An Etymological Dictionary of Astronomy and Astrophysics

A hot star of → apparent visual magnitude V = 5.82 lying in the constellation → Ophiuchus, also known as
HR 6684, HD 163472, and HIP 87812. V2052 Oph is of → subgiant  → spectral type B2 IV-V and belongs to the family of magnetic → Beta Cephei → pulsators. It has a luminosity of 620 Lsol, a radius of 6.83 Rsol and lies about 830 → light-years away.

Three stellar pulsation modes have been detected and studied from ground-based spectroscopy and multi-color photometry. Its magnetic field was deduced to be → dipolar dipolar with a strength of ~ 400 G, inclined 35° to the rotation axis. The star’s → convective overshooting region is small, with no extra mixing in spite of the relatively large rotational velocity. This clearly illustrated the effect of the magnetic field: the inhibition of chemical mixing processes by the magnetic field. This result is in agreement with theoretical criteria, which predict that a surface magnetic field of ~ 70 G is sufficient to limit the overshooting in this star. V2052 Oph is now considered as a prime example of what magneto-asteroseismology can achieve (Neiner et al., 2012, A&A, 537, A148).

See also: → variable star designation.

1. A space completely empty of matter but not achievable in practice on Earth.
   
2. A state of lowest energy in a → quantum field theory.

Etymology (EN): L. vacuum “an empty space, void,” noun use of neuter of vacuus “empty,” related to vacare “to be empty.”

Etymology (PE): Xala’ loan from Ar.

A highly → magnetized vacuum behaving as a prism for the propagation of light, as predicted by → quantum electrodynamics (QED). Attempts to detect this phenomenon in the laboratory have not yet succeeded in the 80 years since it was predicted (Heisenberg & Euler, 1936, Z. Physik, 98, 714). This effect can be detected only in the presence of enormously strong → magnetic fields, such as those around → neutron stars.

Owing to the large inferred magnetic fields (B ~ 10¹³ G, → gauss), radiation from these sources is expected to be substantially polarized, independently of the mechanism actually responsible for the → thermal emission. The strongest magnetic field so far created in a laboratory is less than 10⁶ G lasting only for several tens of milliseconds. A large observed → polarization degree is, however, expected only if QED polarization effects are present in the magnetized vacuum around the star. The detection of a strongly → linearly polarized signal would therefore provide the observational evidence of QED effects in the strong-field regime.

Recently a team of astrophysicists (Mignani et al. 2016, arXiv/1610.08323) have detected → linear polarization toward the neutron star RXJ1856.5-3754 (at a significant degree of around 16%). This finding is likely due to the boosting effect of vacuum birefringence occurring in the area of empty space surrounding the neutron star.

See also: → vacuum; → birefringence.

An enclosure from which air is removed.

See also: → vacuum; → chamber.

In particle physics the lowest energy allowed by field quantization when all fields are in their → ground states. Vacuum energy is predicted to arise from → virtual particles that fluctuate in and out of existence, as manifested by the → Casimir effect. The cosmological → dark energy is postulated to be related to vacuum fluctuations. There is however an enormous discrepancy with the predictions of
quantum field theory. In this theory the value of vacuum energy density is expected to be roughly of the order ρ_v≅ E_max⁴, where E_max is the maximum energy at which the field theory is valid. At energies of the order of the → Planck energy, E_Pl≅ 10¹⁹ GeV, vacuum energy might be roughly: ρ_v≅ E_Pl⁴≅ 10⁷⁶ GeV⁴. On the other hand, the vacuum energy density in standard cosmological model is given by: ρ_Λ = Ω_Λ.ρ_crit, where Ω_Λ is the → density parameter for the → cosmological constant and ρ_crit is the → critical density. More explicitly, ρ_Λ = Ω_Λ . 3 H²/(8πG). Using present-day values of Ω_Λ (0.7) and H (70) leads to ρ_Λ = 10^-46 GeV⁴. Therefore, the discrepancy between the prediction and the observed value is 122 orders of magnitude.

See also: → vacuum; → energy.

A quantum field theory a process in which an electromagnetic field gives rise to virtual electron-positron pairs that in turn exert electromagnetic fields of their own, in a manner similar to classical dielectric polarization.

See also: → vacuum; → polarization.

1. Not clearly or explicitly stated or expressed.
   

2. Indefinite or indistinct in nature or character, as ideas or feelings.
   

3. Not definitely established, determined, confirmed, or known; uncertain (Dictionary.com).

→ ambiguous, → nebulous, → indefinite, → fuzzy, → unclear.

Etymology (EN): M.E., from M.Fr. vague “empty, vacant; wild, uncultivated; wandering,” from L. vagus “wandering, rambling,” of unknown origin.

Etymology (PE): Gereng, from Kalidari gereng “wandering,” Kurd. (Kurmanji) garin, (Sorani) garân “to wander,” Dehxodâ gereng “shattered;” ultimately from Proto-Ir. *gar-an-ka-, from *gar- “to turn, to wind” (Cheung 2007).

A year of 365 days that overlooks the fraction of less than 0.25 days corresponding to the whole length of the → tropical year. The vague year was used in the → calendars of ancient Egypt, Iran, Mayas, and some other civilizations. Typically the vague year was divided into 12 months of 30 days each plus 5 → epagomenal days.

See also: From annus vagus “wandering year;” → vague→ year.

A measure of the number of chemical bonds formed by the atoms of a given element. It represents the relative ability of an atom of an element to combine with other atoms. For example, the valence of O in water, H₂O, is 2.
Also called valency.

Etymology (EN): M.E., from O.Fr., from L valentia “strength, worth,” from valentem (nominative valens), pr.p. of valere “to be strong.”

Etymology (PE): Arzâyi, noun from arzâ, agent noun/adj. from arzidan “to be worth,” arzân “worthy; of small value, cheap,” arj “esteem, honour, price, worth” (Mid.Pers. arz “value, worth,” arzidan “be worth,”
arzân “valuable;” Av. arəjaiti “is worth,” arəja- “valuable,” arəg- “to be worth;” cf. Skt. arh- “to be worth, to earn,” árhant- “worthy person;” Gk. alphanein “to bring in as profit,” alphein “to ear, obtain;” Lith. algà “salary, pay;” PIE base *alg^wh- “to earn; price, value”).

The range of energy states in the spectrum of a solid crystal which includes the energies of all the electrons binding the crystal together.

See also: → valence; → band.

An electron of an atom lying farthest from the nucleus. Valence electrons are shared when atoms combine to form molecules.

See also: → valence; → electron.

Same as → valence.

Logic: Of an argument, if the premises are true, then the
conclusion cannot be denied without contradiction. The truth of the premises logically guarantees the truth of the conclusion.

Etymology (EN): M.E., from M.Fr. valide, from L. validus “strong, effective,” from valere “to be strong.”

Etymology (PE): Pâymand, from pâ, pây “foot; base, foundation, firmness” (Mid.Pers. pâd, pây; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + -mand possession suffix.

Logic: Of an argument, whether or not the conclusion follows logically from the premises and the allowable syllogisms of the logical system being used.

See also: Quality noun from → valid.

A system of canyons located just south of the Martian equator. The system is about 4000 km long. The central individual troughs, generally 50 to 100 km wide, merge into a depression as much as 600 km wide. In places the canyon floor reaches a depth of 10 km, 6 to 7 times deeper than the Grand Canyon on Earth.

See also: L. Valles Marineris “Mariner’s Valleys,” named after the Mars orbiter Mariner 9, which discovered the Martian canyon in 1971-72. → valley.

A long, narrow region of low land between ranges of mountains, hills, or other high areas, often having a river or stream running along the bottom.

Etymology (EN): M.E. valeie, valey, from O.Fr. valee “valley,” from V.L. *vallata, from L. vallis “valley,” of unknown origin.

Etymology (PE): Darré “valley,” from Mid.Pers. dar, darrak “ravine, cleft;” cf. Skt. pradará- “cleft.”

Math.: Magnitude; quantity; a numerical quantity assigned to a mathematical symbol.

Etymology (EN): M.E., from O.Fr. value “worth, value,” noun use of feminine p.p. of valoir “be worth,” from L. valere “be strong, be well, be of value.”

Etymology (PE): Arzeš, verbal noun of arzidan “to be worth,” arzân “worthy; of small value, cheap,” arj “esteem, honour, price, worth;” Mid.Pers. arz “value, worth,” arzidan “be worth,”
arzân “valuable;” Av. arəjaiti “is worth,” arəja- “valuable,” arəg- “to be worth;” cf. Skt. arh- “to be worth, to earn,” árhant- “worthy person;” Gk. alphanein “to bring in as profit,” alphein “to ear, obtain;” Lith. algà “salary, pay;” PIE base *alg^wh- “to earn; price, value.”

Any device that halts or regulates the flow of a fluid through a passage, pipe, etc.

Etymology (EN): From L. valvae “leaves of a door.”

Etymology (PE): Daricé, leterally “small door, window,” → stop. Supâp, loan from Fr. soupape, from O.Fr. sourpape, probably from souspape “a blow under chin,” from sous “under,” from L. subtus, → sub-,

• *pape “jaw,” from paper, from L. pap(p)are “to eat.”

A mechanism proposed by Eddington to explain → stellar pulsations. Same as the → kappa mechanism. In this analogy the stellar layer acts like a heat engine with radiation taking the role of stream. The expanding and contracting layer acts as the piston, and the opacity of the layer behaves as the valve mechanism (Eddington, 1917, The pulsation theory of → Cepheid variables, The Observatory 40, 290).

See also: → valve; → mechanism.

The ring-shaped regions of charged particles surrounding the Earth from 1 to 6 Earth radii into space. The charged particles are trapped in by the Earth’s magnetic field. The inner belt is between 1.2 and 4.5 Earth radii and contains high-energy electrons and protons which originate mainly from interactions between cosmic rays and the upper atmosphere. The outer belt, located between 4.5 and 6.0 Earth radii, contains lower-energy charged particles mainly coming from the solar wind.

See also: Named after James Van Allen (1914-2006), who discovered the belts
in 1958 based on measurements made by Explorer 1, the USA’s first successful artificial satellite; → belt.

In → Young’s experiment of → interference with double apertures, if a monochromatic source is a considerable distance from the → aperture plane and aperture separation is small, → fringe visibility from an extended source is proportional to the → Fourier transform of the source’s spatial distribution. The transform variable is the angular separation of the aperture-plane sampling points divided by the wavelength. The van Cittert-Zernike Theorem is at the heart of → aperture synthesis.

See also: Developed independently by Dutch physicists Pieter Hendrick van Cittert (1889-1959) in 1934 and Frits Zernike (1888-1966) in 1939; → theorem.

A catalog of → reflection nebulae containing 158 objects. The catalog gives information for all BD and CD stars north of δ = -33 deg which are surrounded by reflection nebulosity visible on both the blue and red prints of the → Palomar Observatory Sky Survey .

See also: van den Bergh, S., 1966, AJ, 71, 990; → catalog

An → equation of state that satisfactorily describes the behavior of → real gass over a wide range of temperatures and pressures. It is derived from considerations based on kinetic theory, taking into account to a first approximation the size of a molecule and the cohesive forces between molecules: (P + a / V²) (V - b) = RT, where P, V, and T are pressure, volume, and temperature and R the gas constant. a and b are characteristic constants for a given substance. For a = b = 0, the van der Waals equation reduces to the characteristic equation of an → ideal gas. See also → Dieterici equation.

See also: Named after Dutch physicist Johannes Diderik van der Waals (1837-1923), Nobel Prize in Physics 1910; → equation.

A weak attractive force between neutral atoms and molecules arising from polarization induced in each particle by the presence of other particles. All molecules contain electrically charged particles, and even though the molecule as a whole is electrically neutral there do exist between molecules van der Waals attractive forces of electric origin.

