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

The → Eddington limit of luminosity for a → rotating star in which both the effects of → radiative acceleration and rotation are important. Such objects mainly include → OB stars, → LBV, → supergiants, and → Wolf-Rayet stars. It turns out that the maximum permitted luminosity of a star is reduced by rotation, with respect to the usual Eddington limit (Maeder & Meynet, 2000, A&A, 361, 159).

→ rotational; → Eddington limit.

The → kinetic energy due to the → rotation of and object. Rotational energy is part of the total kinetic energy of the body. It is given by: (1/2)Iω², where I is the → moment of inertia and ω is the → angular velocity. Same as → angular kinetic energy.

→ rotational; → energy.

A consequence of → stellar rotation that deforms the star, triggers instabilities (→ shear turbulence and → meridional currents) leading to → transport of chemical species in the star. The efficiency of rotational mixing (measured for instance by the degree of surface → enrichments at a given → evolutionary stage) increases when the initial mass and rotation increase. This efficiency increases also when the initial → metallicity decreases. This is due to the fact that when the metallicity is lower, the stars are more compact. This makes the → gradients of the → angular velocity steeper in the stellar interiors. Steeper gradients produce stronger shear turbulence and thus more mixing. Rotational mixing can bring to the surface heavy elements newly synthesized in the stellar core. Rotation thus produces an increase of the → opacity of the outer layers and activates strong → mass loss through → radiatively driven winds. This effect may be responsible for the loss of large fractions of the initial mass of the star (Meynet et al. 2007, arXiv:0709.2275).

→ rotational; → mixing.

A very small variation in the surface brightness of a single star due to its rotation. Several types of stars are known to have photospheric spots. Brightness variation occurs as rotation carries star spots or other localized activity across the line of sight.

→ rotational; → modulation.

Of a → rigid body, a motion in which there are always two points of the body which remain motionless.

→ rotational; → motion.

→ rotation period.

→ rotational; → period.

A slight change in the energy level of a molecule due to the rotation of its constituent atoms about their center of mass.

→ rotational; → transition.

The velocity of a → rotational motion; same as → angular velocity.

→ rotational; → velocity.

A device that rotates or causes rotation.

Agent noun from → rotate.

A rotating part of an electrical apparatus, e.g. the armature of a generator, or of a mechanical device.
A system of several flat blades attached to a hub, which rotates either horizontally to give lift and thrust to a helicopter, or vertically to help control it.
Meteo.: The circulation of flow about a horizontal or nearly horizontal axis.

Short for rotator, → rotate, + → -or.

Carxâ agent noun of carxidan, → rotate.

CCD detector: Series of pixels arranged along a line. → column

O.E. ræw "a row, line;" cf. Du. rij "row;" O.H.G. rihan "to thread," riga "line;" Ger. Reihe "row, line, series."

Raj "line, row," variants raž, rak, râk, rezg (Lori), ris, risé, radé, rasté, râsté, related to râst "right, true; just, upright, straight;" Mid.Pers. râst "true, straight, direct;" Soghdian rəšt "right;" O.Pers. rāsta- "straight, true," rās- "to be right, straight, true;" Av. rāz- "to direct, put in line, set," razan- "order;" cf. Skt. raj- "to direct, stretch," rjuyant- "walking straight;" Gk. orektos "stretched out;" L. regere "to lead straight, guide, rule," p.p. rectus "right, straight;" PIE base *reg- "move in a straight line," hence, "to direct, rule."

An integral constraint used to quantify the uncertainty on the extent of → convective overshooting and its effect on models of stars.

Roxburgh, I. 1989, A&A, 211, 361; → criterion.

A member of a large class of → pulsating stars of type A2-F6 with periods less than 1 day. They are similar to → Cepheids, except that their periods are much shorter and are less luminous. RR Lyrae stars belong to → Population II and are often found in → globular clusters (hence one of their older names cluster variables) or elsewhere in the → galactic halo. They are used as distance indicators (→ standard candle) out to more than 200 kpc.

→ Lyra; → star.

A metallic chemical element; symbol Rb. Atomic number 37; atomic weight 85.4678; melting point 38.89°C; boiling point 686°C; specific gravity 1.53 at 20°C. It was discovered in the mineral lepidolite by the German chemist Robert Wilhelm Bunsen and the German physicist Gustav-Robert Kirchoff in 1861. Bunsen isolated rubidium in 1863.

From L. rubidus "deep red," because of the two "deep red lines" in its spectra.

Red form of corundum, Al₂O₃, which owes its color to traces of chromium. Used in laser as a gem stone.

M.E. rubi, from O.Fr. rubi, from M.L. rubinus lapis "red stone," from L. rubeus "red," related to ruber→ red.

Yâqut, yâkand related to Gk. hyakinthos "hyacinth," probably ult. from a non-I.E. Mediterranean language.

A set of astronomical tables created in 1627 by Johannes Kepler (1571-1630) based on observations by Tycho Brahe (1546-1601). These tables allowed Kepler to derive the three laws of planetary motions bearing his name (→ Kelpler's laws). These are the first tables in which → atmospheric refraction has been taken into account. They overruled the → Prutenic Tables.

From the L. title Tabulae Rudolphinae, in memory of Rudolf II (1552-1612), king of Hungary and Bohemia, and Holy Roman Emperor; → table.

A method for finding the value of a → polynomial given by a real number and deriving its → roots. It consists essentially of factoring the polynomial in a nested form. Also known as → nested multiplication.

Named after Paolo Ruffini (1765-1822) and William Horner (1786-1837), who independently elaborated the method; → method.

An → induction coil which was a forerunner of today's automobile ignition coil. It consists of two coils wound on a single → iron core, and uses an → alternating current produced by a break-wheel to induce a high-voltage current in the secondary coil.

After Heinrich Daniel Ruhmkorff (1803-1877), a German-born instrument maker, who settled in Paris in 1819 for the rest of his life; → coil.

1) A law or regulation that governs behaviors, actions, or operations. → Arnett's rule, → commutation rule, → Fleming's rule, → Hund's rule, → left-hand rule, → Maxwell's rule, → right-hand rule, → rigorous selection rule, → selection rule, and → Trouton's rule.
2) A strip of rigid material marked off in units used especially for measuring length.
3) In some → planispheric astrolabes, a bar which rotates across the front of the astrolabe and is used to locate positions on the → rete, and to relate them to the scale of hours marked on the → limb. Also called the → index.

M.E. riule, reule, from O.Fr. riule, from L. regula "straight stick, bar, ruler," related to regere "to rule, straighten, guide;" cognate with Pers. râst "right, straight," razan "rule," as below.

Razan from Av. razan "rule, order," from rāz- "to direct, put in line, set," rasman- "the lines or files of the army;" O.Pers. rāsta- "straight, true," rās- "to be right, straight, true;" Mid.Pers. râst "true, straight, direct;" Soghdian rəšt "right," rây-, râyênitan "to arrange;" Mod.Pers. râst "right, true; just, upright, straight;" raj "line, row," variants raž, rak, râk, rezg (Lori), radé, râdé "line, rule, row," rasté, râsté "row, a market with regular ranges of shops;" ris, risé "straight;" cf. Skt. raj- "to direct, stretch," rjuyant- "walking straight;" Gk. orektos "stretched out;" L. regere "to lead straight, guide, rule," p.p. rectus "right, straight;" PIE base *reg- "move in a straight line," hence, "to direct, rule."

Same as → significance testing and → test of significance.

→ rule; → decision.