

To join, combine, or incorporate so as to form a single whole or unit.

Etymology (EN): M.E. uniten, from L. unitus, p.p. of unire “to unite,” from unus, → one.

Etymology (PE): Âyekidan, from â- nuance/strengthening prefix + yek, → one, + -idan infinitive suffix; ham-yekidan, from ham- “together,” → syn-,

• yekidan.

Formed by or resulting from the union of two or more persons or things.

See also: P.p. of → unite.

The state or condition of being one.
Math.: One.

Etymology (EN): M.E. unite, from O.Fr., from L. unitatem “oneness, sameness, agreement,” from unus, → one.

Etymology (PE): Yeki, noun from yek, → one.

Statistics: Involving only one variable.

See also: → uni-; → variate.

A sequence of measurements of the same variable collected over time. Most often, the measurements are made at regular time intervals.

See also: → univariate; → time; → series.

1a) Of, pertaining to, or characteristic of all or the whole; relating to, affecting, or including everyone in a group or situation.

1b) Of or pertaining to the → Universe, all nature, or all existing things.

2. A trait, characteristic, or property, as distinguished from a particular individual or event, that can be possessed in common, as the care of a mother for her young (Dictionary.com).
   

See also:

→ Coordinated Universal Time, → universal astrolabe, → universal gas constant, → universal IMF, → universal quantifier, → Universal Time.

Related concepts:

→ all, → general, → omni-, → public, → total.

See also: → universe; → -al.

An → astrolabe which could be used at any → latitude.

See also: → universal; → astrolabe.

Same as → gas constant.

See also: → universal; → gas; → constant.

Same as → canonical IMF.

See also: → universal; → initial mass function.

A symbol of → predicate logic which expresses that the statements within its scope are → true for everything, or every instance of a specific thing. The symbol ∀ “for all” is used as the universal quantifier. Universal quantifiers are normally used in logic in conjunction with predicate symbols, which say something about a variable or constant, in this case the variable being quantified.

See also: → universal; → quantifier.

A measure of time that conforms, within a close approximation, to the mean diurnal motion of the Sun and serves as the basis off all civil timekeeping. The local civil time for a position on the Greenwich meridian.

See also: → universal; → time.

Sale as → Coordinated Universal Time (UTC).

See also: → Coordinated Universal Time (UTC).

The character or state of being universal.

See also: → universal; → -ity.

Same as → weak equivalence principle.

See also: → universality; → free; → fall.

The hypothesis whereby there exists a universal parent → distribution function which describes the stellar → initial mass function in individual star forming events. A consensus appears to have emerged in the community that the stellar IMF is largely invariant for star formation conditions as are found throughout the Local Group of galaxies at the present time. For details and discussion see Pavel Kroupa (2012), Recent advances on IMF research, arXiv.org/abs/astro-ph/1210.1211, and references therein.

See also: → universality; → initial mass function.

1. The totality of all matter and energy that exists in the vastness of cosmos whether known to human beings or not. Related concepts: → world,
   → cosmos,
   → multiverse.
   

2a) Math.: A → set made of
→ subsets that contains all the elements relevant to a particular discussion or problem.

2b) Statistics: The entire population under study.

See also: → all, → general, → omni-, → public, → total.

Etymology (EN): M.E., from O.Fr. univers, from L. universum “the universe,” noun use of neuter of adj. universus “all together,” literally “turned into one,” from unus, → one, + versus, p.p. of vertere “to turn,” akin to Pers. gar-, gardidan “to turn, to change” (Mid.Pers. vartitan; Av. varət- “to turn, revolve;” cf. Skt. vrt- “to turn, roll,” vartate “it turns round, rolls;” L. vertere “to turn;” O.H.G. werden “to become;” PIE base *wer- “to turn, bend”).

