An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 13033 Search : far
ultraviolet catastrophe
  نگونزار ِ فرابنفش   
negunzâr-e farâbanafš

Fr.: catastrophe ultraviolette   

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.

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 excess
  فزونی ِ فرابنفش، فرهبود ِ اولتر-بنفش   
fozuni-ye farâbanafš, ferehbud-e ultar-banafš

Fr.: excès ultraviolet   

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.

ultraviolet; → excess.

ultraviolet star
  ستاره‌ی ِ فرابنفش، ~ ِ اولتر-بنفش   
setâre-ye farâbanafš, ~ ultar-banafš

Fr.: étoile ultraviolette   

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

ultraviolet; → star.

umbra
  سایه   
sâyé (#)

Fr.:ombre   

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).

From L. umbra "shade, shadow."

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."

Umbriel
  اومبریل   
Umbriel (#)

Fr.: Umbriel   

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.

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.

un-
  نا-   
nâ- (#)

Fr.: in-, non-   

A prefix meaning "not."

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

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. "not;" L. ne-, in-, un-; Gk. ni; Lith. ; O.C.S. ne "not;" E. un-, as above; PIE *ne-.

unaided eye
  چشم ِ بی‌یاور، ~ ِ برهنه   
cašm-e biyâvar, ~ berehné

Fr.: œil nu   

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

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.

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.

unavoidable
  ناتسنیدنی   
nâtosnidani

Fr.: inévitable   

Impossible to avoid.

avoid; → -ance.

unbiased
  بی‌وَرَک   
bi-varak

Fr.: non biaisé   

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

un- + → biased.

Bi- "without" + varak, → bias.

unbiased estimator
  بر‌آور ِ بی‌وَرَک   
barâvar-e bi-varak

Fr.: estimateur non biaisé   

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

unbiased; → estimator.

unbiased sample
  نمونان ِ بی‌وَرَک   
nemunân-e a bi-varak

Fr.: échantillon non biaisé   

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

unbiased; → sample.

unblanketed model
  مدل ِ ناپتومند   
model-e nâpatumand

Fr.: modèle non blanketé   

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

un-; → blanketed model.

unbound
  نابندیده   
nâbandidé

Fr.: non lié   

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.

un-; → bound.

unbound charge
  بار ِ نابندیده   
bâr-e nâbandidé

Fr.: charge non liée   

Same as → free charge.

unbound; → charge.

unbound cluster
  خوشه‌ی ِ نابندیده   
xuše-ye nâbandidé

Fr.: amas non lié   

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.

unbound; → cluster.

unbound system
  راژمان ِ نابندیده   
râžân-e nâbandidé

Fr.: système non lié   

An association of bodies which are not gravitationally tied together.

unbound; → system.

unbound young stellar system (UYSS)
  راژمان ِ ستاره‌ای ِ جوان ِ نابندیده   
râžmân-e setâre-yi-ye javân-e nâbandidé

Fr.: système stellaire jeune non lié   

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).

unbound; → young; → stellar; → system.

unbounded function
  کریای ِ بیکران   
karyâ-ye bikarân

Fr.: fonction non bornée   

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.

unbound; → function.

uncertainty
  ناتاشتیگی   
nâtâšigi (#)

Fr.: incertitude   

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

un- + → certainty.

uncertainty principle
  پَروَز ِ ناتاشتیگی   
parvaz-e nâtâštigi

Fr.: principe d'incertitude   

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.

uncertainty; → principle.

<< < -es -iv -ti 21- a p abe abs abs acc acc acc act act ada adi ads aes Age Alb Alf ali all alp alt ama amp ana ang ang ann ano ant ant ape apo app app arb are ari art ass ast ast ast Atl ato ato Aug aut Avo azi bac bal ban bar bas bea bel Bet bia Big bin bio bir bla bla blo blu Boh Bon bot Bow bre Bri bro but cal cal Can Cap car cas cat cau cel cen cen cha cha cha che Chi chr cir cir civ Cla clo clu CNO coa coe coi col col Col com com com com com com com com com con con con con con con con con con con con coo Cor Cor cor cos cos cos Cou cou cra Cre cro Cru cub cur cyc cyl dar dat Daw de- Deb dec dec def def deg Del den dep der det deu dew dic dif dif dil dip dir dis dis dis dis dis diu dog Dop dou Dra dua dus dwa dyn e-f Ear ecl eco edu eig Ein ela ele ele ele Ele elo emi emp ene Enl env epi equ equ erg est Euc eva evo exa exc exe exo exp exp ext ext f m fac fal fas fee Fer fib fie fin Fir fix fle flu foc for for For fou Fra fre Fri ful fuz Gal Gal gal Gam gas Gau gen geo geo geo geo Gib glo gov gra gra gra gra gre gro gul H-b hal Ham har Hay hea hel hel hem her het Hig hik hol hom hor hot Hub hum hyb hyd hyd hyp hyp ide ign ima imp imp imp inc ind ind ine inf inf inf ini ins ins ins int int int int Int int int int inv ion iri irr iso iso iso Jea Jos jun K-T Kep key kin Kol lad Lam lan lar las lat lea leg len lev lig lik lin lin lin Lis lob loc log loo low lum lun lun Lym Mac Mag mag mag mag mag Mag mak map Mar mas mat mat mea mea med mem mer MES met met MHD mic mid mil min mis mix mod mol mom Moo mos Mu mul mus nab nar nat neb neg nes neu New New nig nod non non non Nor noz nuc nul num OB obl obs Occ oct off Ohm on- ope Opp opt opt orb orb org Ori ort osm out ove ozo pal pap par par par pas pat pec pen per per per per per pH pha pho pho pho phy pie pla Pla Pla pla pla Plo poi pol pol pol pol por pos pot pow pre pre pre Pre pri pri pri pro pro pro pro pro pro Pro pub pul pyr qua qua qua qua Qui rad rad rad rad rad rad ram ran rat rea rec rec rec red ref ref reg reg rel rel rem rep res res res ret rev rho Rie rim riv rol Ros rot rul S a Sag sam sat sca Sch Sch sco sec sec sec seg sel sem sen set sha Sha sho SI sid sil sim sin sis sky slo sna Soc sol sol sol Sol Som sou spa spa spe spe spe spe sph spi spo squ sta sta sta sta ste ste ste sto sto str str sub sub sub sum sup sup sup sup sur sus sym syn sys tan Tay Tem ten ter The the the the thi thr tic tid tim Tol tor tot tra tra tra tra Tri tri tro tru tur two Typ UHE ult unc uni uni uni upg ura uti val var vec vel ver ver vig vir vis VLT vol wad was wav wav wea wei Whe Wie win WN5 Wol WZ Yag yot zen zir > >>