See also: Named after Dutch physicist Johannes Diderik van der Waals (1837-1923), Nobel Prize in Physics 1910; → force.

A soft, ductile, silver-grey metal; symbol V. → Atomic number 23; → atomic weight 50.9415; → melting point about 1,890°C; → boiling point 3,380°C; → specific gravity about 6 at 20°C; and
→ valence +2,+3, +4, or +5. → It is used in various alloys to increase its shock resistance.

See also: Named 1830 by Swedish chemist Nils Gabriel Sefström (1787-1845), from Old Norse Vanadis, epithet of the goddess Freya, +
→ -ium.

1. A blade attached radially to a central axis, as in a windmill, propeller, revolving fin, or the like. → spider vane.
   

2. → wind vane.
   

3. The flat part of a bird’s → feather.

Etymology (EN): M.E., from O.E. fana “flag;” cognate with Ger. Fahne
“flag,” Gothic fana “piece of cloth.”

Etymology (PE): Parré, from parr, → feather.

1. To disappear from sight, especially quickly; become invisible.
   

2. To disappear by ceasing to exist; come to an end.
   

3. Math.: To become zero or to tend to zero.

Etymology (EN): M.E., from O.Fr. esvanir “disappear; cause to disappear,” from L. evanescere “disappear, pass away, die out,” from → ex- “out” + vanescere “to vanish,” from vanus “empty,” cognate with O.E. wanian “to lessen,” wan “deficient;” O.N. vanta “to lack;” L. vacare “to be empty,” vastus “empty, waste;” see below for Iranian cognates.

Etymology (PE): Venidan, from Kurd. ven “missing; away; absent,” cf. Pers. vang “empty, empty-handed, poor;” related to Av. ūna- “deficient, empty,” ū- “to be incomplete;” akin to Skt. vā- “to disappear, diminish,” ūná- “deficient;” L. vanus “empty,” as above.

1. To → conquer or subdue by superior force, as in battle.
   

   2. To defeat in any contest or conflict; be victorious over (Dictionary.com).

Etymology (EN): O.Fr. venquis-, extended stem of veintre “to defeat,” from L. vincere “to overcome, conquer.”

Etymology (PE): Vânidan, from Mid.Pers. vânidan “to vanquish, to conquer;” cf. Av. van- “to vanquish, to defeat;” Skt. van- “to win, conquer;” O.E. winnan, E. win, PIE *uen- “to win, conquer.”

The gaseous form of substances that are normally in liquid or solid form.

Etymology (EN): M.E. vapour, L. vapor “exhalation, steam, heat,” of unknown origin.

Etymology (PE): Boxâr, loan from Ar. buxâr.

The equilibrium pressure of a vapor above its condensed phases (liquid or solid). In other words, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container. The vapor pressure varies with the temperature. As the temperature increases its the vapor pressure also increases, and conversely. The temperature at which the vapor pressure is equal to the atmospheric pressure is called the → boiling point. For example, at 20 °C, water would boil at a pressure of about 0.023 atm, or about a fortieth of atmospheric pressure.

See also: → vapor; → pressure.

The act of vaporizing. The state of being vaporized.
The rapid change of water into steam.

See also: Verbal noun of → vaporize.

1. To become converted into vapor.
   
2. To cause to change into a vapor.

See also: → vapor + → -ize.

The quality, state, or degree of being variable or changeable.

See also: Quality noun from → variable.

A measure of variability of a star (such as Welch-Stetson variability index) which combines the information from two passbands assuming that changes in the star’s luminosity occur nearly simultaneously at all optical wavelengths.

See also: → variability; → index.

Something that may or does vary. → variable star.
Math.: A symbol, usually a letter, which in a mathematical or logic expression represents any member of a set of possible values or possible objects.

Etymology (EN): M.E., from O.Fr. variable, from L. variabilis “changeable,” from variare “to change,” → vary.

Etymology (PE): Vartandé agent noun from vartidan “to change,” → vary.

A star whose luminosity changes over periods of time; there are many reasons and many types. → cataclysmic variable; → Cepheid;
→ pulsating star; → nova; → long-period variable; → short-period variable; → Luminous Blue Variable (LBV); etc.

See also: → variable; → star.

A set of conventions used for naming → variable stars. Stars with existing → Bayer designations are not given new designations. Alternatively, the letters R through Z are used followed by the Latin genitive of the name of the hosting constellation. Otherwise, two letters of alphabet are used (334 combinations) with the Latin genitive of the name of the constellation. Finally,
the letter V (variable) is used followed by numbers 335, 336, and so on. Some examples are: → P Cygni, → T Tauri, → FU Orionis, → EX Lupi, and → V2052 Oph.

See also: → variable; → star; → designation.

A measure of the scatter of the values of a random variable (X) about its mean (μ). Var(X) = E(X -μ)². The positive square root of the variance is called the → standard deviation.

Etymology (EN): M.E., from O.Fr. variance, from L. variantia, from variare “to change,” → vary.

Etymology (PE): Vartâyi quality noun from vartâ agent noun from vartidan, → vary.

1. Liable to or displaying variation.
   

2. Something that differs from a standard or type. For example, a different form of the same word.

See also: → vary.

A quantity which may take any of the values of a specified set with a specified relative frequency or probability. The variate is therefore often known as a → random variable.

Etymology (EN): L. variatus, p.p. of variare, → vary,

• → -ate.

Etymology (PE): → variable; → random.

1. General: An instance of changing, or something that changes.
   

2. Astro.: The periodic inequality in the Moon’s motion that results from the combined gravitational attraction of the Earth and the Sun. Its period is half the synodic month, that is 14.77 days, and the maximum longitude displacement is 39'29’’.9.
   

See also:

→ calculus of variations, → annual variation, → secular variation.

Etymology (EN): M.E., from O.Fr. variation, from L. variationem (nominative variatio) “difference, change,” from variatus, p.p. of variare “to change,” → vary.

Etymology (PE): Varteš, verbal noun from vartidan, → vary.

Of or describing a → variation.

See also: → variation; → -al.

Any of the physical principles that indicate in what way the actual motion of a state of a mechanical system differs from all of its kinematically possible motions or states. Variational principles that express this difference for the motion or state of a system in each given instant of time are called → differential. These principles are equally applicable to both → holonomic and → nonholonomic systems. Variational principles that establish the difference between the actual motion of a system during a finite time interval and all of its kinematically possible motions are said to be → integral. Integral variational principles are valid only for holonomic systems. The main differential variational principles are: the → virtual work principle and
→ d’Alembert’s principle.

See also: → variational; → principle.

1. The quality or state of having different forms or types.
   

2. A number or collection of different things especially of a particular class.
   

3. Something differing from others of the same general kind.
   

4. Any of various groups of plants or animals ranking below a species (Merriam-Webster.com).

Etymology (EN): M.Fr. variété, from L. varietatem (nominative varietas) “difference, diversity; a kind, variety, species, sort,” from varius, → various.

Etymology (PE): Vartiné, from vartin, → various, + noun/nuance suffix -é.

The → moment of the resultant of a → coplanar system of → concurrent forces about any center is equal to the algebraic sum of the moments of the component forces about that center.

See also: Named after Pierre Varignon (1654-1722), a French mathematician, who outlined the fundamentals of statics in his book Projet d’une nouvelle mécanique (1687).

1. Of different kinds, as two or more things; differing one from another.
   

2. Marked by or exhibiting variety or diversity (Dictionary.com).

Etymology (EN): M.Fr. varieux and directly from L. varius “changing, different, diverse,” → vary.

Etymology (PE): Vartin, from vart “change,” present stem of vartidan, → vary, + adj. suffix -in.

1. To undergo change in form, substance, appearance, etc.
   
2. To change or alter. → covariance.

Etymology (EN): M.E. varien, from O.Fr. varier, from L. variare “change, alter, make different,” from varius “variegated, different, spotted.”

Etymology (PE): Vartidan “to change,” from Mid.Pers. vartitan “to change, turn” (Mod.Pers. gardidan “to turn, to change”); 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.”

Any physical quantity which requires a direction to be stated in order to define it completely, for example velocity. Compare with → scalar.

Etymology (EN): From L. vector “one who carries or conveys, carrier,” from p.p. stem of vehere “carry, convey;” cognate with Pers. vâz (in parvâz “flight”); Av. vaz- “to draw, guide; bring; possess; fly; float,” vazaiti “guides, leads”
(cf. Skt. vah- “to carry, drive, convey,” vahati “carries,”
pravaha- “bearing along, carrying,” pravāha- “running water, stream, river;” O.E. wegan “to carry;” O.N. vegr; O.H.G. weg “way,” wegan “to move,” wagan “cart;” M.Du. wagen “wagon;” PIE base *wegh- “to drive”).

Etymology (PE): Bordâr “carrier,” agent noun from bordan “to carry, transport” (Mid.Pers. burdan;
O.Pers./Av. bar- “to bear, carry,” barəθre “to bear (infinitive);” Skt. bharati “he carries;” Gk. pherein “to carry;” L. ferre “to carry;” PIE base *bher- “to carry”).

The study of → vectors and → vector spaces.

See also: → vector; → analysis.

Of a rotating body, a vector of magnitude ω (→ angular velocity) pointing in the direction of advance of a right-hand screw which is turned in the direction of rotation.

See also: → vector; → angular; → velocity.

In nuclear physics, a → boson with the spin quantum number equal to 1.

See also: → vector; → boson.

The study of vector functions between vector spaces by means of → differential and integral calculus.

See also: → vector; → calculus.

A → tensor density of → order 1.

See also: → vector; → density.

A vector each of whose → components is a → scalar field. For example, the → gradient of the scalar field F, expressed by:
∇F = (∂F/∂x)i + (∂F/∂y)j + (∂F/∂z)k.

See also: → vector; → field.

A function whose value at each point is n-dimensional,
as compared to a scalar function, whose value is one-dimensional.

See also: → vector; → function.

Any particle of unit spin, such as the W boson, the photon, or the rho meson.

See also: → vector; → meson.

The perturbation in the → primordial Universe plasma caused by → vorticity. These perturbations cause → Doppler shifts that result in → quadrupole anisotropy.

See also: → vector; → perturbation.

Of two vectors, a vector whose direction is perpendicular to the plane containing the two initial vectors and whose magnitude is the product of the magnitudes of these vectors and the sine of the angle between them: A x B = C, C = |AB sin α|. The direction of C is given by the → right-hand screw rule. Same as → cross product. See also → scalar product.

See also: → vector; → product.

A system of mathematical objects consisting of a set of (muultidimensional) vectors associated with a set of (one-dimensional) scalars, such that vectors can be added together and vectors can be multiplied by scalars while preserving the ordinary arithmetic properties (associativity, commutativity, distributivity, and so forth).

See also: → vector; → space.

The brightest star in the constellation → Lyra and the 5th brightest star in the sky. It is an A type → main sequence star of visual magnitude 0.03. Vega
is also one of the closer stars to the Earth, lying just 25.0 light-years away. Vega’s axis of rotation is nearly pointing at the Earth, therefore it is viewed pole-on. Fast rotation has flattened Vega at its poles, turning it from a sphere into an oblate spheroid. The polar diameter of Vega is 2.26 times that of the Sun, and its equatorial diameter 2.75 solar. The poles are therefore hotter (10,150 K) than the equator (7,950 K).

See also: Vega, from Ar. al-Waqi’ contraction of an-Nasr al-Waqi’ (النسر‌الواقع) “swooping eagle,” from an-Nasr “eagle, vulture” + al-Waqi’ “falling, swooping.”

1. Any of various herbaceous plants having parts that are used as food, such as peas, beans, cabbage, potatoes, cauliflower, and onions.
   

   2. Botany: Of, relating to, characteristic of, derived from, or consisting of plants or plant material (e.g. vegetable oils) (TheFreeDictionary.com).