Etymology (PE): 1) Giti “world, material world, time,” variants jahân, keyhân, geyhân “world;”
Mid.Pers. gêhân “world,” gêtig “the material world; wordly;” Av. gaēθā- “being, world, matter, mankind,” gaya- “life, manner of living”, root gay- “to live” (present tense jiva-), O.Pers. gaiθā- “live-stock,” cognate with Skt. jīv- “to live,” jīva- “alive, living;” Gk. bios “life,” L. vivus “living, alive,” vita “life;” PIE base *g^wei- “to live” (cf. O.E. cwic “alive;” O.C.S. zivo “to live;” Lith. gyvas “living, alive;” O.Ir. bethu “life,” bith “age;” Welsh byd “world”). The Pers. words zistan “to live,” zendé “alive,” zendegi “life,” and jân “vital spirit, soul; mind” belong to this family.

2. Hargân, from har “every, all, each,” → holo-,

• -gân suffix forming plural entities, from Mid.Pers. -gânag, -gâna, on the model of hamegân, → public.

An institution of higher education composed of several faculties for teaching and research and authorized to grant academic degrees.

Etymology (EN): M.E., from O.Fr. universitei, from M.L. universitatem (nominative. universitas), in L.L. “corporation, society,” from L., “the whole, aggregate,” from universus “whole, entire,” → universe.

Etymology (PE): Dânešgâh, from dâneš, → science,

• -gâh “place; time” (Mid.Pers. gâh, gâs “time;” O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come”).

1. Having one meaning only.
   
2. A word or term that has only one meaning

See also: L.L. univocus, from L. → uni- + voc-, vox, → voice. “voice,”

The character of being → univocal. Same as → monosemy.

See also: → univocal; → ity.

Math.: A → variable or → function whose → value is to be found.

See also: → un-, + known, p.p. of → know.

To remove a mute from (a musical instrument).

See also: → un-; → mute.

Uncommon, unusual. → extraordinary.

See also: → un-; → ordinary.

A light whose electric vector of vibration is randomly oriented. Light is an → electromagnetic wave possessing an electric vector and an associated orthogonal magnetic vector. Both vectors are → transverse to the axis of propagation. In unpolarized light the electric and magnetic vibrations
occur in all possible planes. Ordinary light emitted by the Sun, by a living room lamp,
or by a candle flame is unpolarized light. → polarization.

See also: → un-; → polarized light.

Not qualified; not fit; lacking requisite qualifications (Dictionary.com).

See also: → un-; → qualified.

A star whose light is not affected by→ interstellar absorption. → reddening

See also: → un-; → reddened star.

Describing an image whose constituent or elementary parts are not resolved. → unresolved source.

See also: → un- + → resolved.

A source of radiation whose angular size is too small for details of its structure to be revealed.

See also: → unresolved; → source.

An image in which finer details are not visible. → blurred image

See also: → un-; → sharp image.

A process for amplifying fine details in a registered image. Generally speaking, the process consists of creating a mask which contains larger scale features of the image and then subtracting the mask from the initial image.

See also: → un-; → sharp; → masking.

Not → stable, as in → unstable atom, → unstable equilibrium.

See also: → un-; → stable.

An atom whose nuclei → decay by → radioactivity.

See also: → unstable; → atom.

An equilibrium state of a system in which if a small perturbation away from equilibrium is applied, the system will move farther away from equilibrium state. For example, mechanical equilibrium in which the potential energy is a maximum, as a sphere placed on top of a hill. Mathematically, if the second derivative of the energy with respect to the coordinate of interest is negative,
the system is in an unstable equilibrium. → stable equilibrium.

See also: → unstable; → equilibrium.

The brightest star as well as the Alpha star of → Serpens, thus also known as α Serpentis. It is a third magnitude (V = 2.65) an orange giant of spectral type K2 with a radius 15 times as large as that of the Sun and a surface temperature of 4300 kelvins. It is approximately 73.2 light years from Earth.
Unukalhai is in fact a triple star system. The second star (α Serpentis B) is 58 arcseconds from the primary and has a magnitude of 11.8. The third star (α Serpentis C), 13th magnitude, lies 2.3 arcminutes from A. Other designations: Cor Serpentis, HR 5854, HD 140573.

See also: From Ar. ‘unuq al-hayyah (عنق‌الحیه) “Neck of the Snake,” from ‘unuq “neck” + al-hayyah “snake.”

To, toward, or in a more elevated position.

