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

Of or relating to an epoch earlier than the formation of the Sun.

→ pre-; → solar.

A → refractory → nanoparticle embedded in → meteorites and → interplanetary dust particles whose → isotopic ratios suggest formation earlier than the Solar System.

→ presolar; → grain.

A mathematical analysis, based on → self-similarity, used to predict the → mass function of spherically collapsing → dark matter halos. The formalism assumes that the fraction of mass in halos more massive than M is related to the fraction of the volume in which the smoothed initial density field is above some threshold δ_cρ, where ρ is the average density of the Universe, with the volume encompassing a mass larger than M. A variety of smoothing → window functions and thresholds have been argued, but the most common is a top-hat window in real space and δ_c≅ 1.69. The Press-Schechter formalism provides a relatively good fit to the results of numerical simulations in cold dark matter theories.

First described by William H. Press and Paul Schechter's paper (1974, ApJ 187, 425); → formalism.

The force per unit area.

M.E., from O.Fr. pressure, from L. pressura "action of pressing," from pressus, p.p. of premere "to press, compress."

Fešâr "squeezing, constriction, compression," verb fešordan, fešârdan "to press, squeeze;" phonetic variants Lori xošâr, Aftari xešâr, Qazvini, Qomi xošâl; cf. Khotanese ssarr- "to exhilarate;" loaned in Arm. ôšarak, in Ar. afšaraj "juice."

A broadening of spectral lines caused mainly by the stellar atmospheric density and the surface gravity of the star. The line strength of a spectral line depends on the number of atoms in the star's atmosphere capable of absorbing the wavelength in question. For a given temperature, the more atoms there are, the stronger and broader the spectral line appears. Denser stars with higher surface gravity will exhibit greater pressure broadening of spectral lines.

→ pressure; → broadening.

The pressure difference between two adjacent regions of a fluid that results in a force being exerted from the high pressure region toward the low pressure region.

→ pressure; → gradient.

A force resulting from → pressure gradient that is directed from high to low pressure.

→ pressure; → gradient; → force.

A physical state of dense matter in which the electrostatic field of one atom should influence a neighboring atom and hence disturb atomic levels. In extreme case, such as white dwarfs, electron clouds practically rub and electrons are ionized off the parent atoms.

→ pressure; → ionization.

Same as → p mode

→ pressure; → mode.

A basic ingredient of the → mixing length theory that scales with the → mixing length. It is defined by the relation: H_P = -dr/dln P = -Pdr/dP , where r is the height and P the pressure. See also → scale height.

→ pressure; → scale; → height.

Occurring before something else in time or order.

From L. praevius "leading the way," from prae-, → pre- + via "way."

Pišin, from piš "before," → pre-.

Coming or occurring before something else; prior.

→ previous; → -ly.

A statement concerned with thermodynamic heat exchange, according to which bodies in → thermal equilibrium are simultaneously absorbing and emitting radiant energy. A body radiates in the same way whether other bodies are present or not. Also called Prevost's theory of exchanges.

Named after Pierre Prévost (1751-1839), a Swiss philosopher and physicist, who, in 1791, put forward the statement; → law; → exchange.

1) To seize and devour prey, as an animal does.
2) An animal hunted or seized for food, especially by a carnivorous animal (Dictionary.com).

M.E. preye, from O.Fr. preie, from L. praeda "booty, plunder, game hunted;" ultimately from prehendere "to grasp, seize."

1) Cavidan, related to Khotanese cev- "to get, seize," Ossetic cævyn, cavd "to hit, to strike," Pers. câpidan "to plunder," cafsidan, caspidan "to stick; to plunder;" Proto-Ir. *cap- "to seize."
2) Cavâk, from cav- present stem of cavidan, as above, + -âk suffix denoting object of an action.

An animal that is hunted and killed by a → predator as food.

→ prey; → animal.

In cosmological models, the phenomenon that is supposed to seed the → cosmic microwave background anisotropies and the structure formation of the Universe.

→ primordial; → curvature; → perturbation.

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

→ principle; → action; → reaction.

One consequence of the theory of → special relativity, according to which no two events separated by a distance greater than their separation in time multiplied by the → speed of light may have a → causal influence on each other. Violation of this principle leads to → paradoxes, such as that of an → effect preceding its → cause.

→ principle; → relativistic; → causality.

The first postulate in Einstein's theory of → special relativity whereby all the laws of physics are the same in every → inertial reference frame. In other words, no physical measurement can distinguish one inertial reference frame from another. See also → principle of constancy.

→ principle; → relativity.

An act or a manner of proceeding in any action or process; a particular course or mode of action.

From Fr. procédure "manner of proceeding," from O.Fr. procédér from L. procedere "to go forward, advance," from → pro- "forward" + cedere "to go."

Ravand "going," from raftan "to go, walk;" Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- "to go; to attack."