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

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 3079 Search : on
Scorpius-Centaurus association
  آهزش ِ کژدم-کنتاؤروس   
âhazeš-e Každom-Kentawros

Fr.: association Scorpius-Centaurus   

The nearest → OB association to the Sun. It contains several hundred stars, mostly → B stars which concentrate in the three subgroups: Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux. Upper Scorpius is the youngest subgroup, Upper Centaurus Lupus the oldest subgroup of the association. Isochrone fitting to the Hertzsprung-Russell diagram indicates that the star formation occurred some 5-20 Myr ago. Based on data from the → Hipparcos catalog, it turns out that the Sco-Cen association lies at a distance of 118-145 → parsecs, with the exact value depending on the subgroup of the association. The Sco-Cen association is probably a member of the → Gould Belt (Preibisch & Mamajek, 2008, astro-ph/0809.0407).

Scorpius; → Centaurus; → association.

scotopic vision
  دید ِ تاریکی   
did-e târiki

Fr.: vision scotopique   

Vision that occurs when the eye is dark-adapted. In scotopic vision, the level of luminance is so low that the retinal cones are not stimulated, and there is no color vision. Same as scotopia; → dark adaptation.

Scotopic, from L. Gk. skoto- combining form of skotos "darkness" + -opia akin to ope "view, look," ops "eye, face;" → vision.

Did, → vision; târiki noun from târik "dark," Mid.Pers. târig "dark," târ "darkness," Av. taθra- "darkness," taθrya- "dark," cf. Skt. támisrâ- "darkness, dark night," L. tenebrae "darkness," Hittite taš(u)uant- "blind," O.H.G. demar "twilight."

screen font
  ریختار ِ پرده   
rixtâr-e pardé

Fr.: fonte d'écran   

A character used for on-screen → display. See also → printer font.

screen; → font.

screened Coulomb interaction
  اندرژیرش ِ باپرده‌ی ِ کولن   
andaržireš-e bâparde-ye Coulomb

Fr.: interaction de Coulomb écrantée   

The → Coulomb interaction reduced owing to the presence of other electrons. See → shielding effect.

screen; → coulomb; → interaction.

sea horizon
  افق ِ دریا   
ofoq-e daryâ

Fr.: horizon de mer   

The → apparent horizon formed by the sea.

sea; → horizon.

season
  فصل   
fasl (#)

Fr.: saison   

One of the four periods of the year astronomically defined by the position of the Sun with respect to the equator. As a result of the obliquity of the ecliptic, the angular distance between the Sun and the equator varies in the course of the year. This circumstance gives rise to seasons. The current lengths of the astronomical seasons, around the year 2000, are about: spring 92.76 days, summer 93.65 days, autumn 89.84 days, and winter 88.99 days. The seasons are unequal because the Earth's orbit is slightly elliptical and the Sun is not exactly at the center of the orbit. Moreover, the Earth moves faster when it is close to the Sun than when it is farther away, so the seasons that occur when the Earth is close to the Sun pass more quickly.

M.E. sesoun, seson, from O.Fr. seison "a sowing, planting," from L. sationem (nominative satio) "a sowing," from p.p. stem of serere "to scatter seed over land."

Fasl, from Ar. faSl "cutting, dividing; section."

Secchi classification
  رده‌بندی ِ سکی   
radebandi-ye Secchi

Fr.: classification de Secchi   

A pioneering work in → spectral classification conducted in the 1860s. Secchi divided stars into four main groups based on the visual observation of spectra. Class I: The white and bluish stars with a continuous spectrum crossed by hydrogen bands, the metallic bands being absent or weak. Examples, → Sirius, → Vega. Class II: Yellow stars, with spectra in which the hydrogen bands were less prominent and the metallic lines more strong. Examples, Sun, → Capella. Class III: Red or orange stars, showing bands or flutings. Examples, → Antares, → Betelgeuse. Class IV: Red stars, showing bands similar to Class III, but with the sharp edge of the flutings toward the other end of the spectrum. Secchi's scheme was superseded by the photographic → Harvard classification system.

Pietro Angelo Secchi (1818-1878), Italian astronomer and Jesuit priest; → classification.

second
  ۱) دوم، دومین؛ ۲) ثانیه   
1) dovom (#), dovomin (#); 2) sâniyé (#)

Fr.: seconde   

1) Next after the first in place, time, or value.
2) The unit of time in the → International System of Units; symbol s. It is defined by taking the fixed numerical value of the cesium frequency ΔνCs, the unperturbed → ground state  → hyperfine transition  → frequency of the → cesium-133 atom, to be 9 192 631 770 when expressed in the unit → hertz (Hz), which is equal to s-1.

M.E., from O.Fr. second, from L. secundus "following, next in order," from root of sequi "to follow;" PIE base *sekw- "to follow;" cf. Pers. az from; Mid.Pers. hac "from;" Av. hac-, hax- "to follow," hacaiti "follows" (O.Pers. hacā "from;" Av. hacā "from, out of;" Skt. sácā "with"); Skt. sácate "accompanies, follows;" Gk. hepesthai "to follow;" Lith. seku "to follow."