Etymology (EN): → vegetate; → -able.

Etymology (PE): 1) Sabzijât “class or category of greens,” from sabzi, → green, + -jât a suffix denoting class, group, or division of things.

2. Giyâhi, related to giyâh, → plant.

Of, relating to, or characteristic of plants.

See also: → vegetate; → -al.

1. To grow or sprout as a plant by nutriment imbibed through roots and leaves.
   

   2. To lead a passive life without much physical, mental, or social activity.

Etymology (EN): Ultimately from L. vegetatus, p.p. of vegetare “to enliven, to animate,” from vegetus “vigorous, enlivened, active,” from vegere “to be alive, active;” akin to Skt. vaja- “force, strength;” PIE root *weg- “to be strong, be lively.”

Etymology (PE): Giyâhidan, infinitive forum giyâh, → plant.

1. All the plants or plant life of a place.
   

   2. (Botany) Plant life as a whole, especially the plant life of a particular region.
      

   3. The act or process of vegetating.

See also: Verbal noun from → vegetate.

1. Of, relating to, or characteristic of plants or their growth.
   

   2. Noting the parts of a plant not specialized for reproduction (TheFreeDictionary.com).

Etymology (EN): → vegetate; → -ive.

Etymology (PE): Giyâhvâr, from giyâh, → plant,

• -vâr suffix denoting “resembling, like,” → -oid.

1. A piece of opaque or transparent cloth worn by women as a covering for the head and shoulders.
   

2. Something that covers, screens, or conceals.

Etymology (EN): M.E. veile; O.Fr. voile “a head-covering” (also “sail”),
from L. vela, plural of velum “curtain, covering; sail.”

Etymology (PE): Vâšâmé “veil,” variants vâšâm, bâšâmé, bâšâm, from Mid.Pers. *varšâmak (loaned in Arm. varšamak “veil, scarf”); cf. Sogd. wâršâmé “veil;” from Proto-Ir. *varšāmaka-, from *varšām-, from *varš-, from var- “to cover;” cf. Av. var- “to cover, conceal;” Skt. var- “to cover;” Mid.Pers. warr “garment,” prefixed uzvâr-, uzvârdan “to uncover, show, apprehend, interprete;” Mod.Pers. šalvâr “trousers” (literally “thigh cover;” the first component šal “thigh,” šelang “thigh; step, distance between feet when walking,” cognate with Gk. skelos “leg”); Kurd. (Soriani) barg “cover; clothes,” da barg girtin “to wrap in a cover.”

A part of the → Cygnus loop, an old → supernova remnant.

See also: → veil; → nebula.

1. General: An act of covering with or as if with a veil.
   

2. A weakening of the → spectral lines of a star that occurs when the spectral lines are filled in by → continuum emission or light from a → companion star.

See also: Verbal noun from veil (v.).

Any of the tubes forming part of the blood circulation system of the body, carrying mainly oxygen-depleted blood towards the heart (OxfordDictionaries.com).

Etymology (EN): M.E. veine, from O.Fr. veine “vein, artery, pulse,” from L. vena “a blood vessel,” also “a water course, a vein of metal,” of unknown origin.

Etymology (PE): Siyâhrag, literally “black vessel,” from siyâh, → black, + rag, → vessel.

The Sail. A part of the constellation Argo in the Southern hemisphere, which was later divided into three parts, the others being Carina and Puppis. Vela is situated at about 10h right ascension, -50° declination. Abbreviation: Vel; Genitive: Velorum.

Etymology (EN): From L. vela, plural of velum “sail, curtain, covering,” velare “to cover, veil.”

Etymology (PE): Bâdbân “sail,” literally “wind keeper,” from bâd→ wind + -bân a suffix denoting “keeper, guard,” sometimes forming agent nouns or indicating relation, → host.

A pulsar with a short period (89 milliseconds) associated with the → Vela supernova remnant. It is approximately 1500 light-years distant. The Vela pulsar is one of the few pulsars detectable optically. Its optical flashes, of visual magnitude 26, were detected in 1977. Also named PSR 0833-45.

See also: → Vela; → pulsar.

A → supernova remnant located in the southern Milky Way in the constellation → Vela. It has a large angular diameter of about 8° and lies 250 ± 30 pc away (Cha et al. 1999, ApJ 515, L25). Its overall emission is dominated by the interaction of the → supernova blast wave with the → interstellar medium. This SNR is also notable for a number of protrusions extending well beyond its rim, which were suggested to be fragments of ejecta from the supernova explosion. X-ray spectroscopy has since confirmed several of these protrusions to indeed be strongly enriched with ejecta. The age of the SNR is estimated to be ~11,000 years, based on the spin-down rate of the associated → Vela pulsar, but ages as large as 20,000-30,000 years have also been argued.

See also: → Vela; → supernova remnant.

A compact radio source about 1500 light-years distant associated with the
Vela supernova remnant. It has a nonthermal radio spectrum and is
about 20 percent polarized. It is associated with the Gum Nebula, the
Vela pulsar, and the X-ray source 2U 0832-45.

See also: → Vela; X for → X ray.

The time rate of change of position in a given direction, measured as length per unit time. → speed.

Etymology (EN): L. velocitatem (nominative velocitas) “swiftness, speed,” from velox (genitive velocis) “swift.”

Etymology (PE): Tondâ, from tond “swift, rapid, brisk; fierce, severe” (Mid.Pers. tund “sharp, violent;” Sogdian tund “violent;” cf. Skt. tod- “to thrust, give a push,” tudáti “he thrusts;” L. tundere “to thrust, to hit” (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere “to thrust or bore through;”
PIE base *(s)teud- “to thrust, to beat”) + noun suffix -â.

A plot of the radial velocity of an object against time, derived from the Doppler shift of spectral lines.

See also: → velocity; → curve.

The → standard deviation of a velocity → distribution. It indicates how objects of
the sample move relative to one another. Objects with similar velocities have a small velocity dispersion, whereas objects with very different velocities have a large velocity dispersion.

See also: → velocity; → dispersion.

Fluid Mechanics: The rate at which the velocity changes with the distance across the flow. When a fluid flows past a stationary wall, the fluid right close to the wall does not move. However, away from the wall the flow speed is not zero. Therefore a velocity gradient exists, which is due to adhesive, cohesive, and frictional forces. The amount of the velocity gradient is characteristic of the fluid.

See also: → velocity; → gradient.

In the theory of → radiation-driven winds, an equation that describes the behavior of the → wind velocity of → hot stars as a function of distance from the star. This velocity β-law is given by the expression:

v(r) = v_∞(1 - R/r)^β, where v_∞ is the → terminal velocity, R is the stellar radius, and r the distance from the center. For → O-type stars, the exponent is estimated to be β = 0.8.

See also: → velocity; → law.

A → physical constant which represents the ultimate speed limit for anything moving through space, according to the theory of → special relativity. It is the speed of propagation of → electromagnetic waves in a vacuum, equal to 299,792.458 km/s (nearly 3 x 10⁸ m/s). The velocity of light appears as the connecting link between mass and energy in the → mass-energy relation. Usually denoted by c, from L. celeritas “swiftness,” from celer “swift,” → acceleration.

See also: → velocity; → speed;
→ light.

→ dynamic pressure.

See also: → velocity; → pressure.

A plot of the fluid velocity as a function of position.

See also: → velocity; → profile.

Of a dynamical system, a three-dimensional space which consists of the set of values that the velocity can take (v_x, v_y, v_z). → phase space.

See also: → velocity; → space.

The linear relation wherein all galaxies are moving away from one another,
with velocities that are greater with increasing distance of the galaxy. Same as → Hubble’s law.

See also: → velocity; → distance; → relation.

The location in a → fluid stream where the
→ cross section of the → stream is at a minimum, and fluid velocity is the highest, such as in the case of a → jet issuing out of a → nozzle.

Etymology (EN): L. vena “channel;” contracta, “contracted,” → contract.

Etymology (PE): Šâre-gozar “fluid passage,” → fluid; → passage, terengidé, → contracted.

A schematic diagram using circles to represent sets and the relationships between them. Each circle represents one set. Two or more may be overlapped. The areas of overlap indicate subsets.

See also: Named after John Venn (1834-1923), a British logician and philosopher, who introduced the diagram; → diagram.

The subterranean conduit from the underlying → magma chamber through which a volcano ejects igneous material. Same as volcanic vent.

Etymology (EN): M.E. venten “to furnish (a vessel) with a vent,” from O.Fr. esventer “to air,” from es-, → ex-,

• venter, from vent, from L. ventus “wind.”

Etymology (PE): Dudkaš “chimney,” literally “smoke extractor,” from dud, → smoke,

• kaš “to extract, to draw,” present stem of kešidan/kašidan “to carry, draw, protract,
  trail, drag” (Mid.Pers. kešidan “to draw, pull;” Av. karš- “to draw; to plow,” karša- “furrow;” cf. Skt. kars-, kársati “to pull, drag, plow;”
  Gk. pelo, pelomai “to move, to bustle;” PIE base k^wels- “to plow”).

A → device used to → measure the → quantity of → fluid  → flowing through a → pipe.

See also: Named after Italian physicist Giovanni Battista Venturi (1746-1822); → tube

The second → planet from the → Sun, at a mean distance of roughly 108.21 × 10⁶ km (0.72 → astronomical units). The → greatest elongation of Venus is about 47°, making it visible at most about 3 hours after sunset or before sunrise. Venus has the most circular orbit of any planet in the solar system. Venus is only slightly smaller than Earth (95% of Earth’s diameter, 80% of Earth’s mass). Its chemical composition and density are comparable to those of the Earth. It takes Venus just under 224.401 days to orbit the Sun, compared to the 365 day → orbital period of the Earth. Venus’ rotation is → retrograde, that is it actually rotates from east to west, as opposed to west to east (→ prograde) which is the common rotating direction of most other planets. Seen from Venus, the sun would rise in the west and set in the east. Moreover, it takes about 244 Earth days for Venus to rotate once (→ sidereal rotation). This is longer than its orbital period. The length of its → solar day is about 117 Earth days. → Venus rotation. Its axial tilt is only three degrees, so there are no seasons on Venus. The → atmosphere on the surface of Venus consists mostly of → carbon dioxide, with a small trace of → nitrogen. Venus has a surface pressure about 90 times that of the Earth. See also: → Venus visibility, → transit of Venus.

Etymology (EN): O.E., from L. Venus, the goddess of beauty and love in ancient Roman mythology, from venus “love, sexual desire, beauty, charm;” PIE base *wen- “to desire, love, wish;” cf. Av. vāunuš “lovingly,” vantā- “beloved one, wife;” Skt. van- “to love, desire,” vanánā- “desire,” vanitā- “beloved one, wife;” O.H.G. wunsc(h) “wish,” wunsken “to wish.”

Etymology (PE): Nâhid, planet Venus, Mid.Pers. Anahid; O.Pers. anāhita-
“immaculate, unstained,” goddess of pure waters and fertility, from Av. arədvī-sūra-anāhita “valient and unsustained lord of waters,” from arədvī- (Skt. Saravastī) probably “she who possesses water,” + sūra- “strong, powerful” (Skt. śūra- “valiant, courageous”) + anāhita- “unstained,” from an- negation prefix + āhita “spotted.”

The → sidereal rotation period of Venus, or its → sidereal day, is 243.025 Earth days (retrograde). The length of a → solar day on Venus (that is one entire day-night period) is 116.75 Earth days, that is significantly shorter than the sidereal day because of the retrograde rotation. One Venusian year is about 1.92 Venusian solar days.

See also: → Venus; → rotation.

The conditions under which Venus can be seen from Earth as
it travels in its orbit around the Sun.

