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

A → real number which cannot be exactly expressed as a ratio ^a/_b of two integers. Irrational numbers have decimal expansions that neither terminate nor become periodic. Every → transcendental number is irrational. The most famous irrational number is √ 2.

From ir- a prefix meaning "not," a variant of → in-, + → rational; → number.

The astronomical activities that took place from the 8th to the 14th century in the Middle East, Central Asia, North Africa, and Moorish Spain. The term Islamic should refer to a civilization rather than a religion, because much of the astronomy was secular. In fact more than 90% of "Islamic" astronomy deals with the Greek astronomy → Ptolemaic system, which has obviously nothing to do with religion. Moreover, many non-Muslims within that civilization contributed to this science and must be acknowledged. Apart from these considerations, the term "Islamic astronomy" creates a conceptual disparity. In comparison, the works of European astronomers, such as Copernicus, Galileo, Newton, and others are not placed under "Christian astronomy," and they are indeed not called "Christian scientists." See also → Arabic astronomy, → Islamic calendar.

From Islam, literally "submission" (to God); → astronomy.

A curve on a → Hertzsprung-Russell diagram connecting all stars having the same age.

Isochrone, back formation from isochronal, from Gk. → iso- + khronos "time."

Izozamân, from → iso- + zamân→ time.

Having similar angles.

→ iso- + -gonal, → diagonal

Math.: A curve which intersects every member of a given one-parameter family of curves at one and the same angle.

→ isogonal; → trajectory.

Massive star formation outside → OB associations. Recent observational findings suggest that → massive star formation is a collective process. In other words, massive stars form in → cluster environments and the mass of the most massive star in a cluster is correlated with the mass of the cluster itself. Nevertheless, other observational results give grounds for supposing that massive stars do not necessarily form in clusters but that they can be formed as isolated stars or in very small groups. According to statistical studies nearly 95% of Galactic → O star population is located in clusters or OB associations. This means that a small percentage, about 5%, of high mass stars may form in isolation. Isolation is meant not traceable to an origin in an OB association. This definition therefore excludes → runaway massive stars, which are thought to result from either dynamical interaction in massive dense clusters, or via a kick from a → supernova explosion in a → binary system. Alternatively, isolated massive star has been defined as follows: An O-type star belonging to a cluster whose total mass is < 100 Msun and moreover is devoid of → B stars (Selier et al. 2011, A&A 529, A40 and references therein).

→ isolated; → massive star; → formation.

A → neutron star which does not belong to a → binary system, does not have radio emission, and is not surrounded by a progenitor → supernova remnant. INSs appear to be thermally cooling with no emission outside the → soft X-ray band, except for faint optical/UV counterparts. Although these properties are similar to those of → compact central object (CCO)s, they are a distinct class because they lack any observable associated supernova remnant or nebula. There are presently seven confirmed INSs (sometimes referred to as The Magnificent Seven), six of which have measured weakly modulated X-ray pulsations with periods between 3 s and 11 s, much longer than those of CCOs (A. K. Harding, 2013, Front. Phys. 8, 679).

→ isolated; → neutron; → star.

An act or instance of isolating; the state of being isolated.

Verbal noun of → isolate.

A line drawn through all points on a weather map having the same amount of → cloud cover.

From Gk. → iso- + nephos "cloud," cognate with Pers. nam "humidity, moisture;" Av. napta- "moist," nabās-câ- "cloud," nabah- "sky;" Skt. nábhas- "moisture, cloud, mist;" L. nebula "mist," nimbus "rainstorm, rain cloud;" O.H.G. nebul; Ger. Nebel "fog;" O.E. nifol "dark;" from PIE *nebh- "cloud, vapor, fog, moist, sky."

Izoabr, from izo-, → iso-, + abr → cloud.

One of several nuclides having the same number of neutrons in their nuclei but differing in the number of protons.

Isotone, from → iso- + tone, from Gk tonos "strain, tone, mode," literally, "a stretching," akin to teinein "to stretch," cognate with Pers. tanidan "to spin, twist, weave" (Mid.Pers. tanitan; Av. tan- "to stretch, extend;" cf. Skt. tan- to spin, stretch;" tanoti "stretches," tantram "loom;" Gk. teinein "to stretch, pull tight;" L. tendere "to stretch; PIE base *ten- "to stretch;" Pers. târ "string," tân "thread," tur "fishing net, net, snare," and tâl "thread" (Borujerdi dialect) belong to this family; variants tanta "cobweb," tanadu, tafen, kartané, kârtané, kâtené, Pashtu tanistah "cobweb;" cf. Skt. tantu- "cobweb, thread, string").

A slight difference between the → abundances of → isotopes of the same → chemical element owing to → physical or → chemical  → processes. It results in the → enrichment or → depletion of an isotope. Same as → isotopic fractionation.

→ isotope; → fractionation

Same as → isotope fractionation.

→ isotopic; → fractionation

A computational process involving a succession of approximations, which consists of repeating the operation by inputting the outcome of each preceding operation to improve the final result until a desired accuracy is achieved. Compare → repetition.

Verbal noun of → iterate.

An energy schematic representing the → electronic states of a → molecule and the → transitions between them. The vertical axis shows energies whereas → energy states are grouped horizontally according to their spin → multiplicity. Radiation-less transitions are symbolized by usual arrows, while → radiative transitions are represented by wavy arrows. The vibrational ground states of each electronic state are indicated with thick lines and the higher → vibrational states with thinner lines.

Named after Aleksander Jablonski (1898-1980), a Polish physicist who was an expert in the field of luminescence and atomic optics; → diagram.

Powerful, forward thrust that results from the rearward expulsion of a jet of fluid, especially propulsion by jet engines.

→ jet; → propulsion.

The random fluctuation of voltage across a resistor caused by the thermal excitation of electrons within it, and the dissipation of power associated with these fluctuations. More generally, an intrinsic noise generated by thermal agitation of electrons by all bodies whose temperature is above 0 K. Also called → thermal noise, Johnson noise, or Nyquist noise.

Named after John Bertrand Johnson (1887-1970) and Harry Nyquist (1889-1976) Swedish-born American engineers and physicists, who did important work on thermal noise and information theory. → noise.

A quantum mechanical → tunnel effect allowing the flow of a continuous current across two weakly coupled → superconductors which are separated by a very thin insulating barrier.

Named after the British physicist Brian David Josephson, who predicted the existence of the effect in 1962; → effect.

A type of electronic circuit involving → Josephson effect, capable of switching at very high speeds when operated at temperatures approaching → absolute zero.

→ Josephson effect; → junction.

The proportional relationship of mechanical energy to thermal energy, equal to 4.184 joules per calorie. Also called mechanical equivalent of heat.

→ joule; → constant.

The change in the temperature of a gas in the → throttling process.

→ Joule; → Thomson; → effect.