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

The fraction of time that a → pulsar undergoes → pulse nulling. For most → nulling pulsars this fraction can range from zero (for the → Vela pulsar) to more than 50%.

→ null; → -ing; → fraction.

The difference between the isotopic mass of a nuclide and its mass number divided by its mass number. The packing fraction is a measure of the stability of the nucleus.

Packing, from the verb pack "to put together in a pack," from the noun pack, from M.E. pak, packe, from M.D. pac or perhaps M.L.G. pak; → fraction.

Barxé, → fraction; anbâštegi quality noun of anbâštan, anbârdan "to fill, to replete;" Mid.Pers. hambāridan "to fill;" from Proto-Iranian *ham-par-, from prefix ham- + par- "to fill;" cf. Av. par- "to fill," parav-, pauru-, pouru- "full, much, many;" O.Pers. paru- "much, many;" Mid.Pers. purr "full;" Mod.Pers. por "full, much, very;" PIE base *pelu- "full," from *pel- "to be full;" cf. Skt. puru- "much, abundant;" Gk. polus "many," plethos "great number, multitude;" O.E. full.

The ratio expressed by P = (I_par - I_per) / (I_par + I_per), where I_par and I_per are the light intensities with the electric field vector respectively parallel and perpendicular to the incident beam.

→ polarization; → fraction.

Newton's third law of motion, which states that for every action there must be an equal and opposite reaction.

→ principle; → action; → reaction.

The principle that, for a system whose total mechanical energy is conserved, the path to be taken for the system from one configuration to another is the one whose action has the least value relative to all other possible paths and from the same configurations. Also called Maupertuis' principle, least-action principle.

→ principle; → least; → action.

A → thermonuclear reaction in which two protons collide at very high velocities and combine to form a → deuterium. See also → proton-proton chain.

→ proton; → reaction.

A nuclear reaction that takes place at high densities and relatively low temperatures. Pycnonuclear reactions are almost temperature independent and occur even at zero temperature. These reactions are extremely slow at densities typical for normal stars but intensify with increasing density. For example, carbon burns into heavier elements at densities over 10¹⁰ g cm^-3.

Pycnonuclear, from pycno- a combining form meaning "dense, thick," from Gk. pyknos "dense, solid" + → nuclear; → reaction.

Vâžireš, → reaction; cagâl-hasteyi, from cagâl, → dense, + hasteyi, → nuclear.

Since → Planck's constant has the dimension of → energy × → time, its sometimes called the quantum of → action.

→ quantum; → action.

The state of being rarefied, less dense.
An instantaneous reduction in density of a gas resulting from passage a sound wave; opposite of → compression.

M.E. rarefien, from M.Fr. rarefier, from L. rarefacere "make rare," from rarus "loose, wide apart, thin, infrequent."

Verbal noun from âlar present stem of âlaridan→ rarefy + -š, a suffix.

A pressure wave in a fluid generated by rarefaction. It travels in the opposite direction to that of a shock wave in the medium.

→ rarefaction; → wave.

1) General: Action in response to some influence, event, stimulus.
2) An equal and opposite force exerted by a body against a force acting upon it. → principle of action and reaction.
3) A → nuclear reaction; → chain reaction.
4) → chemical reaction.

Verbal noun of → react.

The speed with which a → chemical reaction takes place. In other words, the change in → concentration of a → substance divided by the → time interval during which the change is observed.

→ reaction; → rate.

1) Optics: The change of direction which a beam of light undergoes as it enters a medium of different → refractive index.
2) Acoustics: The change in direction in sound waves on reaching the boundary between two media.
3) Seismology: The bending of a seismic wave as it enters a material of different density.

From L.L. refractionem (nominative refractio) "a breaking up," from L. refractus, p.p. of refringere "to break up," from re- "back" + combining form of frangere "to break," from PIE base *bhreg- "to break" (cf. Goth. brikan, O.E. brecan "to break;" Lith. brasketi "crash, crack").

Šekast, past stem of šekastan "to break, split;" Mid.Pers. škastan "to break;" Av. scind-, scand "to beak, cleave;" Proto-Iranian *skand- "to break, cleave;" PIE sken- "to cut off."

A nuclear reaction whose probability is enhanced at an energy corresponding to an energy level of one of the nuclei. → resonance capture.

Resonant, verbal adj. of → resonate; → reaction.

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

→ screen; → coulomb; → interaction.

A fraction in which the → numerator and → denominator are positive → integer numbers. Compare → compound fraction.

→ simple; → fraction.

The act or instance of removing the contribution of non-related, intervening foreground light to the object.

→ sky; → subtraction.

The interaction between quarks that is transmitted by gluons. The characteristic range of the strong interaction is 10^-13 cm, and the time scale over which it operates is on the order of 10^-23 second. Also called → strong force.

→ strong; → interaction.

The operation of finding the difference between two numbers or quantities.

Verbal noun of → subtract.

An operation of tensor algebra that is obtained by setting unlike indices equal and summing according to a summation convention.

→ contraction; → tensor.