An Etymological Dictionary of Astronomy and Astrophysics

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.”