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

A phase transitions that occurs at zero temperature as a function of a non-thermal parameter like → pressure, → magnetic field, or → chemical composition. In contrast to ordinary → phase transitions, which are associated with passage through a critical temperature, quantum phase transitions are associated with → quantum fluctuations, a consequence of → Heisenberg's uncertainty principle. For example, see → Bose-Einstein condensation.

→ quantum; → phase; → transition.

A phase transition, predicted by cosmological models, to have occurred at approximately 10^-5 seconds after the Big Bang to convert a plasma of free quarks and gluons into hadron.

→ quark; → hadron; → phase; → transition.

A measure of the amount of radiation emitted from a point expressed as the radiant flux per unit solid angle leaving this source.

→ radiant; → intensity.

The constant related to the total energy radiated by a → blackbody and defined as: a = 4σ/c, where σ is the → Stefan-Boltzmann constant and c the → speed of light. Its value is a = 7.5657 x 10^-15 erg cm^-3 K^-4. Same as → radiation constant.

→ radiation; → density; → constant.

A transition between two states of an atomic or molecular entity, the energy difference being emitted or absorbed as photons.

→ radiative; → transition.

The flux of → radio waves that falls on a → detector per → unit → surface area of the detector per unit → bandwidth of the radiation per unit → time. The unit is → jansky.

→ radio; → flux; → density.

The ratio of the density of a substance to the density of a given reference material. For a solid or liquid, it is the density (at 20°C) relative to the maximum density of water (at 4°C). For a gas is its density divided by the density of hydrogen (or sometimes dry air) at the same temperature and pressure. Also called → specific density.

→ relative; → density.

A → spectroscopic phenomenon observed when either an → eclipsing binary's → secondary star or an → extrasolar planet is seen to → transit across the face of the → primary body. Because of the rotation of the star, an asymmetric distortion takes place in the → line profiles of the stellar spectrum, which changes during the transit. The measurement of this effect can be used to derive the → alignment of the → orbit of the transiting exoplanet with respect to the → rotation axis of the star.

Named after Richard Alfred Rossiter (1886-1977) and Dean Benjamin McLaughlin (1901-1965), American astronomers.

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.

A → tensor density of → order 0.

→ scalar; → density.

An → atomic transition whose probability is reduced by a factor of the order of 10⁶ because of → selection rules. Same as → interconnection line.

→ semi-; → forbidden; → transition.

Endowed with sensation; having perception through the senses. Responding to a stimulus.

From M.Fr. sensitif, from M.L. sensitivus "capable of sensation," from L. sensus, p.p. of sentire "feel perceive," → sense.

hessmand, from hess, → sense + -mand possession suffix.

1) The required brightness for an object in order to be detected by an observing instrument. A highly sensitive telescope can detect dim objects, while a telescope with low sensitivity can detect only bright ones.
2) Of a radio receiver or similar device, the minimum input signal required to produce a specified output signal having a specified signal-to-noise ratio.

State noun from → sensitive.

1) To rest with the body supported by the buttocks or thighs; be seated.
2) To be located or situated.

Nešastan "to sit down; to settle down; to sink;" Mid.Pers. nišastan "to sit;" O.Pers. nišādayam [1 sg.impf.caus.act.] "to sit down, to establish," hadiš- "abode;" Av. nišasiiā [1 sg.subj.acr.] "I shall sit down," from nihad- "to sit down," from → ni- "down; into" + had- "to sit;" PIE base *sed- "to sit;" cf. Skt. sad- "to sit," sidati "sits;" Gk. hezomai "to sit," hedra "seat, chair;" L. sedere "to sit;" O.Ir. suide "seat, sitting;" Welsh sedd "seat;" Lith. sedmi "to sit;" Rus. sad "garden;" Goth. sitan, Ger. sitzen; E. sit. See also: → reside, → settle.

The position or location of a building, observatory, etc. especially as to its environment. → astronomical site.

M.E., from L situs "position, arrangement, site," from sinere "to let, leave alone, permit," cognate with Av. šiti- "place, abode, residence," as below.

Sit, from Av. šiti- "place, abode, residence," šitāy- "habitation, dwelling," from ši- "to live;" cognate with Skt. ksay- "to live, to stay," kséti "he dwells;" Gk. ktizein "to inhabit, build;" L. situs "position, site; situated."

The process of choosing a site for an astronomical observatory based on meteorology, seeing conditions, and access to the site.

→ site; → selection.

To place in a site or context; to locate.

From M.L. situatus, p.p. of situare "to place, locate," from L. situs "place, position."

Sitidan, from sit, → site, + -idan infinitive suffix.

Having a site, situation or location.

P.p. of → situate.

1) The manner of being placed with respect to surroundings.
2) Momentary state; a set of circumstances.

Verbal noun of → situate.

The total → radiant energy, in all wavelengths, emitted by the Sun in all directions. It is 3.828 × 10²⁶ W or 3.828 × 10³³ erg sec^-1 (International Astronomical Union, Resolution B3, 14 August 2015, Honolulu, USA). This is the luminosity unit conventionally used to give the luminosities of stars. See also: → solar constant. When the Earth first formed, 4.56 billion years ago, the Sun radiated 30% less energy than it does today, thus giving rise to the so-called → faint early Sun paradox. Ever since then, its power has increased by 7% every billion years (I. Ribas, 2009, arXiv:0911.4872).

→ solar; → luminosity.