Etymology (EN): M.E. up(pe) (adv.), O.E. up(p) “to a higher position;” cf.
O.Frisian up, O.S. up, M.Du. up, op, O.N. upp;
O.H.G. uf; (Ger. auf), Goth. iup.

Etymology (PE): Bâlâ “up, above, high, elevated, height” (related to boland “high,” borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz),
Lori dialect berg “hill, mountain;” Mid.Pers. buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf.
Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place;”
Ger. Burg “castle;” Goth. baurgs “city;” E. burg, borough; Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high.”

1a) An incline going up in the direction of movement.

1b) An increase or improvement.

1c) A new version, improved model, etc.

2a) To promote to a higher grade or rank.

2b) To improve or enhance the quality or value of (Dictionary.com).

See also: → up; → grade.

The process by which an area of Earth’s crust slowly rises either due to increasing upward force applied from below or decreasing downward force (weight) from above.

Etymology (EN): From → up + → lift.

Etymology (PE): Bâlâmad, literally “coming up,” from bâlâ, → up, + âmad past stem of âmadan, → come.

To send data from one’s computer to another computer. See also: → loading, → download.

Etymology (EN): → up-; → load.

Etymology (PE): Bârgozâštan, literally “to put load,” from bâr, → load, + gozâštan “to put, to place,” → passage.

Higher, as in place, position, pitch, or in a scale.

Etymology (EN): Upper, from → up, → hyper-.

Etymology (PE): Bâlâ, → up; zabarin, → superior.

The part of the → arm between the → shoulder and the → elbow.

Etymology (EN): → upper; → arm.

Etymology (PE): Bâzu “arm,” from Mid.Pers. bâzûk “arm;” Av. bāzu- “arm;” Mod.Pers. bâhu “stick, staff; arm;”
cf. Skt. bāhu- “arm, forearm;” Gk. pechys “forearm, arm, ell;”
O.H.G. buog “shoulder;” Ger. Bug “shoulder;” Du. boeg; O.E. bôg, bôh “shoulder, bough;” E. bough " a branch of a tree;" PIE *bhaghu- “arm”).

The general term applied to the atmosphere above the → troposphere.

See also: → upper; → atmosphere.

Same as → superior culmination.

See also: → upper; → culmination.

In atomic physics, an initial energy state in an emission transition.

See also: → upper; → level.

Of an integral operator, the point at which the integration ends.

See also: → upper; → limit.

A → main sequence star with a mass above 1.5 Msun. Upper main sequence stars have high central temperatures so that they produce their energy through the → CNO cycle. The outward energy flux is very high and therefore this flux cannot be maintained by → radiative transfer. Thus, upper main sequence stars have → convective cores. Outside the core, there is a → radiative zone. The mass of the convective core gradually diminishes as the hydrogen is consumed. The surface hydrogen is fully ionized and the → opacity is due to → electron scattering The opacity due to electron scattering.

See also: → upper; → main; → sequence.

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.

See also: → upper; → mantle.

Same as → upper mass limit.

See also: → 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.

See also: → upper; → mass; → limit.

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

See also: → upper; → transit.

1. Toward or in the higher part of a stream; against the current. → downstream
   

2. Directed upstream; situated upstream.

Etymology (EN): → up; → stream.

Etymology (PE): 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.

See also: 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.

See also: → uranium; → conversion.

A black crystalline solid (UO_{2</SUB) which occurs in several minerals including → pitchblende,
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.}

See also: → 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.

See also: → 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.

See also: → 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.

See also: → uranium; → oxide.

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

See also: → 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.

See also: → uranium.

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

See also: → 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.

See also: 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.

See also: 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).

Etymology (EN): 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.

Etymology (PE): 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.

See also: Noun from → urgent.

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

See also: 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.

Etymology (EN): 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.

Etymology (PE): 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.”

A moving star cluster containing over 100 stars that are scattered over an area of sky more than 1,000 minutes of arc in diameter. The five brightest stars of the → Big Dipper as well as → Sirius belong to this → dynamical stream.

See also: → Ursa Major; → star; → cluster.