The → synodic period of Venus, that is the time Venus takes to be seen again from the Earth in the same position with respect to the Sun, is 583,92 days
or just over 19 months. When Venus is between Earth and Sun (→ inferior conjunction) or on the far side of the sun (→ superior conjunction), it is invisible in the Sun’s glare. Since its → greatest elongation from the Sun is never more than 47°, Venus appears only as “the morning star” and “the evening star.” So at its greatest → western elongation Venus will rise about three hours ahead of the Sun and at its greatest → eastern elongation it will set about three hours after sunset. Its entire cycle is as follows:

Day 0: Superior conjunction, “full Venus.”
Day 35: Venus appears in evening sky.
Day 221: Greatest → eastern elongation, “last quarter.”
Day 271: Retrogression of Venus begins.
Day 286: Disappearance from the evening sky.
Day 292: Inferior conjunction, “new Venus.”
Day 298: Venus appears in morning sky.
Day 313: Retrogression ends.
Day 362: Greatest → western elongation, “first quarter.”
Day 549: Disappearance from morning sky.
Day 584: Superior conjunction, “full venus.”

Therefore, Venus is visible as an evening star for 286 Earth days,
as a morning star for 251 days, and is invisible for 47 days.

In addition, the orbital periods of Earth and Venus are closely correlated. After 8 Earth years or 13 Venus orbits, the two planets assume almost the same relative positions – just 0.032 percent away from a perfect orbital resonance of 8:13. After this period of about 2920 Earth days, Venus appears just 1.5° (about 22 hours) in advance of its former position.

Moreover, Venus exhibit → phases because its orbit lies within the Earth’s. When Venus situated on the far side of the Sun from Earth, the planet is fully illuminated from our point of view. But its disk is small, just 10’’ across, because it is nearly 300 million km away. When Venus is almost closest to Earth, on the near side of the Sun, it’s about 60 million km away. Then it appears as a slender but much brighter crescent with a disk nearly 50’’ across. See also → transit of Venus.

See also: → Venus; → visibility.

A new kind of optical telescope with a 8.4-m diameter → primary mirror currently under construction in Chile and scheduled to begin operations in October 2023. Initially named Large Synoptic Survey Telescope (LSST), it will use a 3200 megapixel camera and an automated data processing system.

It will have a large → field of view almost 10 square degrees of sky, or 40 times the size of the full moon. The LSST will move quickly between images
to rapidly → survey the sky.

From its mountain top site in the Andes (Cerro Pachon, a 2,682-m high mountain in Coquimbo Region), the LSST will take more than 800 panoramic images each night with its 3.2 billion-pixel camera, recording the entire visible sky twice each week. Each patch of sky it images will be visited 1000 times during the survey, each of its 30-second observations will be able to detect objects 10 million times fainter than visible with the human eye.

The LSST’s combination of telescope, mirror, camera, → data processing, and survey will capture changes in billions of faint objects. Hence, the data it provides will be used to create an animated, three-dimensional cosmic map with unprecedented depth and detail. This map will serve many purposes, from locating the → dark matter and characterizing the properties of the → dark energy, to tracking transient objects, to studying our own Milky Way Galaxy in depth. It will even be used to detect and track → potentially hazardous asteroids that might impact the Earth.

See also: Named after Vera C. Rubin (1928-2016) whose work on galaxy rotation rates supported the existence of dark matter in galactic halos.

A member of a major category of words that refers to an action or a state. Verbs present a complex system of forms in Indo-European languages. The set of → inflectional forms of a verb is called a → conjugation. Verbs are usually distinguished for person and number along with tense and mood (if applicable).

Etymology (EN): M.E., from O.Fr. verbe from L. verbum “verb,” originally “a word,” from PIE root *wer- “to speak;” cf. Av. urvāta- “command;” Skt. vrata-
“command, vow;” Gk. rhetor “public speaker,”
eirein “to speak, say;” Lith. vardas “name;” Goth. waurd, O.E. “word.”

Etymology (PE): Karvâz, literally “action word,” from kar- present stem of kardan “to do, make” (Mid.Pers. kardan;
O.Pers./Av. kar- “to do, make, build,” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make”) + vâz “word,” variants vâ&#382, âvâz, vâj, vât, vâ, → voice.

In exactly the same words; word for word.

Etymology (EN): From M.L. verbatim “word for word,” from L. verbum “word,” → verb.

Etymology (PE): Vâž-be-vâž, literally “word for word,” from vâž, → word.

Optics: A measure of the convergence or divergence of a pair of light rays, defined as the reciprocal of the distance between the point of focus and a reference plane.
Ophthalmology: The turning motion of the eyeballs toward or away from each other.

Etymology (EN): back formation from → convergence and → divergence, ultimately from L. vergere “to turn, bend, be inclined;” cognate with Pers. gardidan “to turn, to change,” → version.

Etymology (PE): Gerâyi, from gerâyidan “to incline toward; to intend; to make for.”
Gerâ may be a variant of Mod.Pers. kil “bent, inclined” (k/g and l/r interchanges),
from PIE base *klei- “to lean, incline,” cognate with L. clinare “to bend” (E. declination, inclination, etc.), Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” P.Gmc. *khlinen (Ger. lehnen, E. lean).

In logical positivism philosophy, the claim that a statement is literally meaningful (it expresses a proposition) if and only if it either actually has been verified or could at least in principle be verified.

See also: Quality noun from → verifiable; → principle.

That can be verified. → verify; → verification; → verifiability principle.

See also: → verify; → -able.

The act of verifying. The state of being verified.
The process of research and examination required to establish correctness, authenticity, or validity.

See also: Verbal noun of → verify.

To ascertain the truth or correctness of, as by examination, research, or comparison.

Etymology (EN): M.E. verifien, from M.Fr. verifier, from M.L. verificare “to make true,” from L. verus “true;” → -fy.

Etymology (PE): Râst-jostan, literally “to seek the truth, to seek the right,” from râst “right, true; just, upright, straight” (Mid.Pers. râst “true, straight, direct;” Soghdian rəšt “right;” O.Pers. rāsta- “straight, true,” rās- “to be right, straight, true;” Av. rāz- “to direct, put in line, set,” razan- “order;”
cf. Skt. raj- “to direct, stretch,” rjuyant- “walking straight;” Gk. orektos “stretched out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” PIE base *reg- “move in a straight line,” hence, “to direct, rule”)

• jostan/juyidan “to seek, strive for” (Proto-Iranian *iud- “to struggle for something, to fight;” Av. yūδ- “to fight, struggle;” Mod.Pers. justan, juy- “to search, seek, ask for;” cf. Mid.Pers. vijuyihitan “to search, seek”).

Of or pertaining to spring. → vernal equinox.

Etymology (EN): From L. vernalis “of the spring,” from vernus “of spring,” from uēr “spring,” cognate with Pers. bahâr, as below.

Etymology (PE): Bahâri of or pertaining to bahâr “spring;” Mid.Pers. wahâr “spring;” O.Pers. vāhara- “spring time,” θūra-vāhara- “name of a spring month;” Av. vaηhar “spring;” cf. Skt. vasara- “relating or appearing in the morning;”
Gk. ear “spring;” L. uēr “spring,” vernus “of spring;” O.N. vār “spring;” Lith. vasara “summer;” O.C.S. vesna “spring.”

The point of intersection between the ecliptic and the celestial equator at which the Sun passes from south to north of the celestial equator during its apparent annual motion. The instant of this event. It occurs on March 20, 21 or rarely 19. At the vernal equinox, as with the → autumnal equinox, night and day are equal in length world over. Several thousands years ago the vernal equinox was in Aries, but because of precession it has now slid west into Pisces. Right ascension and celestial longitude are measured from the vernal equinox. Also known as spring equinox.
→ First Point of Aries.

See also: → vernal; → equinox.

The intersection point between the → celestial equator and the → ecliptic from where the Sun apparently enters into the northern celestial hemisphere.

See also: → vernal; → point.

The time interval between two successive passages of the Sun, when the true longitude of the Sun is considered.
In other words, the interval during which the Sun’s true longitude increases by 360 degrees. Its mean length for the epoch J2000.0 is 365.24236460 real solar days (approximately 365.2424 days). The vernal-equinox year, on which the Iranian calender is based, should not be confused with → tropical year. See also: A concise review of the Iranian calendar. → Iranian calendar

See also: → vernal; → equinox; → year.

A small movable scale running parallel to the main graduated scale in certain measuring instruments, such as the → sextant, used to obtain a fractional reading of one of the divisions on the main scale. Also known as Vernier scale.

See also: After the French mathematician Pierre Vernier (1580-1637), who invented the scale in 1631.

A particular form or variant of something.
An account of something, given from a specific point of view.

Etymology (EN): M.E., from M.Fr. version, from M.L. versionem (nominative versio) “a turning,” from p.p. stem of L. vertere “to turn;” cognate with Pers. gardidan “to change,” as below.

Etymology (PE): Gardâk, present stem of gardidan “to change, to turn” (Mid.Pers. vartitan “to change, to turn;” 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”) + -âk noun suffix.

Plural form: vertices.

1. Astro.: A point in the → celestial sphere toward which or from which the common motion of a group of stars is directed.
   

2. Math.: The → point where two → sides of a → plane figure or an → angle  → intersect.
   

3. In → graph theory, any of the points of which graphs are formed. Same as → node.

Etymology (EN): From L. vertex “highest point,” literally “the turning point,” originally “whirlpool,” from vertere “to turn,” cognate with Pers. vartidan, gardidan, → version.

Etymology (PE): Târak, variant târ “top, vertex, head, the middle of the head;” cf. Sogd. târ “summit, forehead;” Yaghnobi tôr(k) “the back of the head;” Yazghulami tur “summit, head;” Proto-Ir. *starH- “to spread,” → expand; PIE *ster- “spread, extend.”

1. The apparent → direction of → gravity at the point of observation.
   

2. Being in a position or direction → perpendicular to the → plane of the → horizon. See also → normal.

Etymology (EN): Vertical, literally “of or at the vertex, directly overhead,” from M.Fr. vertical, from L.L. verticalis “overhead,” from L. vertex (genitive verticis) “highest point”

Etymology (PE): Hajin, from haj, variant hac “anything held vertical, erected in the manner of a spear” (Dehxodâ), may be from Proto-Ir. *hac- “to follow;” cf. Av. (+ *ā-) hac- “to stick to;” Mid.Pers. hâz- “to lead, guide;”
→ associate.

The greater circle on the celestial sphere which passes through → zenith, → nadir,
and the star and whose plane is perpendicular to the plane of horizon. Same as → azimuth circle.

See also: → vertical; → circle.

In computer science, a scaling in which the processing power of the same node/system is increased by increasing/decreasing its resources (CPU, memory, etc.). See also → horizontal scaling.

See also: → vertical; → scaling.

Plural of → vertex.

See also: → vertex.

In a high degree. → Very Large Telescope (VLT); → very massive star; → very high frequencies (VHF).

Etymology (EN): M.E.; O.Fr verai “true;” L. verax (genitive veracis) “truthful,” from verus “true.”

Etymology (PE): Besyâr, from bas “many, much” (Mid.Pers. vas “many, much;” O.Pers. vasiy “at will, greatly, utterly;” Av. varəmi “I wish,” vasô, vasə “at one’s pleasure or will,” from vas- “to will, desire, wish”).

Radio frequencies in the range 30 to 300 mega Hz.

See also: → very; → high; → frequency.

A radio interferometer consisting of 27 antennas, each 25 m in diameter, in a Y-shaped configuration. It is located about 100 km west of Socorro, New Mexico, and is operated by the United States National Radio Astronomy Observatory. The VLA has the resolution of a single antenna 36 km wide and the sensitivity of a dish 130 m across.

See also: → very; → large; → array.

