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

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.

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

→ classification; → B[e] star.

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

→ un-; → clear.