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

Math.: A succession of numbers or quantities in which there is a constant relation between each member and the one succeeding it. See also → arithmetic progression, → geometric progression, → harmonic progression.

From O.Fr. progression, from L. progressionem "a going forward," from progressus, p.p. of progredi "go forward," from → pro- "forward" + gradi "to step, walk," from gradus "step."

Farâyâzi, from farâ-, → pro-, + yâzi, verbal noun of yâzidan "to stretch out the arms; grow up;" Parthian Mid.Pers. y'd "to reach a goal, come to, stretch out;" Av. yat- to reach, take one's place," yaiiata "places,' frā-iiatāt "has reached;" cf. Skt. yat- "to be in place, put in place, line up;" PIE base *iet- "to be in place."

A person who directs a project.

→ project; → director.

The temperature in the → solar wind, as derived from the mean kinetic energy of protons: mv²/2 = (3/2)kT_p, where k is → Boltzmann's constant. There are two types of proton temperature: parallel temperature, measured from protons moving parallel to the magnetic field, and perpendicular temperature relating to protons at right angles to the magnetic field. The proton temperature is usually derived using particle detectors on board space probes that determine the velocity → distribution function of the particles from their energies (N. Meyer-Vernet, 2007, Basics of the Solar Wind, Cambridge Univ. Press). See also → electron temperature.

→ proton; → temperature.

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 dense zone of magnetized → plasma surrounding a → pulsar. The magnetosphere, lying between the surface of the → neutron star and the → light cylinder, corotates with the pulsar like a rigid body under the effect of strong magnetic field. The magnetosphere's thickness is determined by the constraint that the corotation velocity of its upper surface should not exceed the → speed of light.

→ pilsar; → magnetosphere.

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.

The proposition that the → square of the → hypotenuse of a → right triangle is equal to the → sum of the squares of the other two sides: a² + b² = c².

After Pythagoras (c570 BC-c495BC), Greek philosopher and mathematician; → theorem.

Any group of three → integers that satisfy the relations specified by the → Pythagorean theorem. Some examples: 3, 4, and 5; 5, 12, and 13; 8, 15, 17.

→ Pythagorean theorem; → triple; → triplet.

1) General: The process of making something square; the act of squaring.
2) Math.: The act or process of constructing a → square with an area equal to that of a specified surface, especially a surface bounded by a curve.
3) Astro.: The position of a planet or the Moon when it makes a 90° angle with the Sun as seen from Earth.

From L. quadratura, from quadrat(us) p.p. of quadrare "to make square."

1) Cârušeš verbal noun of cârušidan "to square," from câruš "square," from Av. caθruša-, → quadratic.
Câruši quality noun of câruš "square," as preceding.
2) Navadân, from navad "ninety," referring to the longitude difference of 90° (Mid.Pers. nawad; Av. nauuaiti- "ninety," from nauua- "nine" (Mod.Pers. noh); cf. Skt. navati- "ninety") + -ân suffix of place and time.

Constructing a square whose area equals that of a given circle. This was one of the three geometric problems of antiquity. It was finally proved to be an impossible problem when π was proven to be transcendental by Lindemann in 1882. Same as → squaring the circle.

→ quadrature; → circle.

In quantum physics, a situation where an object's wave property is split in two, and the two waves coherently interfere with each other in such a way as to form a single state that is a superposition of the two states. This phenomenon is based on the fact that atomic particles have wave-like properties. Quantum coherence is in many ways similar to → quantum entanglement, which involves the shared states of two quantum particles instead of two quantum waves of a single particle. Quantum coherence and quantum entanglement are both rooted in the → superposition principle.

→ quantum; → coherence.

The frequency at which an oscillation is intermittently quenched, as in a super-regenerative receiver.

→ quench; → frequency.

The → momentum carried by → photons to a surface exposed to → electromagnetic radiation. Stellar radiation pressure on big and massive objects is insignificant, but it has considerable effects on → gas and → dust particles. Radiation pressure is particularly important for → massive stars. See, for example, → Eddington limit, → radiation-driven wind , and → radiation-driven implosion. The → solar radiation pressure is also at the origin of various physical phenomena, e.g. → gas tails in → comets and → Poynting-Robertson effect.

→ radiation; → pressure.

The temperature of a source calculated assuming that it behaves as a → blackbody that radiates with the same intensity at the same frequency. Compared to the → effective temperature, the radiation temperature is measured over a narrow region of the → electromagnetic spectrum.

→ radiation; → temperature.

A process in which a molecule relaxes without emitting a → photon.

→ radiation; → -less; → relaxation.

Capture of a free electron by an ion with the subsequent emission of photons; also called → radiative recombination.

→ radiative; → capture.

The process by which an ionized atom binds a free electron in a → plasma to produce a new atomic state with the subsequent radiation of photons.

→ radiative; → recombination.

The → electromagnetic radiation with the frequency range between 3 → kiloherz (kHz) to 300 → gigahertz (GHz). See also → radio wave.

→ radio; → frequency.

A → recombination line whose wavelength lies in the radio range of the electromagnetic spectrum. Radio recombination lines are the result of electronic transitions between high energy levels (n > 50) in an atom or ion.

→ radio; → recombination line.

The pressure exerted on a body moving through a → fluid medium. For example, a → meteor traveling through the Earth's atmosphere produces a → shock wave generated by the extremely rapid → compression of air in front of the → meteoroid. It is primarily this ram pressure (rather than → friction) that heats the air which in turn heats the meteoroid as it flows around the meteoroid. The ram pressure increases with → velocity according to the relation P = (1/2)ρv², where ρ is the density of the medium and v the relative velocity between the body and the medium. Similarly, → ram pressure stripping produces → jellyfish galaxies. Same as → dynamic pressure.

→ ram; → pressure.