A network of ten 25-m radio telescopes for
→ very-long-baseline interferometry (VLBI), operated by the U.S. National Radio Astronomy Observatory. Eight of the VLBA telescopes are distributed
across the continental United States, while the other two are in Hawaii and the Virgin Islands, giving a maximum baseline of about 8,000 km and a resolution better than a milliarcsecond at its shortest wavelength.

See also: → very; → large; → baseline; → array.

An observing facility consisting of four 8.2 m telescopes, with the combined collecting area of a 16 m mirror, owned and operated by the European Southern Observatory at an altitude of 2635 m at the Paranal Observatory in Chile. The four reflecting unit telescopes are called Antu “Sun” in the language of Chile’s indigenous Mapuche people, Kueyen “Moon,” Melipal “Southern Cross,” and Yepun “Venus.” Each unit is equipped with several sophisticated instruments. The light of the individual telescopes can be combined using interferometric techniques to achieve superior resolution. → VLT Interferometer (VLTI). The wavelength range covered by the VLT is extremely wide, ranging from deep ultraviolet to mid-infrared.

See also: → very; → large; → telescope.

In evolutionary models of → post-asymptotic giant branch stars,
the occurrence of the helium shell burning when the star has reached the → white dwarf cooling track.
This leads to the possibility of a violent → helium shell flash and expansion on a time-scale of ≤ 10 years. The rapid expansion and prompt change in surface composition observed in → Sakurai’s Object are thought to be due to such a very late thermal pulse.

See also: → very; → late; → thermal; → pulse.

A star with a typical mass of one-hundredth of the mass of the Sun (→ solar mass) and a luminosity of about one-millionth that of the Sun (→ solar luminosity). Same as
→ brown dwarf.

See also: → very; → low; → mass; → star.

A star of mass around 100 → solar masses. See also:
→ supermassive star, → massive star, → canonical upper limit.

See also: → very; → massive; → star.

A special type of carbonaceous → interstellar dust grains
with a size ranging from 10 to 150 Å and consisting of tens to hundreds of atoms. In contrast to → big grains, VSGs are not in → thermal equilibrium. They can be heated to very high temperatures (~ 1000 K) by the absorption of a single photon. It is thought that VSGs are clusters of → PAH.

See also: → very; → small; → grain.

A technique in radio interferometry in which the individual telescopes are not directly
connected together, but instead make their observations separately with very accurate timings. The data are later sent to a central correlator to be combined. With this technique the individual telescopes can be arbitrarily far apart, and so the technique provides the highest resolution images in astronomy, typically down to a few milliarcseconds.

See also: → very; → large; → baseline; → interferometry.

1. A tube or duct, as an artery or vein, containing or conveying blood or some other body fluid.
   

   2. Botany: A duct formed in the xylem, composed of connected cells that have lost their intervening partitions, that conducts water and mineral nutrients (Dictionary.com).

Etymology (EN): M.E., from O.Fr. vessel “container, receptacle; ship,” from L.L. vascellum “small vase or urn,” also “a ship,” alteration of L. vasculum, diminutive of vas “vessel.”

Etymology (PE): 1) Rag “blood vessel, vein; lineage, race, stock,” from Mid.Pers. rag, from Proto-Iranian *raha-ka-, from *raha- “liquid, essence,” + suffix -ka-; cf. Av. ranhā- “name of a mythical stream;” Skt. rása- “juice (of plants), liquid, essence,” rásavant-, rasin- “juicy;” L. ros “dew;” Lith. ràsa “dew;” O.C.S. rosa “dew.”

2. Âvand “vessel,” contraction of âbvand, literally “water utensil,” from âb, → water, + vand “vessel, vase, cup,” from bastan “to bind, shut,” → shutter.

To oscillate with a continuing periodic change relative to a fixed reference point or state of equilibrium. → oscillate.

Etymology (EN): From L. vibratus, p.p. of vibrare “to move quickly to and fro, shake” (cf. Lith. wyburiu “to wag the tail,” Dan. vippe, Du. wippen “to swing,” O.E. wipan “to wipe”).

Etymology (PE): Šividan “to vibrate, move to and fro, to tremble,” related to šodan, šow- “to go; to become;” Av. šiyav-, š(ii)auu- “to move, go,” šiyavati “goes,” šyaoθna- “activity; action; doing, working;” O.Pers. šiyav- “to go forth, set,” ašiyavam “I set forth;” Skt. cyu- “to move to and fro, shake about; to stir,” cyávate “stirs himself, goes;” Gk. kinein “to move;”
Goth. haitan “call, be called;” O.E. hatan “command, call;” PIE base *kei- “to move to and fro.”

1. The act of vibrating. The state of being vibrated.
   

2. A continuing → periodic  → oscillation relative to a fixed reference point or → equilibrium state.

See also: Verbal noun from → vibrate.

Of or pertaining to → vibration. → vibrational mode.

See also: → vibration; → -al.

The energy due to the vibration of the molecules making up atoms (→ molecular vibration). A molecule in space can have energies in various forms: → rotational energy, vibrational energy, or electronic energy. These energies of molecules are → quantized and a particular molecule can exist in different rotational and vibrational → energy levels. The molecules can move from one level to another level only by a jump involving a finite amount of energy. → Quantum mechanics predicts that any molecule can never have zero vibrational energy, that is atoms can never be completely at rest relative to each other. The harmonically oscillating molecules can undergo vibrational changes determined by simple selection rules obtained from → Schrödinger equation.

See also: → vibrational; → energy.

The frequency at which the atoms in a molecule vibrate. The frequencies of → molecular vibrations in diatomic molecules are in the order of 10^-12 to 10^-14 Hz. In such molecules, the only → vibrational mode available is along the bond. More complicated molecules have many types of vibration and stretching modes.

See also: → vibrational; → frequency.

Any of the ways in which a → molecule vibrates. Each vibrational mode has a different → frequency frequency. The number of vibrational modes of a molecule is determined by the number of atoms in it. The number of vibrational modes for a non-linear molecule is 3N - 6, where N is the number of atoms making up the molecule. For a linear molecule it is 3N - 5.

See also: → vibrational; → mode.

A transition between two → quantized  → levels of a → molecule that have different vibrational energies.

See also: → vibrational; → transition.

A slight change in the → energy level of a → molecule due to → vibrational transition and/or → rotational transition.

See also: → vibrational; → rotational; → transition.

A person or thing that suffers harm or death, from another or from some adverse act or circumstance.

Etymology (EN): M.Fr. victime, from L. victima “sacrificial animal.”

Etymology (PE): Lišé, from Mid.Pers. lyš- / rêš- “to wound, hurt;” Pers. riš, rêš “wound;” Av. raēš- “to get hurt, suffer damage;” cf. Skt. reṣ “to get harmed.”

A series of 532 years, arising from the cycles of the Sun and Moon multiplied into one another. It was used by the Western churches for many years, in computing the time of Easter, till the → Gregorian calendar was established.

See also: Named after Victorius (a French clergyman), who invented the period about the middle of the 5th century.

1. (n.) The visual part of a television broadcast.
   Something that has been recorded on videotape, especially a movie or music performance.
   
2. (adj.) Of or pertaining to the electronic apparatus for producing the television picture.

Etymology (EN): From L. video “I see,” from videre→ vision.

Etymology (PE): Didâré, didâri, from didâr “vision, sight,” 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 → lunisolar calendar used now in Vietnam mainly for determining seasonal holidays and cultural events. It is in fact the → Chinese calendar computed for Hanoi. It has 12 months of 29 or 30 days each (→ synodic month) and the year totals 355 days. The → lunar year is therefore 11 days shorter than its solar counterpart. To keep up with the solar pace, every 19 years seven extra months are added to the calendar.
In practice, approximately every third year an → embolismic month is included. The New Year, called Tet, begins at the second → new moon after the → winter solstice. The Vietnamese calendar has some minor differences with the Chinese calendar. For example, it uses the cat and buffalo instead of the Chinese rabbit and cow respectively in the → zodiac.

See also: From Vietnam “Viets of the South,” from Viet the people’s name + nam “south;” → lunar; → calendar.

1. An instance of seeing or beholding; visual inspection.
   

   2. Sight; vision (Dictionary.com).

Etymology (EN): M.E. v(i)ewe, from M.Fr. veue “sight,” from V.L. *viduta, from *vidutus, from L. visus, p.p. of videre “to see,” → vision.

Etymology (PE): Did, → vision.

The maximum angle at which a display, such as a TV screen, can be viewed with acceptable visual performance.

Etymology (EN): Viewing, from view, M.E. v(i)ewe (n.); M.Fr. veue “sight,” feminine p.p. of veoir “to see,” from L. videre “to see,” → vision; → angle.

Etymology (PE): Zâviyé, → angle; did→ vision.

The numeral system based on → twenty.

Etymology (EN): From L. vigesimus, variant of vicesimus, vicensimus “twentieth,” from vigniti “twenty” + → -al.

The gradual reduction in energy through an optical system as the off-axis angle increases, resulting from limitations of the clear apertures of elements within the system.

Etymology (EN): From vignette “an unbordered picture, often a portrait, that shades off into the surrounding color at the edges;” “softening the edges of a picture in vignette style;” from Fr. vignette, O.Fr., diminutive of vigne “vineyard;” from L. vinea “vine, vineyard,” from vinum “wine.”

Etymology (PE): Labe-puš, literally “limb covering,” from labé “limb,” from lab “lip;” (Mid.Pers. lap; cf. L. labium; O.E. lippa; E. lip; Ger. Lefze) + puš present stem of pušidan “to cover; to put on” (Mid.Pers. pôšidan, pôš- “to cover; to wear;” cf. Mid.Pers. pôst; Mod.Pers. pust “skin, hide;” O.Pers. pavastā- “thin clay envelope used to protect unbaked clay tablets;” Skt. pavásta- “cover,” Proto-Indo-Iranian *pauastā- “cloth”).

A small group of dwellings in a rural area, usually ranking in size between a hamlet and a town.

Etymology (EN): M.E. village, from O.Fr. village, from L. villaticus, ultimately from L. villa “country house,” related to vicus “village, group of houses,” vecinus “neighbor;” cf. Mid.Pers. wis “village,” Manichean Mid.Pers. wys “to repose;” Khotanese bäsā “house;” O.Pers. viθ- “settlement;” Av. vis- “to enter;” cf. Skt. vis- “house;” Albanian vis- “place;” Gothic weihs “village;” E. suffix wich, -wick (as in Norwich and Brunswick); Iranian dialects/languages Xonsâri hos “house, home;” Lori, Laki hoš, höš, hovš “house, home;” Baluci ges “house, home;” Kermânshahi, Ilâmi, Lori huz “clan, family, tribe;” Parachi γos, γus “house, home;” Wakhi wiš- / wišt- “to set [of sun];” Yaghnavi wes-, ves- “to go down;” PIE *ueik- “to settle (down).”

Etymology (PE): Deh, dehkadé, from Mid.Pers. dêh “village, settlement, country;” loaned into Armenian deh “district;” O.Pers./Av. dahyu- “country.”

A star located in the → Virgo constellation, also called ε Virginis. It is a yellow → giant of apparent magnitude 2.83 and → spectral type G8 III. Vindemiatrix lies about 102 → light-years from Earth, has a luminosity 83 times the → solar luminosity, and a → surface temperature about 5,000 K.

Etymology (EN): L. Vindemiatrix “grape-harvestress,” feminine of vindemiator “grape-hervester,” translation of Gk. names Protrugeter, Protrugetes, and Trugeter used by Ptolemy, Plutarch, and other Gk. authors. The first of these words denoted “Fruit-plucking Herald.” In Gk. trugos is the process of collecting the grapes. It has been argued that the first visibility of the star in morning light was the time of gathering the grapes. The original Gk. name was translated in Ar. as al-Mutaqaddim lil-Qaţāf (المتقدم للقطاف) “Harvest Precursor,” and later to Almuredin maybe with the same meaning, but the etymology is not clear.

