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

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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spherical harmonic
  هماهنگ ِ کره‌ای   
hamâhang-e kore-yi

Fr.: fonction harmonique sphérique   

A solution of some mathematical equations when → spherical polar coordinates are used in investigating physical problems in three dimensions. For example, solutions of → Laplace's equation treated in spherical polar coordinates. Spherical harmonics are ubiquitous in atomic and molecular physics and appear in quantum mechanics as → eigenfunctions of → orbital angular momentum. They are also important in the representation of the gravitational and magnetic fields of planetary bodies, the characterization of the → cosmic microwave background anisotropy, the description of electrical potentials due to charge distributions, and in certain types of fluid motion.

The term spherical harmonics was first used by William Thomson (Lord Kelvin) and Peter Guthrie Tait in their 1867 Treatise on Natural Philosophy; → spherical; → harmonic.

spintronics
  اسپین‌ترونیک   
espintronik

Fr.: spintronique   

A new area of science and technology which exploits the intrinsic → spin of electrons and its associated → magnetic moment, in addition to its fundamental electronic charge, in solid-state devices. In brief, spin-based electronics. For example, information could be transported or stored through the spin-up or spin-down states of electrons. Spintronics techniques are capable of much higher speed while requiring less power than the conventional method of using electron charges to represent data. The first use of spintronics was in the late 1980s with the development of → giant magnetoresistance (GMR) read heads for disk drives

Short for → spin + → electronics.

spline function
  کریای ِ اسپلین   
karyâ-ye splin

Fr.: fonction spline   

A function consisting of several segments, usually → polynomials, joined smoothly together at specific points with an explicitly stated degree of accuracy. Spline functions are used to approximate a given function on an interval.

From East Anglian dialect, maybe related to O.E. splin and to modern splint. A spline was originally a slat or a thin strip of wood. A later meaning was "a long, thin, flexible strip used as a guide for drawing arcs of curves;" → function.

splint bone
  نازک-نی   
nâzok-ney (#)

Fr.: péroné   

fibula.

M.E., from M.Du. or M.L.G. splinte, splente "thin piece of iron," related to M.Du. splinte "splint," probably literally "thin piece cut off."

fibula.

spontaneous
  سرخود   
sarxod (#)

Fr.: spontané   

Arising from internal forces or causes; independent of external agencies; self-acting.

From L.L. spontaneus "willing, of one's free will," from L. (sua) sponte "of one's own accord, willingly," of unknown origin.

Sarxod, literally "by himself/herself," from sar "head" (soru, sorun "horn;" karnâ "a trumpet-like wind instrument," variant sornâ "a wind instrument;" Mid.Pers. sar "head," sru "horn;" Av. sarah- "head," srū- "horn, nail;" cf. Skt. śiras- "head, chief;" Gk. kara "head," karena "head, top," keras "horn;" L. cornu "horn," cerebrum "brain;" P.Gmc. *khurnaz (Ger. Horn, Du. horen; cognate with E. horn, as above, from PIE *ker- "head, horn;" O.E. horn "horn of an animal," also "wind instrument;" E. horn); PIE base *ker- "head, horn, top, summit") + xod "self" (Mid.Pers. xwad "self; indeed;" Av. hva- "self, own").

spontaneous combustion
  سوزش ِ سرخود   
suzeš-e sarxod

Fr.: combustion spontanée   

The self-ignition of a substance that produces sufficient heat within itself, by a slow oxidation process, for ignition to take place without the need for an external high-temperature source. The produced heat energy is absorbed by the substance raising its temperature slowly until the → ignition temperature is reached. Same as spontaneous ignition.

spontaneous; → combustion.

spontaneous emission
  گسیل ِ سرخود   
gosil-e sarxod

Fr.: émission spontanée   

The emission of electromagnetic radiation from an atom or molecule that does not depend on the presence of external fields.

spontaneous; → emission.

spontaneous symmetry breaking
  شکست ِ سرخود ِ همامونی   
šekast-e sarxod-e hamâmuni

Fr.: brisure spontanée de symétrie   

A physical phenomenon whereby a symmetric system becomes permanently asymmetric. A simple example is a ball lying on top of a hill in equilibrium. The hill-ball system is symmetric about the vertical axis through the top of the hill. Moreover, there is no preferred horizontal direction to the system. However, its state is unstable, since the slightest perturbing force will cause the ball to roll down the hill in some particular direction. The system becomes permanently asymmetric because the ball will not roll uphill by itself. Symmetry breaking is found in several fields of physics, for example in → magnetism (→ ferromagnetism), → thermodynamics (→ crystallization), and → particle physics, where it constitutes the basis of → electroweak interactions. In cosmology, according to the → Big Bang model, the fundamental forces of the Universe split off from one another in a form of spontaneous symmetry braking. If a single, unified force existed with a certain symmetry just after the Big Bang, if that symmetry were somehow broken so that the unified force were fractured, then the result might be several fundamental forces. See also → grand unified theory, → theory of everything, → phase transition.

