An Etymological Dictionary of Astronomy and Astrophysics
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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 → supernova resulting from the → pair instability that generates several successive explosions. According to models, a first pulse ejects many solar masses of hydrogen layers as a shell. After the first explosion, the remaining core contracts and searches for a stable burning state. When the next explosion occurs a few years later, several solar masses of material are again ejected, which collide with the earlier ejecta. This collision can radiate 10⁵⁰ erg of light, about a factor of ten more than an ordinary → core-collapse supernova. After each pulse, the remaining core contracts, radiates neutrinos and light, and searches again for a stable burning state. Later ejections have lower mass, but have higher energy. They quickly catch up with the first shell, where the collision dissipates most of their kinetic energy as radiation. The first SNe from → Population III stars are likely due to pulsational pair instability (Woosley et al. 2007, Nature 450, 390). See also → pair-instability supernova.

→ pulsational; → pair; → instability.

A linear → Hermitian operator associated with a physical quantity.

→ quantum; → mechanics; → operator.

In a dynamical system, a form of motion that is regular but never exactly repeating. Quasiperiodic motion appears when the system contains two or more incommensurate frequencies.

→ quasi-; → periodic; → motion.

The last two million years of geologic time, comprising the Pleistocene and Holocene glacial epochs. Estimates of the date of the beginning of the Quaternary vary between 2.5 and 1.6 million years ago.

Quaternary, from L. quaternarius "consisting of four," from quatern(i) "fourt at a time" + -arius "-ary;" → period.

Dowrân, → period; cahârom "fourth," from cahâr "four" + -om "-th."

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.

Statistics: Experiments in which results will not be essentially the same even though conditions may be nearly identical.

→ random; → experiment.

The operation of a computer during the actual time that the related physical processes take place so that the results can be used to guide the physical processes.

→ real; → time; → operation.

A supergiant star with spectral type K or M. Red supergiants are the largest stars in the Universe, but not necessarily the most massive. Betelgeuse and Antares are the best known examples of a red supergiant.

→ red; → supergiant.

→ magnetic permeability.

→ relative; → permeability.

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

→ rotation; → period.

→ rotation period.

→ rotational; → period.

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 variation of planetary orbital elements which is always in the same direction as time increases.

→ secular; → perturbation.

The richest → supercluster of galaxies in the nearby → Universe at a → redshift going from z ~0.03 to z ~0.05 (680 million → light-years), and extending over several square degrees on the plane of the sky. It lies behind the → Centaurus supercluster. Also called the Shapley concentration, it is made up of 25 → galaxy clusters with a total mass of about 10¹⁶→ solar masses. At the core of the Shapley supercluster is a remarkable complex formed by several rich clusters of galaxies from the → Abell catalog; the central and most massive of them is A3558.

→ Shapley's wing; → supercluster

A comet with a period less than 200 years. Same as → periodic comet.

→ short; → period; → comet.

A variable star that has a relatively short period with respect to stars of similar types.

→ short; → period; → variable.

→ sidereal revolution period; → sidereal rotation period.

→ sidereal; → period.

The time taken by a planet or satellite to complete one revolution about its primary with respect to stars. For Earth, same as → sidereal year. Sidereal periods of the solar system planets, interms of the sidereal year, are as follows: Mercury 0.240846 (87.9691 days); Venus 0.615 (225 days); Earth 1 (365.25636 solar days); Mars 1.881; Jupiter 11.86; Satrurn 29.46; Uranus 84.01; and Neptune 164.8. That of the Moon is 0.0748 (27.32 days) and for → Sedna 12050.

→ sidereal; → revolution; → period.

The rotation period of a celestial body with respect to fixed stars. For Earth, same as → sidereal day.

→ sidereal; → rotation; → period.