The Lesser Bear. A constellation situated in the northern hemisphere, at about 15h right ascension, +79° declination. Its two brightest stars, both of 2nd magnitude, are → Polaris, the present pole star, and → Kochab. Abbreviation: UMi, Genitive: Ursae Minoris.

Etymology (EN): Ursa,from L. ursus “bear,” cognate with Pers. xers, as below; Minor “smaller, less,” from minus related to L. minuere “make small;” cf. Gk. meion “less,” minuthein “to lessen;” Skt. miyate “diminishes, declines;” O.E. minsian “to diminish;” PIE base *mei- “small.”

Etymology (PE): Xers “bear,” dialectal Tabari aš;
Mid.Pers. xirs, Av. arša- “bear;” cf. Skt. rksa- “bear;” Gk. arktos; L. ursus; PIE base *rtko- “bear;” kucak “small;” Mid.Pers. kucak “small,” related to kutâh “short, small, little,” kudak “child, infant,” kutulé, → dwarf; Mid.Pers. kôtâh “low,” kôtak “small, young; baby;” Av. kutaka- “little, small.”

A dwarf spheroidal galaxy of faint luminosity that is a member of the Local Group.

See also: → Ursa Minor; → system.

An annual → meteor shower whose → radiant lies in the constellation → Ursa Minor and is active between December 17 and December 24. The shower usually peaks around December 23.

See also: → Ursa Minor; → -ids.

An → impact cratrer on → Ceres which is the third largest crater on this → dwarf planet. Urvara is located south of → Occator and is about 160 km wide and 6 km deep. It has a prominent central peak that is about 3 km high.

See also: Named for the ancient Indo-Iranian personification of fertility, Av. urvarā- “plant,” often defied, Skt. urvárā- “land, soil, fertile field.”

A person who uses or exploits something, such as a computer.

Etymology (EN): From use; M.E. usen from O.Fr. user “to use, employ,” from V.L. *usare “to use,” from L. uti “to use.”

Etymology (PE): Kârbar “user,” from bé kâr bordan “to use.”

A committee whose members represent the astronomers who use the facilities of an observatory and which is intended to improve the interaction between the observatory and the users.

Etymology (EN): → user; committee, M.E., from O.Fr. commettre “to commit,” from L. committere “to bring together,” from → com- + mittere “to put, send.”

Etymology (PE): Komité, loan from Fr.; kârbarân plural of kârbar→ user.

1. Of, relating to, or in the interests of utility.
   

2. Of, characterized by, or adhering to → utilitarianism.

Etymology (EN): Coined by Jeremy Bentham (1748-1832) from → utility + -arian.

Philosophy: A doctrine according to which the virtue of a thing or an action is determined by its utility. The goal of utilitarian ethics is to promote the greatest happiness for the greatest number of people. The founders of this philosophical school were Jeremy Bentham (1748-1832) and James Mill (1773-1836).

See also: → utilitarian; → -ism.

1. The state or quality of being useful; usefulness.
   

2. Having, or made for, useful practical purposes. → utility software.

Etymology (EN): M.E. utilite, from O.Fr. utilite “usefulness,”
earlier utilitet, from L. utilitatem “usefulness, profit,” from utilis “usable,” from uti “to use.”

Etymology (PE): Hudemandi, from hudemand “utile,” from hudé “use” (as in bihudé “useless, vain, absurd”), from Mid.Pers. hudâg “good, useful, beneficent;” Av. hūdā- “doing good, producing wealth,” from hū-, hu-, → eu-, + Av./O.pers. dā- “to give, grant, put,” dadāiti “he gives;” Mid.Pers./Mod.Pers. dâdan “to give, put”
(cf. Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
L. dare “to give, offer;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do”) + -mand(i)

A part of the system software designed to support the operation of application software and is used to manage the computer files. Examples of utility software are disk diagnosis program, backup software, password generation software, and virus protection software. Also called utilities.

See also: → utility; → software.

The act or process of utilizing.

See also: Verbal noun of → utilize.

To make practical or worthwhile use of.

Etymology (EN): From Fr. utiliser, from It. utilizzare, from utile “usable,” from L. utilis “usable,” from uti “to use.”

Etymology (PE): Hudidan, from hudé, hudag “use,” → utility, + -idan.