1) Dovom, dovomin "ordinal number of do, two" (Mid.Pers. do; Av. dva-; cf. Skt. dvi-; Gk. duo; L. duo; (Fr. deux; E. two; Ger. zwei).
2) Sâniyé, from Ar. sâniyat (feminine) "second."

second approximation
  نزدینش ِ دوم   
nazdineš-e dovom

Fr.: deuxième approximation   

Math: In calculus, limiting an equation to its → second derivative, for example: ex≅ 1 + x + x2/2. Also called linear approximation. → first approximation.

second; → approximation.

second collapse
  رمبش ِ دوم   
rombeš-e dovom

Fr.: deuxième effondrement   

An early evolutionary period in the process of star formation which succeeds the → first collapse. When the mass of the → first core has increased by about a factor 2 and the radius has decreased by a similar factor, the central temperature of the core reaches about 2000 K. At this point the → molecular hydrogen begins to dissociate into atoms. This reduces the → adiabatic index (γ) below the critical value 4/3, with the result that the material at the center of the core becomes unstable and begins to collapse. Most of the gravitational energy generated by this collapse goes into the → dissociation of H2 molecules, so that the temperature rises only slowly with increasing density. In this second collapse phase, as in the first, the density distribution in the collapsing region becomes more and more sharply peaked at center, and the time scale becomes shorter and shorter with increasing central density. The central collapse of the core continues until the hydrogen molecules are nearly all dissociated and γ again rises above 4/3. The central pressure then rises rapidly and once again becomes sufficient to decelerate and stop the collapse at the center. A small core in the → hydrostatic equilibrium then arises, bounded by a shock front in which the surrounding infalling material is suddenly stopped. The initial mass and radius of the second core are about 3 x 1030 g (1.5 x 10-3Msun) and 9 x 1010 cm (1.3 Rsun) respectively, and the central density and temperature are about 2 x 10-2 g cm-3 and 2 x 104 K, respectively. The second core will evolve into a → young stellar object (R. B. Larson, 1969, MNRAS 145, 271).

second; → collapse.

second contact
  پرماس ِ دوم   
parmâs-e dovom

Fr.: deuxième contact   

The beginning of the total phase of a solar eclipse when the leading edge of the Moon touches the eastern edge of the Sun completely obscuring the Sun.

second; → contact.

second core
  مغزه‌ی ِ دوم   
maqze-ye dovom

Fr.: deuxième cœur   

A hydrostatic object predicted to result from the → second collapse of a → molecular cloud in an early stage of star formation.

second; → core.

second derivative
  واخنه‌ی ِ دوم   
vâxane-ye dovom

Fr.: dérivée seconde   

In → calculus, the → derivative of a → first derivative. It is usually written as f''(x), d2y/d2x, or y''.

second; → derivative.

second derivative test
  آزمون ِ واخنه‌ی ِ دوم   
âzmun-e vâxane-ye dovom

Fr.: test de la dérivée seconde   

A method, used in → calculus, for determining whether a given → stationary point of a → function is a → local minimum or → local maximum.

second; → derivative; → test.

second dredge-up
  برونکشید ِ دوم   
borunkašid-e dovom

Fr.: deuxième dragage   

A → dredge-up process that occurs after core helium burning, in which the convective envelope penetrates much more deeply, pushing hydrogen burning shell into close proximity with the helium burning shell (→ first dredge-up). This arrangement is unstable and leads to burning pulses. The reason is that the hydrogen shell burns out until there is enough helium for the helium combustion to occur and all the helium is rapidly burnt. Afterward the hydrogen shell again burns outward and the process repeats.

second; → dredge-up.

second generation star
  ستاره‌ی ِ آزانش ِ دوم   
setâre-ye âzâneš-e dovom

Fr.: étoile de deuxième génération   

A star whose formation is induced by an older star itself formed previously in the same region. See also → stimulated star formation, → sequential star formation, → triggered star formation.

second; → generation; → star.

second law of black-hole mechanics
  قانون ِ دوم ِ مکانیک ِ سیه‌چال   
qânun-e dovom-e mekânik-e siyah-câl

Fr.: deuxième loi de la mécanique des trous noirs   

The surface area of a black hole's horizon can never decrease.

second; → law; → black hole; → mechanics.

second law of thermodynamics
  قانون ِ دوم ِ گرماتوانیک   
qânun-e dovom-e garmâtavânik

Fr.: deuxième loi de la thermodynamique   

1) Heat cannot be transferred from a colder to a hotter body without some other effect, i.e. without → work being done. Expressed in terms of → entropy: the entropy of an → isolated system tends toward a maximum and its available energy tends toward a minimum.
2) In language of → statistical physics, an isolated physical system will tend toward an equilibrium → macrostate with as large a total → entropy as possible, because then the number of → microstates is the largest. See also → Kelvin's postulate, → Clausius's postulate.

second; → law; → thermodynamics.

second quantization
  کو‌آنتومش ِ دوم   
kuântomeš-e dovom

Fr.: deuxième quantification   

In quantum mechanics, the quantization of the field that replaces potential in Newtonian mechanics, whereby the field variables become operators from which the creation (of particle) operators and destruction operators can be constructed.

second; → quantization.

second-order logic
  گوییک ِ رایه‌ی ِ دوم   
guyik-e râye-ye dovom

Fr.: logique du seconde ordre   

An n extension of → first-order logic that quantifies not only → variables that range over → individuals, but also quantifies over → relations.

second; → order; → predicate; → logic.

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