Etymology (PE): Angurcin “grape harvester,” from angur “grape” (related to quré “unripe grape,” angordé “a single grape, a berry;” cf. Skt. ankurá- “buds, sprout, shoot, blossom, swelling”) + cin present stem of cidan “to gather, collect,” related to gozidan “to choose, select”
(Mid.Pers. cyn- “to gather, collect;” Av. ci- “to heap up, gather”

Any of various plants, especially the grapevine, having long flexible stems that creep along the ground or climb by clinging to a support by means of tendrils, leafstalks, etc (Dictionary.com).

Etymology (EN): M.E., from O.Fr. vigne “vine, vinyard,” from L. vinea “vine, vineyard,” from vinum “wine,” from PIE *win-o- “wine.”

Etymology (PE): Mow, tâk, raz “vine,” Persian words of unknown origin.

To break, infringe, or transgress (a law, rule, agreement, promise, instructions, etc.). → parity violation.

Etymology (EN): M.E., from L. violatus p.p. of violare “to treat with violence, violate,” from violentus “violent, " from vis “force, violence.”

Etymology (PE): Enâhidan infinitive from enâh, from Av. aēnah- “violence, mischief, crime, outrage,” from aēn- “to do violence to, to violate, to injure, to offend;” cf. Skt. énas- “offence, mischief, crime, sin;” Gk. ainos “terrible.”

The act of violating. The state of being violated. → parity violation

See also: Verbal noun from → violate.

Intense in force, effect; severe; extreme.

See also: → optically violent variable (OVV) quasar, → violent galaxy, → violent interstellar environment, → violent relaxation, → violent star formation.

Etymology (EN): M.E., from O.fr. violent, from L. violentus “vehement, forcible.”

Etymology (PE): Surâ, from Av. sūra- “strong, powerful, mighty;” cf. Skt. śūra- “strong, powerful, valiant.”

A type of galaxy that releases a tremendous amount of energy, on the average
10⁵⁸ ergs, compared with a supernova release of 10⁴⁹ ergs. Violent galaxies include quasars and exploding galaxies. About 1 percent of the galaxies are classified as violent. The nearest violent galaxy is Cen A.

See also: → violent; → galaxy.

A description of the interstellar medium justified by the presence of supersonic shock waves, massive star winds, turbulence, supernova explosions, etc.

See also: → violent; → interstellar; → environment.

A process in which a dynamical system made up of many objects (star cluster, galaxy cluster) rapidly relaxes from a chaotic initial state to a quasi-equilibrium.

See also: → violent; → relaxation.

The concept of star formation pertaining to a variety of systems (OB associations, giant H II regions, H II galaxies, massive star clusters, etc.) that are believed to have formed large numbers of stars in a very short time.

See also: → violent; → star formation.

A color at the opposite end of the visible spectrum from red, an effect of light with a wavelength between 4000 and 4500 Å. → ultraviolet.

Etymology (EN): M.E., from O.Fr. violete, diminutive of viole “violet,” from L. viola, akin to Gk. ion “violet.”

Etymology (PE): Banafš “violet,” related to banafšé “violet flower;” Mid.Pers. vanafšak “violet flower.”

A layer of particles in the upper Martian atmosphere that scatter and absorbs electromagnetic radiation at shorter wavelengths, making the atmosphere opaque to blue, violet, and ultraviolet light.

See also: → violet; → layer.

The Maiden. A large constellation of the Zodiac, situated at the celestial equator with 13h right ascension, 2° south declination. The brightest star is the first magnitude → Spica, and there are seven others brighter than fourth magnitude. Because of the presence of a background galaxy cluster, → Virgo cluster, this constellation is especially rich in galaxies. Eleven of the brighter galaxies are listed in the → Messier catalog.
Abbreviation: Vir; Genitive: Virginis.

Etymology (EN): L. virgo “maiden, unwedded girl or woman.”

Etymology (PE): Dušizé “maiden, virgin;” Mid.Pers. dôšizag “maiden, virgin,” related to duxtar, duxt “daughter” (O.Pers. *duxçi-;
Av. dugədar-, duγdar-; cf.
Skt. duhitár-, Gk. thygater;
E. daughter; Ger. Tochter; PIE base *dhug(h)əter-); Manichean Mid.Pers. duxš “princess, girl of noble birth;” O.Pers. duxši- (Elamite du-uk-ši-iš) “royal princess” + diminutive suffix -izag, -izé (as in suffixed Fr. demoiselle, from V.L. *dominicella, from domin(a) “lady” + -i- + -cella diminutive suffix).

The most powerful radio source in the constellation Virgo, among the thousands of galactic systems comprising the → Virgo Cluster. Optically, it is an elliptical galaxy (M87) with a luminous blue jet about 1500 pc long. It is also an X-ray source (3C 274, Virgo X-1, 2U 1228+12).

See also: → Virgo.

The largest and nearest galaxy cluster to the Local Group. The Virgo cluster spans 120 square degrees on the sky and contains on the order of 2,000 galaxies. It is located at a distance of about 60 million light-years. It is an irregular cluster with no central concentration. The giant elliptical galaxy M87 is the most massive in the cluster.

See also: → Virgo; → cluster.

A → Michelson interferometer using → laser beams designed to detect → gravitational waves. It consists of two 3-km-long arms, which house the various machinery required to form a → laser interferometer. The gravitational waves searched for have frequencies between 10 Hz and 10 kHz. Virgo has been designed and built by a collaboration between the French Centre National de la Recherche Scientifique (CNRS) and the Italian Istituto Nazionale di Fisica Nucleare (INFN). It is now operated and improved in Cascina, a small town near Pisa on the site of the European Gravitational Observatory (EGO), by an international collaboration of scientists from France, Italy, the Netherlands, Poland, and Hungary.

The initial Virgo detector observed the sky between 2007 and 2011 together with the two interferometers of the → Laser Interferometer Gravitational-Wave Observatory (LIGO), located in the United States. Virgo underwent a major upgrade after a long shutdown period. The “Advanced Virgo” overhaul lasted 5-year and costed 23 million Euros. The upgraded observatory was inaugurated on 20 February 2017 and, notably, detected the → GW170817 event.

See also: Named after the → Virgo cluster of galaxies whose stellar explosions it aims to detect; → interferometer.

The irregular supercluster that contains the Virgo cluster and the Local Group. At least 100 galaxy groups and clusters are located within its diameter of 110 million light-years. It is one of millions of superclusters in the observable Universe.

See also: → Virgo; → supercluster.

Of or pertaining to the interactive forces between components of a system, such as particles or molecules in a gas or stars in a cluster.
→ virial equation of state, → virial equilibrium, → virial mass, → virial parameter, → virial radius, → virial temperature, → virial theorem, → virialization.

Etymology (EN): From L. vires, plural of vis “strength,” and by extension
“force” or “energy,” first used by Rudolf Clausius in the investigation of problems in molecular physics.

Etymology (PE): Viriyâl, loan from E., as above.

In thermodynamics, a generalized → equation of state obtained when the → compression factor Z is expanded in terms of a power series, e.g.: Z = 1 + B(T) / V_m + C(T) / V_m² + …

See also: → virial; → equation of state.

The condition of a physical system which obeys the → virial theorem.

See also: → virial; → equilibrium.

The mass of a cluster of stars or galaxies in statistical equilibrium derived by using the → virial theorem.

See also: → virial; → mass.

A dimensionless parameter that measures the ratio of thermal plus kinetic energies to gravitational energy of a physical system, such as a molecular cloud. The virial parameter is expressed as: α_vir = 5σ²R / GM, where R and M are the radius and mass of the cloud respectively, σ is the one-dimensional → velocity dispersion inside the cloud, and G the → gravitational constant. It indicates whether a cloud could be bound or not. For molecular clouds that are confined by their surface pressure and for which self-gravity is unimportant, α_vir is much larger than unity, whereas α_vir
is ~ 1 when the gravitational energy of a clump becomes comparable to its kinetic energy. See, e.g., Bertoldi & McKee, 1992 (ApJ 395, 140). See also → virial theorem.

See also: → virial; → parameter.

The radius centered on a galaxy containing matter at 200 times the → critical density of the Universe.

See also: → virial; → radius.

The mean temperature at which a gravitationally → bound system would satisfy the → virial theorem. For a system of mass M and radius R with constant density, the gravitational energy per unit mass is W = GM/R. The kinetic energy per unit mass is E = (3/2)kT_vir/μ, where k is → Boltzmann’s constant and μ the mean molecular weight. According to the virial theorem, E = W/2, which leads to the virial temperature
T_vir = (1/3)(GM/kR).

See also: → virial; → temperature.

A general equation applicable to a gravitationally → bound system of equal mass objects (stars, galaxies, etc.), which is stable against → dynamical disruption.
It states that in such a system the average → gravitational potential energy (W_vir) is twice the average → kinetic energy (K_vir) of the system: W_vir = -2K_vir. This general proposition, first derived by Rudolf Clausius (1822-1888),
has important applications in a variety of fields ranging from statistical mechanics to astrophysics. See also → virialization, → virial equilibrium, → virialized.

See also: → virial; → theorem.

The process whereby a system of gravitationally interacting particles attains stability. The comparable mass components interact with each other, but the whole system does not expand or collapse. Virialization occurs when the → potential energy is twice the negative → kinetic energy:

• W_vir = 2 K_vir (→ virial theorem). In the case of a → galaxy cluster,
  when the cluster is virialized the merging process and the collapse of matter have finished and the formation process of the galaxy cluster is considered to be done. A cluster has formed by → hierarchical clustering. Virialized clusters, in other words finished clusters, can be found by looking at their radius and density. A cluster is virialized when it satisfies the condition:

R_vir ~ R_max/2,

where R_vir is the radius when the cluster is virialized and
R_max is the radius when the collapse starts. From this condition it follows that the object is 8 times denser at virialization than when the collapse started.

See also: Verbal noun of → virialize.

To undergo → virialization.

See also: → virial; → -ize.

That has undergone → virialization.

See also: Past participle of → virialize.

1. General: Having the efficacy without the material part; unreal but capable of being considered as real for some purpose.
   

2. Computers: Simulated by a computer (for reasons of experiment or
   convenience) of an entity that lacks some elements of total reality.

Etymology (EN): M.E., from M.L. virtualis, from L. virtus “manliness, excellence, potency, efficacy,” from vir “man, human, husband, soldier,” cf. Mid.Pers. vīr, wīr “man, hero;” Av. vīra- “man, human;” Skt. vīrá- “man, hero;”
Lith. vyras “man, husband;” O.Ir. fer “man;” Goth. wair “man;” O.E. wer “man.” In Roman philosophy, virtue became associated with virility and strength of character.

Etymology (PE): Virâgin from vir “intellect, mind, memory,” variants bar, bir (Mid.Pers. vir, varm, vârom “mental faculty, memory, mind;” Av. vārəma, vārəm “according to one’s wishes,” from var- “to choose”) + -âgin a
suffix denoting “consisting of, similarity, possession.”

In → analytical mechanics, any infinitesimal change in the configuration of a material system, consistent with any constraints acting on the system at a given instant. If the constraints are stationary (→ scleronomous), then the actual displacement of the system, in an
infinitesimal length of time dt, coincides with one of its virtual displacements. In the case of time-dependent (→ rheonomous) constraints, the actual displacement of the system does not coincide with any of the virtual ones, since the conditions imposed by the constraints vary during the time dt.

See also: → virtual; → displacement.

Optics: An image formed inside an instrument at the point where diverging rays would cross if they were extended backward into the instrument. Such an image cannot be obtained on a screen placed at its apparent position, since the rays do not pass through that point. → real image.

See also: → virtual; → image.