spontaneous; → symmetry; → break.

spontaneous transition
  گذرش ِ سرخود   
gozareš-e sarxod

Fr.: transition spontanée   

An → atomic transition that gives rise to a → spontaneous emission.

spontaneous; → transition.

spring constant
  پایای ِ فنر   
pâpâ-ye fanar

Fr.: constante de rappel du ressort   

A characteristic of a spring which is defined as the ratio of the force affecting the spring to the displacement caused by the force. In other words, the spring constant is the force applied if the displacement in the spring is unity. It is expressed by the equation k = -F/x (from → Hooke's law), where F = force applied, x = displacement by the spring. The spring constant is usually expressed in Newton per meter (N/m).

spring; → force.

staff astronomer
  اخترشناس ِ استب   
axtaršenâs-e estab

Fr.: astronome résident   

A professional astronomer who works within a specified observatory or research group.

staff; → astronomer.

stagnation
  ناروانی   
nâravâni

Fr.: stagnation   

The state or condition of not flowing or running. → stagnation point, → stagnation pressure.

L. stagnatum, stagnatus, p.p. of stagnare "to stagnate," from stagnatum "standing water," from PIE root *stag- "to seep drip."

Nâravâni, literally "not flowing," from nâ- negation prefix, → un-, + ravân "flowing, running," pr.p. of raftan "to go, walk; to flow;" (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- "to go; to attack").

stagnation point
  نقطه‌ی ِ ناروانی   
noqte-ye nâravâni

Fr.: point de stagnation   

A point where the → flow  → velocity is → zero. For example a point around an obstacle where a → flow tube splits into two portions.

stagnation; → point.

stagnation pressure
  فشار ِ ناروانی   
fešâr-e nâravâni

Fr.: pression de stagnation   

The sum of → static pressure and → dynamic pressure in the → Bernoulli equation.

stagnation; → pressure.

stallion
  نریان   
naryân (#)

Fr.: étalon   

An uncastrated adult male horse, especially one used for breeding.

M.E. stalon, from O.Fr. estalon, "uncastrated male horse," cognate with O.H.G. stal "stable," cf. O.H.G. stall "stand, place, stable, stall," Ger. Stall "stable," Stelle "place"), from PIE root *stel- "to put, stand," with derivatives referring to a standing object or place; akin to Pers. istâdan "to stand," → station.

Nariyân, from nar "male," → masculine.

standard deviation
  کژرفت ِ استانده   
kažraft-e estândé

Fr.: écart-type   

The most widely used measure of dispersion of a frequency distribution. It is equal to the positive square root of the → variance. Same as → standard error. Not to be confused with the → root mean square error.

standard; → deviation.

star formation
  دیسش ِ ستاره   
diseš-e setâré

Fr.: formation d'étoiles   

The process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy, star formation includes the study of the interstellar medium and molecular clouds as precursors to the star formation process as well as the study of young stellar objects.

star; → formation.

star formation efficiency (SFE)
  کارایی ِ دیسش ِ ستاره   
kârâyi-ye diseš-e setâré

Fr.: efficacité de formation d'étoiles   

The degree to which stars form in a system, such as a → molecular cloud or a → galaxy. It is given by the ratio of the total mass of stars to the initial gas mass: εSFE = Mstars / (Mstars + Mgas).

star formation; → efficiency.

star formation history
  تاریخ ِ دیسش ِ ستاره   
târix-e diseš-e setâré

Fr.: histoire de formation d'étoiles   

The → star formation rate as a function of time.

star; → formation; → history.

star formation quenching
  اسرش ِ دیسش ِ ستارگان   
osereš-e diseš-e setâregân

Fr.: assèchement de formation d'étoiles   

The premature termination of star formation process in some galaxies. The ultimate quenching of star formation is caused by stripping of the gas reservoir which will finally turn into stars. A wide variety of mechanisms have been proposed to provide quenching. For example, → major mergers can transform spiral galaxies into ellipticals, and may also quench future star formation by ejecting the → interstellar medium from the galaxy via starburst, → active galactic nucleus, or shock-driven winds. In rich clusters, where merging is less efficient because of the large relative velocities of galaxies, rapid encounters or fly-bys may cause the formation of a bar and growth of a spheroidal component instead of larger scale star formation. Also, cold gas can be stripped out of the galaxy both by tidal forces and ram pressure in the intracluster medium. Similarly, the hot halo that provides future fuel for cooling and star formation may be efficiently stripped in dense environments, thus quenching further star formation (see, e.g., Kimm et al., 2009, MNRAS 394, 1131, arXiv:0810.2794).

star; → formation; → quench.

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