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

Any of a set of equations that defines the coordinates of the dependent variables of a curve or surface in terms of one or more independent variables or parameters.

→ parametric; → equation.

An atmospheric optical phenomenon, seen as a bright spot sometimes appearing at either side of the → Sun, often on a luminous ring or → halo and at the same angular elevation as the Sun. Parhelia are caused by the → refraction and → reflection of → sunlight by → ice crystals suspended in the Earth's → atmosphere. Also called mock Sun or sundog.

From Gk. parhelion "a mock Sun," from → para- "beside" + helios "sun," → helio-.

Parâhur, from parâ-, → para-, + hur "sun," → helio-.

In quantum mechanics, the condition of parity in strong and electrodynamic interactions, where it remains constant and does not change with time. In other words, parity conservation implies that Nature is symmetrical and makes no distinction between right- and left-handed rotations or between opposite sides of a subatomic particle. Thus, for example, two similar radioactive particles spinning in opposite directions about a vertical axis should emit their decay products with the same intensity upward and downward. Same as → parity symmetry.

→ parity; → conservation.

In quantum mechanics, the condition of → parity in the → weak interaction. For example, the emitted → beta particles in → radioactive decay of → cobalt-60 nuclei are not equally distributed between the two poles of cobalt-60. More beta particles emerge from one pole than the other, and it would be possible to distinguish the mirror image nuclei from their counterparts.

→ parity; → violation.

A catalog of 397 radio sources between declinations +20° and +27° which were compiled from a finding survey made at 635 MHz with the 64m radio telescope at the Australian National Radio Astronomy Observatory, Parkes, N.S.W. and published in 1968. Originally abbreviated PSR, this catalog, which is also called PKS, replaces and improves on four earlier lists (1964 to 1966).

Parkes, proper noun; selected, p.p. of → select; → region.

A type of differential equation involving an unknown function (or functions) of several independent variables and its (or their) partial derivatives with respect to those variables.

→ partial; → differential; → equation.

One of several zones of the stellar interior where increased → opacity can provide the → kappa mechanism to drive → pulsations. See also → Kramers' law. In these zones where the gases are partially ionized, part of the energy released during a layer's compression can be used for further ionization, rather than raising the temperature of the gas. Partial ionization zones modulate the flow of energy through the layers of the star and are the direct cause of → stellar pulsation. The partial ionization zones were first identified by the Russian astronomer Sergei A. Zhevakin (1916-2001) in the 1950s. In most stars there are two main ionization zones. The hydrogen partial ionization zone where both the ionization of neutral hydrogen (H ↔ H⁺ + e^-) and the first ionization of helium (He ↔ He⁺ + e^-) occurs in layers with a characteristic temperature of 1.5 x 10⁴ K. The second, deeper zone is called the He⁺ partial ionization zone, and involves the second ionization of helium (He⁺↔ He⁺⁺ + e^-), which occurs deeper at a characteristic temperature of 4 x 10⁴ K. The location of these ionization zones within the star determines its pulsational properties. In fact if the → effective temperature of the star is ≥ 7500 K, the pulsation is not active, because the ionization zones will be located very near to the surface. In this region the density is quite low and there is not enough mass available to drive the oscillations. This explains the blue (hot) edge of the instability strip on the → H-R diagram. Otherwise if a star's surface temperature is too low, ≤ 5500 K, the onset of efficient convection in its outer layers may dampen the oscillations. The red (cool) edge of the instability strip is believed to be the result of the damping effect of convection. He⁺ ionization is the driving agent in → Cepheids. See also → gamma mechanism.

→ partial; → ionization; → zone.

An act or instance of participating. The fact of taking part.

Verbal noun of → participate.

Of partial differential equations, the solution which can be obtained from the general solution by particular choice of the arbitrary functions. → general solution; → singular solution.

→ particular; → solution.

1a) A division into or distribution in portions or shares.
1b) A separation; something that separates or divides; a part, division, or section.
2a) The act or process of dividing something into parts. The state of being so divided.
2b) Math.: → integer partition; → set partition.
2c) Computers: → disk partition.
3) To divide into parts or portions. To divide or separate by a partition.

M.E., from O.Fr. particion, from L. partition- "division, partition, distribution," from p.p. stem of partire "to part, divide," from PIE root *per- "to grant, allot," related to → part and akin to pake, as below.

Parke, from Kermâni parke "piece, part, fragmant," related to pâr "→ part;" parkidan, infinitive from parke; parkeš, verbal noun of parkidan.

See → integer partition.

→ partition; → function.

The ratio of the → concentration of a → solute in a single definite form in the stationary phase to its concentration in the same form in the other phase at equilibrium.

→ partition; → ratio.

A chemical separation process whereby a solute is distributed between two phases.

→ partition; → -ing.

The act or fact of being partitioned.

→ partition; → -ment.

A fraction of a whole number in units of 1/1000,000. It is usually used to describe chemical concentrations, very small amounts of pollutants in air, water, body fluids, and uncertainty. For example 30 ppm is 3 x 10^-5 or 0.003%.

→ part; → per; → million.

A quantum mechanical principle according to which no two identical → fermions can share the same → quantum state. Also known as → exclusion principle.

In honor of Wolfgang Pauli (1900-1958), Austrian theoretical physicist, who formulated the principle in 1925; → exclusion; → principle.

A planet formation concept that concerns the → accretion of small objects of negligible gravitational mass (ranging from centimeters up to meters in diameter) onto large, gravitating bodies: → planetesimals, → protoplanets, or → planets. In a more narrow sense, pebble accretion is an accretion process where (gas) drag and gravity play major roles. Simply put, this means that the pebble has to be aerodynamically small and the planet to be gravitationally large (C. W. Ormel, in Formation, Evolution, and Dynamics of Young Solar Systems, p. 197, eds. M. Pessah, O. Gressel, Springer, 2017).

→ pebble; → accretion.

1) The true motion of a star with respect to the Local Standard of Rest. → proper motion.
2) The motion of a cosmological object other than the apparent recession caused by the expansion of the Universe.

→ peculiar; → motion.

1) Depreciation; a lessening in worth, quality, etc.
2) Semantic change in a word to a lower, less approved, or less respectable meaning.

Verbal noun of → pejorate.

The process of recognizing or identifying something. Usually employed of sense perception, when the thing which we recognize or identify is the object affecting a sense organ.

Verbal noun of → perceive.