An Etymological Dictionary of Astronomy and Astrophysics
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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 general purpose software package for the reduction and analysis of astronomical data. It is aimed specifically at the reduction of imaging and spectroscopy data obtained using → CCD detector systems. IRAF is developed by the National Optical Astronomy Observatories (NOAO).

Short for Image Reduction and Analysis Facility.

The most accurate solar calendar in use, which is based on two successive passages of the Sun through the true vernal equinox. The year length, defined by an ingenious intercalation system devised by the mathematician Omar Khayyâm (A.D. 1048-1131), is 365.2424.. solar days, in perfect agreement with the → vernal-equinox year of 365.24236 solar days (epoch +2000). This interval should not be confounded with the → tropical year of 365.2422 solar days. The most remarkable feature of the calendar is Nowruz, the spring festival, which has its profound roots in the Zoroastrian worldview. Same as → Persian calendar. Click here for more details.

Iranian, of or pertaining to Iran "(land of) the Aryans," as below; → calendar.

Gâhšomâr, → calendar; Irâni adj. of Irân, from Mid.Pers. Êrân "(land of) the Aryans," pluriel of êr "noble, hero," êrîh "nobility, good conduct;" Parthian Mid.Pers. aryân; O.Pers. ariya- "Aryan;" Av. airya- "Aryan;" cf. Skt. ārya- "noble, honorable, respectable."

The condition or state of being → iridescent; exhibition of colors like those of the → rainbow.

From L. iris (genitive iridis) "rainbow," + → -escence.

Producing a display of lustrous, rainbow-like colors.

→ iridescence

A metallic chemical element; symbol Ir. Atomic number 77; atomic weight 192.22; melting point about 2,410°C; boiling point about 4,130°C; specific gravity 22.55 at 20°C. Iridium is a very hard, usually brittle, extremely corrosion-resistant silver-white metal with a face-centered cubic crystalline structure. The unusually high concentration of iridium found in the thin clay layer that marks the boundary between the Cretaceous and Tertiary rocks is attributed to an asteroid impact with Earth 65 million years ago.

Iridium coined 1804 by its discoverer, E. chemist Smithson Tennant (1761-1815) from Gk. → iris "rainbow;" so called for the varying color of its compounds.

1a) The circular diaphragm forming the colored portion of the eye and perforated by the pupil in its center. → pupil.
1b) A diaphragm forming an adjustable opening over a lens in an optical instrument.
2) Asteroid 7, discovered in 1847 by E. astronomer John Russell Hind (1823-1895).
3) Botany: A plant having showy flowers, typically of purple, yellow, or white, and long thin leaves.

Iris, M.E., from L. irid-, iris "colored part of the eye, rainbow, iris plant, a precious stone," from Gk. iris, iridos "rainbow, iris plant, iris of the eye," initially "a messenger of the gods, regarded as the goddess of the rainbow." The eye portion was so called for being the colored part.

Titak, from Kermâni, Tâleši, variants Lori tiya, Dehxodâ dictionary tuk, probably from didan "to see," Mid.Pers. ditan "to see, regard, catch sight of, contemplate, experience;" O.Pers. dī- "to see;" Av. dā(y)- "to see," didāti "sees;" cf. Skt. dhī- "to perceive, think, ponder; thought, reflection, meditation," dādhye; Gk. dedorka "have seen."
Zanbaq, from Pers. zanba "white rose."

A mechanical device, consisting of thin overlapping plates, designed to smoothly vary the effective diameter of a lens, thereby controlling the amount of light allowed through.

→ iris; → diaphragm.

Same as → NGC 7023.

→ iris; → nebula.

A metallic → chemical element occurring abundantly in combined forms and used alloyed in a wide range of important tools and structural materials; symbol Fe. → Atomic number 26; → atomic weight 55.845; → melting point about 1,535°C; → boiling point about 2,750°C; → specific gravity 7.87 at 20°C; → valence +2, +3, +4, or +6. Iron is of critical importance to life, i.e. plants, humans, and animals. It occurs in hemoglobin, a molecule that carries → oxygen from the lungs to the tissues and then transports → carbon dioxide (CO₂) back from the tissues to the lungs.
Iron has the highest nuclear → binding energy of all elements, and is therefore the most stable element. It is synthesized in → massive stars, and its occurrence ends the process of → thermonuclear reaction in stars. The resulting energy crisis leads to the destruction of the star through a → supernova explosion. It has several → radioactive isotopes with half-lives from 6 min (⁶¹Fe) to about 3 x 10⁵ years (⁶⁰Fe).

Iron, from O.E. isærn, from P.Gmc. *isarnan (cf. O.S. isarn, O.N. isarn, M.Du. iser, O.H.G. isarn, Ger. Eisen) "holy metal" or "strong metal," probably an early borrowing of Celt. *isarnon (cf. O.Ir. iarn, Welsh haiarn), from PIE *is-(e)ro- "powerful, holy," from PIE *eis "strong" (cf. Skt. isirah "vigorous, strong," Gk. ieros "strong").
The chemical symbol Fe, from L. ferrum "iron."

Âhan, Kurd. âsan, Mid.Pers. âhan; Av. aiianhaēna- "made of metal," from aiiah- "metal;" cf. Skt. áyas- "iron, metal;"  L. aes "brass;" Goth. aiz "bronze;" O.H.G. ēr "ore" (Ger. Erz "oar"); O.E. ora "ore, unworked metal," ar "brass, copper, bronze."

The period generally occurring after the → Bronze Age, marked by the widespread use of iron. Its date and context vary depending on the country or geographical region. The Indo-European Hittites are the first people to work iron, in the Asia Minor, from about 1500 BC.

→ iron; → age.

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) Electromagnetism: A bar of → soft iron that passes through a coil and serves to increase the → inductance of the coil.
2) The innermost part of some planets, such as Mercury, Venus, and Earth, which have a molten iron-rich core.
3) The end point in the evolution of stars with a mass above ~ 10 → solar masses. Such a star evolves in several stages over millions of years during which various → thermonuclear reactions take place in the star core. Each stage results in a core composed of heavier elements. The process ends when → silicon burning produces a core of iron-nickel. Since iron has the maximum → binding energy per → nucleon, the → nuclear fusion cannot proceed further. The iron core shrinks and heats up. It is maintained against → gravitational collapse by → electron degeneracy pressure, but it continues to grow as Si burning adds more iron. When the core reaches its → Chandrasekhar limit, it becomes unstable and undergoes the → core collapse.

→ iron; → core.

A meteorite which is composed mainly of iron mixed with smaller amounts of → nickel. Iron meteorites make up about 4.4% of all meteorites. See also → stony meteorite, → stony-iron meteorite.

→ iron; → meteorite.

A bump appearing in the plot of stellar → opacity versus temperature. The ionization of the heaviest → chemical elements, especially → iron, which is the most abundant heavy metal, produces a large number of weak spectral → absorption lines. These lines dominate the stellar opacity in the temperature range 10⁵-10⁶ K and furnish two local opacity peaks: a large peak around 2 × 10⁵ K and a smaller one around 1.5 × 10⁶ K (Rogers & Iglesias, 1992, ApJS 79, 507; Iglesias et al. 1992, ApJ, 397, 717).

→ iron; → opacity; → peak.