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

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.

The concentric colored rings formed by an electrical → spark when a → discharge takes place on a metallic plate.

Joseph Priestley (1733-1804), English chemist, author, and clergyman; → ring.

1) First or highest in rank or importance; chief; principal.
2) Abbreviation for → primary body, → primary star, or → primary mirror.

M.E., from L. primarius "of the first rank, principal," from primus "first."

Naxostân, from naxost, from Mid.Pers. naxust "the first," Parthian Mid.Pers. nxwšt, from naxu, Manichean Parthian nwx "beginning" + -ist superlative suffix, Av. -išta-, cf. Skt. -istha-, Gk. -istos, O.H.G. -isto, -osto, O.E. -st, -est, -ost.

The body that is being orbited; such as the → Sun in the → solar system. As regards → multiple star systems, the most massive, or → primary star. See also: → secondary body.

→ primary; → body.

A → cell in which the electrochemical action producing the current is not normally reversible. Such a cell cannot be recharged by an electric current. → secondary cell.

→ primary; → cell.

The → cosmic rays which arrive on the Earth's → atmosphere from the outer space. The primary cosmic rays are very high energy → protons and to a lesser extent heavier nuclei which rain upon the Earth from all diretions in the outer space. They contain about 90% protons, 7% → alpha particles and about 1% still heavier nuclei of amost all the atoms from Li to Ni ( → mass number< 60). See also: → secondary cosmic rays.

→ primary; → cosmic; → ray.

Of a transiting → exoplanet, the event and the interval of time during which the planet passes in front of its host star. The planet occults a portion of the stellar disk, and a fraction of light from the star is seen after traversal through the atmosphere around the planet's limb. → secondary eclipse.

→ primary; → eclipse.

In a → reflecting telescope, the first mirror that collects the light and focuses it to the → focal plane.

→ primary; → mirror.

The main rainbow that forms between about 40° and 42° from the → antisolar point (or about 50° from the → antisolar point), as viewed by the observer. The light path involves → refraction and a → single  → reflection inside the water → droplet. If the drops are large, 1 millimeter or more in diameter, red, green, and violet are bright but there is little blue.

As the droplets get smaller, red weakens.

Rainbows are not seen in midday since the whole 42° circle is below the horizon at most latitudes. So rainbows tend to be seen most in the later afternoon when a thundershower has passed and the Sun is illuminating from the west.

→ primary; → rainbow.

In a → multiple star system, the most massive → component. In other words, the star nearest to the system's → center of garvity.

→ primary; → star .