An international initiative by the astronomical community to allow global electronic access to the available astronomical data archives of space and ground-based observatories. It also aims to enable data analysis techniques through a coordinating entity that provides common standards, wide-network bandwidth, and state-of-the-art analysis tools. The Virtual Observatory is also intended for re-using data for scientific objectives different from the original ones, in order to optimize the science return of astronomical observations. The Virtual Observatory’s capabilities are enabled through the use of standard protocols for registering the existence and location of data and for requesting data that satisfies the user’s interests. These standards are developed on an international basis through the → IVOA. The cornerstone of the Virtual Observatory is → interoperability.

See also: → virtual; → observatory.

A subatomic particle that, according to the uncertainty principle, comes into being out of energy fluctuations of the “vacuum” and lasts for extremely short periods of time. An electron-positron pair can exist only about 4 x 10^-21 seconds. The lifetime increases as the mass and energy involved decreases. Virtual particles are real and have measurable effects, but cannot be directly observed, according to the uncertainty principle. → vacuum polarization.

See also: → virtual; → particle

In → analytical mechanics, an element of work performed in a → virtual displacement by the → forces acting on all n particles of a → holonomic system with s degrees of freedom (→ degree of freedom).

See also: → virtual; → work.

In → analytical mechanics, a principle whereby it is necessary and sufficient for the equilibrium of any material system with ideal constraints that the sum of the elements of work, performed by the applied forces acting on the system in any virtual displacement, be equal to zero (if all constraints are bilateral) or less than zero (if some of the constraints are unilateral).

See also: → virtual; → work; → principle.

1. An ultramicroscopic (20 to 300 nm in diameter), metabolically inert, infectious agent that replicates only within the cells of living hosts, mainly bacteria, plants, and animals: composed of an RNA or DNA core, a protein coat, and, in more complex types, a surrounding envelope (Dictionary.com).
   

2. → computer virus.

See also: From L. virus “poison, sap of plants, slimy liquid, a potent juice,” ultimately from PIE *ueis- “fluidity, slime, poison;” cf. Pers. bīš “a poisonous plant;” Mid.Pers. wiš “poison;” Av. viš-, viša- “poison;” Skt. visa- “venom, poison, poisonous;” L. viscum “sticky substance, birdlime;” Gk. ios “poison,” ixos “mistletoe, birdlime;” O.C.S. višnja “cherry;” O.Irish fi “poison;” Welsh gwy “poison.”

An instrument used to measure the → viscosity of a liquid. Same as viscosimeter.

See also: → viscosity; → -meter.

Same as → viscometer.

See also: → viscosity; → -meter.

The property of a → fluid that resists the force tending to cause the fluid to flow. Viscosity may be thought of as the internal → friction of two fluid layers which flow parallel to each other at different speeds. The cause of viscosity is the transport of → momentum by the molecules from one layer to the other. Viscosity is given by η = φ.u.λ.ρ, where φ is a coefficient which depends on the nature of the interaction between the molecules, u is the average velocity of thermal motion of the molecules, λ is the → mean free path, and ρ the → density of the fluid. Also called → dynamic viscosity or → absolute viscosity. See also → kinematic viscosity.

See also: Noun from → viscous; → -ity.

Having the property of → viscosity. See also: → viscous dissipation, → viscous decretion disk, → viscous fluid, → viscous force, → nonviscous.

Etymology (EN): M.E., from M.Fr. viscous, from L. viscosus “sticky,” from viscum “anything sticky; mistletoe.”

Etymology (PE): From vošk “a kind of sticky gum” + -sân suffix of similarity, from sân “way, manner.”

A model for explaining several observational features of → circumstellar disks around → Be stars. According to this model, the central star provides → angular momentum to the disk at the innermost radius, and then it is redistributed over the whole disk via → viscosity. Thus, in isolated Be stars, the equatorial disk can spread out to a large distance as long as the star can give angular momentum to the disk.

The VDD model, first introduced by Lee et al. (1991, MNRAS, 250, 432) and further developed by several other researchers, is now widely accepted as the best physical model for describing the circumstellar disks of Be stars. Among the growing evidence supporting the VDD model is the confirmation that the disks rotate in a Keplerian way (→ Keplerian orbit), allowing for the identification of viscosity as the mechanism that makes the disk grow (see, e.g., Klement et al., 2015, A&A 584, A85).

See also: → viscous; → decretion; → disk.

A degradation of → mechanical energy that is irreversibly converted to → thermal energy due to → viscous forces in the → fluid. Viscous dissipation occurs in → turbulent flows.

See also: → viscous; → dissipation.

A fluid whose viscosity is sufficiently large to make the viscous forces a significant part of the total force field in the fluid.

See also: → viscous; → fluid.

The force per unit volume or per unit mass arising from the action of tangential stresses in a moving → viscous fluid.

See also: → viscous; → force.

1. General: The state or fact of being visible. → crescent Moon visibility, → Venus visibility.
   

2. Optics: → fringe visibility. See also: → visibility function, → visibility plane.
   

3. Meteo.: A measure of transparency of the atmosphere. The maximum horizontal distance at which objects can be identified. → turbidity.

See also: → visible + → -ity.

The → Fourier transform of a source’s → brightness distribution, weighted by the characteristics of the → interferometer’s antennas.

See also: → visibility; → function.

In → interferometry, the projection of a → baseline onto the plane normal to the source direction defining a vector in (u,v) space, measured in wavelength units.

See also: → visibility; → plane.

1. Capable of being seen by, or perceptible to, the human eye.
   

2. → visible light.

Etymology (EN): M.E., from O.Fr. visible, from L. visibilis “that may be seen,” from visus, p.p. of videre “to see;” cognate with Pers.
bin, present stem of didan “to see”
(Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
cf. Skt. veda “I know;” Gk. oida “I know,” idein “to see;” PIE base *weid- “to know, to see”).

Etymology (PE): Diyâr “visible” in several dialects, e.g. štiyâni, Malâyeri, Širâzi, Tabari, related to 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.”
Padidâr (→ phenomenon) and peydâ are both also related to dā(y)-
(with prefixes).

That line where Earth and sky appear to meet, and the projection of this line upon the celestial sphere. Also known as the apparent horizon.

See also: → visible; → horizon.

The portion of the → electromagnetic radiation
that can be seen by the human → eye. The → wavelengths extend from about 400 nm (violet) to 750 nm (red). The wavelengths of various colors of the visible spectrum are as follows: → violet: 390-455 nm; → blue: 455-492 nm; → green: 492-577; → yellow: 577-597; → orange: 597-622; → red: 622-780 nm.

See also: → visible; → light.

The portion of → electromagnetic spectrum corresponding to → visible light.

See also: → visible; → spectrum.

The act or power of sensing with the eyes. → averted vision; → acuity of vision.

Etymology (EN): M.E., from O.Fr. vision, from L. visionem (nominative visio) “act of seeing, sight, thing seen,” from p.p. stem of videre “to see,” cognate with Pers. bin, present stem of didan “to see”
(Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
cf. Skt. veda “I know;” Gk. oida “I know,” idein “to see;” PIE base *weid- “to know, to see”).

Etymology (PE): Negâh, “look,” from Mid.Pers. nikâh “look, glance, observation;” Proto-Iranian *ni-kas- “to look down,” from ni- “down, below,” → ni- (PIE),

• *kas- “to look, appear;” cf. Av. nikā-, nikāta- (in the name of the 15-th nask) “that which is observed,” ākas- “to look;” Mid.Pers. âkâh, Mod.Pers. âgâh “aware, knowing;” Skt. kāś- “to become visible, appear;” Ossetic kast/kaesyn “to look;” did, 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”).

1. Of or pertaining to seeing or sight.
   

2. → optical.

Etymology (EN): M.E., from L.L. visualis “of sight,” from L. visus “sight,” from visus, .pp. of videre “to see;” cognate with Pers.
bin, present stem of didan “to see”
(Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
cf. Skt. veda “I know;” Gk. oida “I know,” idein “to see;” PIE base *weid- “to know, to see”).

Etymology (PE): Didi, of or pertaining to did, 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”); didgâni, adj. of didgân, plural of didé, did “sight, eye; seen;” didâri, from didâr, from didan.

Same as → acuity of vision.

See also: → visual; → acuity.

A → binary system of stars whose components can be resolved telescopically and which have detectable orbital motion.

See also: → visual; → binary.

The → extinction in the visual range of the electromagnetic radiation.

See also: → visual; → extinction

The → apparent magnitude of a celestial body in the color sensitivity region of the human eye at a wavelength of 5600 Å. Visual magnitude is now essentially synonymous with V magnitude, which is determined photometrically.

See also: → visual; → magnitude.

1. The act of visualizing (→ visualize).
   

2. The presentation of → information on a → screen.

Etymology (EN): Verbal noun of → visualize.

Etymology (PE): Cašm-did, Mid.Pers. cašm-did “visible,” Mod.Pers. cašm-didâr by Tusi, in Pers. translation of Sufi’s “Book of Fixed Stars,” from cašm “eye” (Mid.Pers. cašm, Av. cašman- “eye,” ākas- “to look,” from prefix ā- + Proto-Iranian *kas- “to look, appear,” cf. Skt. cáksus- “seeing”) + did past stem of 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”).

1. To make → visual or → visible.
   

2. To present → image or → information on a → screen, usually a → computer  → display.

See also: From → visual + → -ize; → visualization.

The science, production, and study of → grapes.

Etymology (EN): From L. viti(s) “vine” + → culture.

Etymology (PE): Mowkâri, from mow→ vine + kâri noun from kâštan “to cultivate,” → culture.

An interferometer using a combination of the four 8.2 m VLT telescopes
with the assistance of one or more of the the four 1.8 m Auxiliary Telescopes in order to achieve a very high spatial resolution. The system works in the visible and near- and mid-infrared wavelengths.

See also: → Very Large Telescope (VLT); → interferometer.

A set of words used by a → language, group, individual, or work, or in a field of → knowledge.

Etymology (EN): M.L. vocabularium “a list of words,” from L. vocabulum “word, name, noun,” from vocare “to name, call;” cognate with Pers. vâž, → word.

Etymology (PE): Vâžgân, from vâž, → word, + -gân suffix forming plural entities, from Mid.Pers. -gânag, -gâna, from Proto-Iranian *kāna-ka-.

The sharp edge of a fold of mucous membrane stretching along either wall of the larynx from the angle between the laminae of the thyroid cartilage to the vocal process of the arytenoid cartilage. Vibrations of these cords are used in voice production (The American Heritage).

See also: → voice; → cord.

The internal structure and all observable characteristics of a star
(such as luminosity and temperature) are determined uniquely by its mass, chemical composition, and age. Same as → Russell-Vogt theorem.

See also: Named after the German astronomer Heinrich Vogt (1890-1968) and the American astronomer Henry Norris Russell (1877-1957); → theorem.

Sounds made when speaking or singing.

Etymology (EN): M.E., from O.Fr. voiz, from L. vox “voice, sound, utterance, cry, call, speech, sentence, word,” related to vocare “to call;” akin to Pers. âvâz “voice,” as below.

Etymology (PE): Âvâz “voice, sound, song,” related to âvâ “voice, sound, song” (both prefixed forms), bâng “voice, sound, clamour” (Mid.Pers. vâng), vâžé “word,” variants vâj-, vâk-, vâ-, vâz-, vât-;
Av. vacah- “word,” vaocanghê “to decalre” (by means of speech), from vac- “to speak, say;” cf. Skt. vakti “speaks, says,” vacas- “word;”
Gk. epos “word;” L. vox “voice;” PIE base *wek- “to speak.”

1. An empty space; a gap or opening; emptiness. → vacuum.
   

2. A large region of cosmic space without galaxies. The first of these voids to be discovered lies in the direction of → Bootes
   and is some 300 million → light-years across. It is estimated that voids take up about 98% of the volume of the Universe, with clusters of galaxies concentrated in the thin walls that surround them.

