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

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 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.

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.

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."

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.

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. 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.

→ 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.

Impossible to avoid.

→ avoid; → -ance.

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

→ un- + → biased.

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.

→ unbiased; → estimator.

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

→ unbiased; → sample.

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

→ 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.

→ un-; → bound.

Same as → free charge.

→ 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.

→ unbound; → cluster.

An association of bodies which are not gravitationally tied together.

→ 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).

→ 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.

→ unbound; → function.

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

→ 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.

→ uncertainty; → principle.