An Etymological Dictionary of Astronomy and Astrophysics
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The spatial distribution of ionic species around an ionization source according to their → ionization potentials. The higher the ionization potential, the nearer to the source the corresponding ions will be.

→ ionization; → stratification.

An H II region whose → exciting star(s) do not have enough → Lyman continuum photons to ionize the whole region. → density-bounded H II region.

→ ionization; → bounded; → H II region.

To change into ions. Verbal form of → ionization.

From → ion + → -ize.

Converted into ions.

P.p. of → ionize.

A gas composed partially or totally of → ions.

→ ionized; → gas.

Same as → H II region.

→ ionized; → hydrogen; → region.

A cloud of matter in the → interstellar medium consisting of → ionized gas, mainly → hydrogen, and → dust. Same as → H II region.

→ ionized; → nebula.

A photon that has enough energy to remove an electron from an atom or molecule, thus producing an ion and free electrons.

Ionizing, adj. from → ionize; → radiation.

The region of the Earth's upper atmosphere containing a small percentage of free electrons and ions produced by photoionization of the constituents of the atmosphere by solar ultraviolet radiation.

→ ion + → sphere.

A → multiple star system in the → Orion constellation. Also known as → Hatsya, → Na'ir al-Saif, and HR 1899. It is the brightest star of → Orion's Sword, located at the sword's tip, with an → apparent visual magnitude of 2.8. From parallax measurements, it is located at a distance of roughly 1,330 → light-years (410 parsecs) from the Sun. The system has three components designated Iota Orionis A, B and C. Iota Orionis A is itself a massive spectroscopic binary, with components Iota Orionis Aa and Ab.

Iota, Greek letter ι used in the → Bayer designation of star names; Orionis, genitive of → Orion.

A → convective zone close to the surface of → hot stars caused by a peak in the → opacity due to iron recombination. A physical connection may exist between → microturbulence in hot star atmospheres and a subsurface FeCZ. The strength of the FeCZ is predicted to increase with → metallicity and → luminosity, but decrease with → effective temperature. The FeCZ in hot stars might also produce localized surface magnetic fields. The consequence of the FeCZ might be strongest in → Wolf-Rayet stars. These stars are so hot that the → iron opacity peak, and therefore FeCZ, can be directly at the stellar surface or, better said, at the → sonic point of the wind flow. This may relate to the very strong → clumping found observationally in Wolf-Rayet winds, and may be required for an understanding of the very high → mass loss rates of Wolf-Rayet stars (See Cantiello et al. 2009, A&A 499, 279).

→ iron; → convection; → zone.

1) Exposure to any kind of radiation or atomic particles.
2) An optical effect that makes a bright object appear larger than it really is when viewed against a darker background.

Irradiation, from ir- variant of → in- (by assimilation) before r + → radiation.

1) Tâbešdehi, tâbešgiri;, from tâbeš→ radiation + giri verbal noun of gereftan "to take, seize" (Mid.Pers. griftan, Av./O.Pers. grab- "to take, seize," cf. Skt. grah-, grabh- "to seize, take," graha "seizing, holding, perceiving," M.L.G. grabben "to grab," from P.Gmc. *grab, E. grab "to take or grasp suddenly;" PIE base *ghrebh- "to seize"); dahi verbal noun of dâdan "to give," Mid.Pers. dâdan "to give" (O.Pers./Av. dā- "to give, grant, yield," dadāiti "he gives;" Skt. dadáti "he gives;" Gk. tithenai "to place, put, set," didomi "I give;" L. dare "to give, offer," facere "to do, to make;" Rus. delat' "to do;" O.H.G. tuon, Ger. tun, O.E. don "to do;" PIE base *dhe- "to put, to do").
2) Nurgostard, from nur, → light, + gostard past stem of gostardan "to expand; to spread; to diffuse" (Mid.Pers. wistardan "to extend; to spread;" Proto-Iranian *ui.star-; Av. vi- "apart, away from, out" (O.Pers. viy- "apart, away;" cf. Skt. vi- "apart, asunder, away, out;" L. vitare "to avoid, turn aside") + Av. star- "to spread," starati "spreads;" cf. Skt. star- "to spread out, extend, strew," strnati "spreads;" Gk. stornumi "I spread out," strotos "spread, laid out;" L. sternere "to spread;" Ger. Strahlung "radiation," from strahlen "to radiate," from Strahl "ray;" from M.H.G. strāle; from O.H.G. strāla "arrow," stripe; PIE base *ster- "to spread").

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.

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.

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

Verbal noun of → isolate.

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.

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

→ jet; → propulsion.

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.