Etymology (EN): M.E. voide, from O.Fr. voide “empty, vast, wide, hollow,” from L. vocivus “unoccupied, vacant,” related to vacuus “empty,” → vacuum.

Etymology (PE): Tohi “empty” (variants in dialects Tabari tisâ,
Saraxsi, Lâsgardi, Sangesari tusâ,
Aftari tussâ); Mid.Pers. tuhig; Av. taoš- “to become empty,” pres. tusa-, caus. taošaya-, tusən “they lose their posture;” cf. Skt. tuccha-, tucchya- “empty;” L. tesqua, tesca “deserted place;” Rus. tošcij “hollow;” PIE base *teus- “to empty.”

Double refraction occurring when a strong → magnetic field is applied to a vapor through which light is passing perpendicular to the field.

See also: Named after Woldemar Voigt (1850-1919), a German physicist (1908, Magneto- und Elektro-optik, B. G. Teubner, Leipzig); → effect.

A spectral profile in which a → spectral line is broadened by two types of mechanisms, one of which alone would produce a & rarr; Gaussian profile (usually, as a result of the → Doppler broadening), and the other would produce a → Lorentzian profile.

See also: After Woldemar Voigt (1850-1919), a German physicist; → profile.

The Flying Fish. A constellation in the southern hemisphere at 7h 40m right ascension, -70° declination. Originally called Piscis Volans, and invented by Johann Bayer (Uranometria, published in 1603).
Abbreviation: Vol; Genitive: Volantis.

Etymology (EN): L. Volans “flying,” from volare “to fly.”

Etymology (PE): Mâhi-ye Parandé, from mâhi “fish” (Mid.Pers. mâhik; Av. masya-; cf. Skt. matsya-; Pali maccha-) + parandé “flying, flier,” from
paridan “to fly” (Mid./Mod.Pers. par(r) “feather, wing,” Av. parəna- “feather, wing;” cp. Skt. parna “feather,” E. fern; PIE *porno- “feather”).

A substance that vaporizes at relatively low temperatures (e.g. H₂O, CO₂, CO, CH₄, NH₃, and so forth). The opposite of volatile is → refractory.

Etymology (EN): M.E., from M.Fr. volatile, from L. volatilis “fleeting, transitory, flying,” from p.p. stem of volare “to fly,” of unknown origin.

Etymology (PE): Parrâ “flying,” from paridan “to fly in the air,” → Volans.

In → planetary science, any of a group of → chemical elements and → chemical compounds with relatively low → boiling points that are associated with a planet’s or moon’s → crust and/or → atmosphere. For example, H, He, C, N, O are underabundant (relative to the solar → photospheric values) in all types of → meteorites, including the C1 → carbonaceous chondrites. Any heating of the meteorite parent body subsequent to its formation would tend to drive the volatile elements out of the rock, whence it sublimated into → interplanetary medium.

See also: → volatile; → element.

Of or relating to a volcano. Characterized by volcanoes.

See also: → volcano; → -ic.

The explosive ejection of superheated matter from a → volcano.

See also: → volcanic; → eruption.

A logarithmic scale, ranging from 1 to 8, used to measure the intensity of volcano eruptions. The VEI is based on several factors: the degree of fragmentation of the volcanic products released by the eruption, the amounts of sulfur-rich gases that form stratospheric aerosols, the volume of the eruptions, their duration, and the height is reached. The largest eruptions (8) produce an amount of bulk volume of ejected → tephra of ~ 1,000 km³.

See also: → volcanic; → explosivity; → index.

→ vent.

See also: → volcanic; → vent.

An opening in the Earth’s → crust from which → lava, → ash, and hot → gases flow or are → ejected during an → eruption.

Etymology (EN): From It. vulcano, from L. Vulcanus, → Vulcan.

Etymology (PE): Âtašfešân, literally “fire disperser, dispersing fire,” from âtaš,
→ fire, + fešân contraction of afšân, from afšândan “to spread, scatter,” Mid.Pers. afšân “to spread, to scatter;” ultimately from Proto-Ir. *apašan-, from root *šan- “to shake” (Cheung 2007).

The SI unit of potential difference, defined as the difference of potentials across the ends of a conductor in which a power 1 watt is liberated when a current of 1 ampere flows through it.

See also: In honor of the Italian scientist Alessandro Volta (1745-1827), known for his pioneering work in electricity and the invention of the first battery.

The electric potential difference expressed in volts.

See also: From → volt.

Of, relating to electricity or electric currents, especially when produced by chemical action, as in a cell. → photovoltaic detector.

See also: Alessandro Volta (1745-1827), Italian physicist, known for his pioneering work in electricity.

The amount of space occupied by a three-dimensional object or region of space, expressed in cubic units.

Etymology (EN): M.E. volum(e), from O.Fr. volume, from L. volumen (genitive voluminis) “roll (as of a manuscript), coil, wreath,” from volvere “to turn around, roll.”

Etymology (PE): Gonj “volume,” gonjidan “to be contained or held; to hold;”
gonjâyeš “capacity, holding, containing;”
Mid.Pers. winj- “to be contained;” Proto-Iranian *uiac-/*uic-; cf. Skt. vyac- “to contain, encompass,” vyás- “extent, content, extension;” L. uincire “to bind.”

A survey in which the observed objects are contained in a given volume of space.

See also: → volume; → limited; → survey.

A → rotating star cannot simultaneously achieve → hydrostatic equilibrium and → rigid body rotation. The paradox can be solved if → baroclinic flows (essentially a → differential rotation and a → meridional circulation) are included. For a broader view of the subject see: M. Rieutord, 2006, in Stellar Fluid Dynamics and Numerical Simulations: From the Sun to Neutron Stars, ed. M. Rieutord & B. Dubrulle, EAS Publ., 21, 275, arXiv:astro-ph/0608431.

See also: → von Zeipel theorem; → paradox.

A theorem that establishes a relation between the → radiative flux at some → colatitude on the surface of a → rotating star and the local → effective gravity
(which is a function of the → angular velocity and colatitude). For a rotating star in which → centrifugal forces are not negligible, the → equipotentials where gravity, centrifugal force, and pressure are balanced will no longer be spheres. The theorem states that the radiative flux is proportional to the local effective gravity at the considered colatitude, F(θ) ∝ g_eff (θ)^α, where α is the → gravity darkening coefficient. As a consequence, the stellar surface will not be uniformly bright, because there is a much larger flux and a higher → effective temperature at the pole than at the equator (T_eff (θ) ∝ g_eff (θ)^β, where β is the → gravity darkening exponent. In → massive stars this latitudinal dependence of the temperature leads to asymmetric → mass loss and also to enhanced average → mass loss rates. Also called → gravity darkening. See also → von Zeipel paradox; → meridional circulation; → baroclinic instability;
→ Eddington-Sweet time scale.

See also: Named for Edvard Hugo von Zeipel, Swedish astronomer (1873-1959), who published his work in 1924 (MNRAS 84, 665); → theorem.

Same as the → von Zeipel theorem.

See also: → von Zeipel theorem; → law.

Meteo.: A whirling mass of water or air.

A pattern of rotation in which the rotation direction rotates by 360° along any path which surrounds the centre of the vortex.

Etymology (EN): From L. vortex, variant of vertex “whirlpool; whirlwind, an eddy of water, wind, or flame;” from stem of vertere “to turn,” cognate with Pers. gardidan, as below.

Etymology (PE): Gerdšâr (on the model of gerdâb “whirlpool” and gerdbâd “whirlwind”), from gard present stem of gardidan “to turn, to change” (Mid.Pers. vartitan “to change, to turn;” 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”) + šâr, from šâré, → fluid.

Of or pertaining to a → vortex.

See also: Adj. from → vortex.

In fluid mechanics, a measure of the rate of rotational spin in a fluid. Mathematically, vorticity is a vector field defined as the curl of the velocity field:
ω = ∇ x v.
Meteo.: The rotation of air around a vertical axis.

Etymology (EN): From L. vortic-, from → vortex + → -ity.

Etymology (PE): Gerdšârigi, from gerdšâr→ vortex + -igi, → -ity.

A speech sound that is produced as a stream of air that is not obstructed or blocked in any way by the vocal organs, but only modulated by the position of the tongue, lips, etc.

Etymology (EN): M.E., from O.Fr. vouel, from L. vocalis shortening of littera vocalis, literally “vocal letter,” from  → vox “voice,” cognate with Pers. âvâ, → phone, vâžé, → word.

Etymology (PE): Xodâvâ, literally “self-voice,” as Ger. Selbstlaut, from xod-, → self-, + âvâ, → phone.

A hypothetical small planet proposed in the 19-th century to exist in an orbit between Mercury and the Sun. In order to explain some characteristics of Mercury’s orbit, the French astronomer Urbain Jean Joseph Le Verrier (1811-1877) hypothesized the presence
of another planet, which he named Vulcan. Those particularities of Mercury’s orbit were later explained by Albert Einstein’s theory of general relativity.

See also: L. Vulcanus in Roman mythology the blacksmith god of fire and volcanoes, a word of Etruscan origin

1. Characterized by ignorance of or lack of good breeding or taste.
   

2. Spoken by, or being in the language spoken by, the people generally; vernacular (Dictionary.com).

Etymology (EN): From L. vulgaris, volgaris “of or pertaining to the common people, common, vulgar, low, mean,” from vulgus “the common people, multitude, crowd, throng” (cognates: Skt. varga- “a company, group, section,” Pers. jarga, as below, Gk. eilein “to press, throng,” Welsh gwala “sufficiency, enough”).

Etymology (PE): Jargvâr, from jarg, jarga, jargé “a group of people, a ring of men or beasts;” cf. Lori, Laki jarga “group, throng,” jarra “bundle;” Kurd. jerge, cerge “assembly, club;” + -vâr a suffix denoting “suiting, befitting, resembling, in the manner of, possession.”

Same as → common fraction.

Etymology (EN): M.E., from L. vulgaris, from vulgus “the common people,” + -aris, → -ar.

Etymology (PE): → common; → fraction.

The Little Fox. A faint constellation in the northern hemisphere, at 20h right ascension, +25° declination, between Cygnus and Aquila. Abbreviation: Vul; Genitive: Vulpeculae.

Etymology (EN): L. Vulpecua, from vulpes “fox,” cognate with Pers. rubâh, as below, + diminutive suffix -cula, variant of → -ule.

Etymology (PE): Rubâhak “little fox,” from rubâh

A → red supergiant star which is one of the largest and most intrinsically luminous stars in the sky. It has a radius of about 1,420 solar radii (→ solar radius) and a luminosity of 3 × 10⁵&nbspLsun (→ solar luminosity). It is about 3,900 → light-years from the Earth. Excluding solar system bodies, VY CMa is the third brightest object in the sky at 10 µm. It has a large optical → reflection nebula, visible through even small telescopes, created by its prodigious mass loss (> 10^-4&nbspMsun per year). Its initial mass is estimated to be about 25 Msun and its current mass 17 Msun. Recent observations have shown the presence of submicron → dust grains, with an average radius of ~ 0.5 μm, in the ejecta. These grains are 50 times larger than those found in the diffuse → interstellar medium (P. Scicluna et al., 2015, A&A, 584L, 10S).

See also: V and Y, following the → variable star designation scheme; → Canis Major.

A class of → O-type stars whose spectra show He II 4686 absorption line much stronger than any other He line especially He II 4541 or He I 4471. Vz stars are thought to be young stars lying close to the → ZAMS (Walborn & Parker 1992, ApJ 399, L87). Alternatively, the Vz characteristics may be related to the wind properties rather than to the youth of the star (Martins et al., 2005, A&A 441, 735).

See also: V indicating → main sequence, z for “zero age;” → star.