An Etymological Dictionary of Astronomy and Astrophysics
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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.

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.

Same as → significance testing and → test of significance.

→ rule; → decision.

A term inserted into a formula for the energy of a single electron in the outermost shell of an atom to take into account the failure of the inner electron shells to screen the nuclear charge completely.

→ rydberg; → correction.

An equation that gives the number of atoms of a given species in various stages of → ionization that exist in a gas in → thermal equilibrium as a function of the temperature, density, and ionization energies of the atoms.

Named after the Indian astrophysicist Megh Nad Saha (1894-1956), who first derived the equation in 1920; → equation.

A → supergiant star of visual magnitude 2.06 and → spectral type B0.5 Ia marking the right knee of Orion. It is about 700 light-years away.

Saiph "sword," from Ar. as-saiph al-jabbâr (سیف الجبار) "the Sword of the Giant."

The three conditions that are necessary for the generation of a → baryon asymmetry in the → early Universe. These conditions are:
1) The → baryon number should not be → conserved.
2) The → charge conjugation and → CP symmetry should be → violated, and
3) Departure from → thermal equilibrium.

Named after Andrei Sakharov (1921-1989), who in 1967 described these three minimum conditions (A. D. Sakharov, 1967, Zh. Eksp. Teor. Fiz. Pis'ma 5, 32; 1967, JETP Lett. 91B, 24); → condition.

The first mathematical description of the → initial mass function (IMF) of newly formed stars of solar to → intermediate-masses. It is proportional to M ^-2.35, where M is the stellar mass. → Salpeter slope.

Named after the Austrian-Australian-American astrophysicist Edwin Ernest Salpeter (1924-2008); → function.

A solution which can exist in equilibrium with excess of solute. The saturation concentration is a function of the temperature.

→ saturate; → solution.

Physics: Degree of magnetization of a substance which cannot be exceeded however strong the applied magnetizing field.
Detectors: The condition of a detector or a pixel when it is submitted to a signal so strong that it cannot handle it properly; the result is a non-linear, useless response.

Verbal noun of → saturate.

The maximum current that can be obtained in a specific circuit under specified conditions.

→ saturation; → current.

The maximum intrinsic magnetic induction possible in a material.

→ saturation; → induction.

In radar, a signal whose amplitude is greater than the dynamic range of the receiving system.

→ saturation; → signal.

The energy density fluctuations in the → photon-baryon plasma that bring about hotter and colder regions. This perturbation creates velocity distributions that are out of phase with the acoustic density mode. The fluid velocity from hot to cold regions causes blueshift of the photons, resulting in → quadrupole anisotropy.

→ scalar; → perturbation.

A mathematical expression that describes the → luminosity function of galaxies. The function correctly reflects the facts that the luminosity function decreases with increasing luminosity and that the decrease is particularly marked at high luminosities. It is expressed as: φ(L) = φ^*(L/L^*)^α exp (-L/L^*), which has two parts and three parameters: φ^* is an empirically determined amplitude, α is an empirically derived exponent, and L^* is a characteristic luminosity which separates the low and high luminosity parts. For small luminosities (L much smaller than L^*) the Schechter function approaches a power law, while at high luminosities (L much larger than L^*) the frequency of galaxies drops exponentially. φ^*, L^*, and the faint-end slope α depend on the observed wavelength range, on the → redshift, and on the environment where the galaxies are observed.

Named after the American astronomer Paul Schechter (1948-), who proposed the function in 1976 (ApJ 203, 297); → function.