An Etymological Dictionary of Astronomy and Astrophysics
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A condition for the onset of → instability in multilayer fluids which compares the balance between the restoring force of → buoyancy and the destabilizing effect of the → shear.

Named after the British meteorologist Lewis Fry Richardson (1881-1953), who first arrived in 1920 to the dimensionless ratio now called → Richardson number. The first formal proof of the criterion, however, came four decades later for → incompressible flows (Miles, J. W. 1961, J. Fluid Mech., 10, 496; Howard, L. N., 1961, J. Fluid Mech., 10, 509). Its extension to → compressible flows was demonstrated subsequently (Chimonas 1970, J. Fluid Mech., 43, 833); → criterion.

A coordinate in the → equatorial system measured from the → vernal equinox eastward to the point where the object → hour circle intersects the → celestial equator. Right ascension (symbol α) is expressed in hours, minutes, and seconds. See also: → declination.

→ right; ascension, M.E. ascencioun, from O.Fr., from L. ascendere "to climb up," from → ad- "to" + scandere "to climb."

Râst, → right; afrâz present stem of afrâzidan, afrâštan "to raise, exalt, extole," from Mid.Pers. abrâstan, abrâz- "to lift, raise," from ab-, from O.Pers./Av. abiy-/aiwi- "to, upon, against;" cf. Skt. abhi-, Gk. amphi- + râst "straight, direct, true;" from 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;" Ger. recht; E. right; PIE base *reg- "move in a straight line," hence, "to direct, rule."

A → selection rule obeyed by → discrete transitions. Among them are: rigorous selection rules for → electric dipole transitions (→ permitted) requiring: 1) ΔJ must be 0 or ± 1 with J = 0 ↔ 0 forbidden. 2) ΔM_J = 0, ± 1. 3) → Parity change, i.e. even ↔ odd.

→ rigorous; → selection; → rule.

An empirical rule discovered before the advent of quantum mechanics which states that it is possible to find pairs of spectral lines, which have the property that the sum of their wavenumbers is also an observed spectral line.

Named after Walther Ritz (1878-1909), a Swiss theoretical physicist; → combination; → principle.

The motion of a body about its axis.

Verbal noun of → rotate.

The imaginary line around which an object rotates. Same as → rotational axis and → axis of rotation.

→ rotation; → axis.

A plot of the variation in → orbital velocity of stars and → interstellar matter with distance from the center of a → galaxy. A "flat" rotation curve indicates that the mass of the galaxy increases linearly with distance from its center. See also: farsi→ Keplerian rotation curve

Rotation; → curve.

The → kinetic energy of rotational motion of an object. It is expressed by E_R = (1/2)Iω², where I is the → moment of inertia and ω → angular velocity (2π/P).

→ rotation; → energy.

1) The number of rotations per unit time of a rotating object.
2) The number of → stellar rotations per unit time. The reciprocal of the → rotation period. This parameter usually refers to the equator of the star, because stars do not rotate as solid bodies.

→ rotation; → frequency.

The interval of time during which an object turns once about its axis.

→ rotation; → period.

A position parameter used in → stellar magnetic field studies. Its zero value represents the moment when, during → stellar rotation, the positive → magnetic pole is nearest to the → line of sight.

→ rotation; → phase.

A type of → turbulence with motions more vigorous in the horizontal than in the vertical direction occurring in internal radiation zone of → rotating stars. Same as → shear turbulence.

→ rotation; → induced; → turbulence.

A → neutron star that is spinning down as a result of → torques from → magnetic dipole radiation and particle emission. RPPs derive their energy primarily from the → rotation of the neutron star. The energy from their → spin-down appears as broad-band pulsations from → radio to → gamma-ray wavelengths and as a → wind of energetic particles flowing into their surrounding → pulsar wind nebulae. Since the discovery of RPPs through their radio → pulsations in 1967, more than 2000 → radio pulsars are now known with periods ranging from a few milliseconds to several seconds (A. K. Harding, 2013, Front. Phys. 8, 679).

→ rotation; → power; → pulsar.

The spectrum of a molecule resulting from the simultaneous rotation and vibration of its constituent atoms.

→ rotation; → vibration; → spectrum.

Of or pertaining to → rotation.

Rotational, adj., from → rotation + → -al.

The → angular momentum of a body rotating about an axis. The rotational angular momentum of a solid homogeneous sphere of mass M and radius R rotating about an axis passing through its center with a period of T is given by: L = 4πMR²/5T.

→ rotational; → angular; → momentum.

An imaginary line about which a solid object rotates. Same as → rotation axis and → axis of rotation.

→ rotational; → axis.

The spectral line broadening caused by stellar rotation. Light from two rims of the star will be Doppler shifted in opposite directions, resulting in a line broadening effect. The line broadening depends on the inclination of the star's pole to the line of sight. The derived value is a function of v_e. sini, where v_e is the rotational velocity at the equator and i is the inclination, which is not always known. The fractional width (Δλ/λ) is of the order of 10^-3 for B stars.

→ rotational; → broadening.

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.