A → plain in the northern hemisphere of Mars that was chosen as the landing site of the Viking II space probe on September 3, 1976.

Etymology (EN): Mod.L. Utopia, literally “nowhere,” coined by Thomas More (1516), from Gk. ou “not” + topos “place;” planitia, from planus, → plain.

Etymology (PE): Dašt, → plain; nâkojâ “nowhere,” from nâ-, → un-, + kojâ “where?; a place;” Mid.Pers. kugiyâg, from kū “where; that; than” + giyâk “place” (O.Pers. ā-vahana- “place, village;” Av. vah- “to dwell, stay,” vanhaiti “he dwells, stays;” Skt. vásati “he dwells;” Gk. aesa (nukta) “to pass (the night);”
Ossetic wat “room; bed; place;” Tokharian B wäs- “to stay, wait;” PIE base ues- “to stay, live, spend the night”); nâkojâ âbâd literally “city of nowhere, habitation of nowhere,” from nâkojâ, as explained, + âbâd “city; habitation; cultivated” (Mid.Pers. âpât, âpâtân “cultivated, inhabitated;” Proto-Iranian *ā-pāta- “protected,” from prefix ā + pā- “to protect, guard” (Mod.Pers. pâyidan), → observe.

1. To give audible expression to; speak or pronounce.
   

2. Phonetics: To produce (speech sounds, speech-like sounds, syllables, words, etc.) audibly, with or without reference to formal language (Dictionary).

Etymology (EN): M.E. outren from M.Du. uteren or M.L.G. utern “to turn out, show, speak,” from uter “outer,” comparative adj. from ut, → out.

Etymology (PE): Vâpidan, from Proto-Ir. *uab-/*uaf- “to utter (sing, speak), to call;” cf. Av. uf- “to sing;” Sogd. w’β, w’b “to say, speak;” Baluci gwâpt/gwâp- “to summon, call together;” Mod.Pers. gap, gab “word, chit-chat,” zand-bâf “nightingale,” buf “owl.”

1. An act of uttering; a spoken word, statement, or vocal sound.
   

2. Something uttered; a word or words uttered; a cry, animal’s call, or the like.
   

3. Linguistics: Any speech sequence consisting of one or more words and preceded and followed by silence: it may be coextensive with a sentence (Dictionary.com).

See also: → utter; → -ance.

A geometric plane defined for the analysis/processing of → interferometer observations, for which the axes are conventionally termed u and v.

The plane is at right angles to a direction from which radiation is being received from the sky.

Unlike a typical telescope, a → radio interferometer cannot produce an image of the sky directly. Instead it measures the → Fourier transform of the sky brightness distribution in the uv plane. The Fourier transform relates the → interference pattern to the intensity on the sky. A measurement on a particular → baseline corresponds to a point on this plane with coordinates u and v, at a distance from the origin equal to the projected length of the baseline measured in wavelengths. Two telescopes form one baseline which represents one point in uv plane;

three telescopes create three baselines and are therefore represented by three points in uv plane.

Good uv coverage requires many simultaneous baselines amongst many antennas, or many sequential observations from a few antennas.

Good coverage of points is essential for high-quality

→ aperture synthesis mapping.

See also: u and v conventional symbols; → plane.

A four-color stellar → photometric system devised by B. Strömgren. It is based on measurements in the ultraviolet (3500 Å), violet (4100 Å), blue (4670 Å), and yellow (5470 Å) regions of the spectrum. The filters bandwidths are 340, 200, 160, and 240 Å respectively. Also known as Strömgren four-color photometry.

See also: u, v, b, and y referring to ultraviolet, violet, blue, and yellow
respectively; → system.

A star that shows large irregular brightness variations and belongs to the Herbig Ae/Be family, i.e. pre-main sequence stars of intermediate mass. Typically a decrease of 2-3 magnitudes in the visible occurs for a few days to a couple of weeks. Current theories explain this behavior as being an obscuration of the central star by orbiting dust clouds, as well as contribution to the total luminosity by unsteady accretion onto the central star.
Also called UXOr.

See also: U and X letters of alphabet, Ori, → Orion; → star.