An Etymological Dictionary of Astronomy and Astrophysics

1. The fifth of → Jupiter’s known moons and the third largest. It is the innermost of the → Galilean satellites. With a diameter of 3630 km, Io is slightly larger than Earth’s Moon. It revolves at a mean distance of 422,000 km from Jupiter. Its mass is 8.93 x 10²² kg (about 1.2 Earth Moons) and its → orbital period 1.8 Earth days.
   The mean → surface temperature of Io is -155 °C. Io’s yellow color derive from → sulfur and molten → silicate rock. The unusual surface of Io is kept very young by its system of active → volcanoes. The intense → tidal force of Jupiter stretches Io and damps wobbles caused by Jupiter’s other Galilean moons. The resulting friction greatly heats Io’s interior, causing molten rock to explode through the surface. Io’s volcanoes are so active that they are effectively turning the whole moon inside out. Some of Io’s volcanic lava is so hot it glows in the dark.
   

2. Also the name of an → asteroid numbered 85.

See also: In Gk. mythology, Io was a maiden who was seduced by Zeus (Jupiter). When Hera came upon their rendez-vous, Zeus transformed the maiden into a white heifer.

A nonmetallic chemical element; symbol I; atomic number 53; atomic weight 126.9045; melting point 113.5°C; boiling point 184.35°C.

Etymology (EN): Iodine, coined 1814 by British chemist Sir Humphry Davy from Fr. iode “iodine,” coined 1812 by Fr. chemist Joseph Louis Gay-Lussac (who proved it was an element) from Gk. ioeides “violet-colored,” because of its violet vapors. Despite the priority rights dispute between Davy and Gay-Lussac, both acknowledged Courtois as the discoverer of the element.

Etymology (PE): Yod, from Fr. iode, as above.

An atom that has lost or gained one or more electrons and has become electrically charged as the result.

Etymology (EN): Ion (introduced in 1834 by E. physicist and chemist Michael Faraday), from Gk ion " going," neut. pr.p. of ienai “to go,” from PIE base *ei- “to go, to walk,” eimi “I go;” cf. Pers. ây-, â- present stem of âmadan “to come;” O.Pers. aitiy “goes;” Av. ay- “to go, to come,” aēiti “goes;” Skt. e- “to come near,” eti “arrival;” L. ire “to go;” Goth. iddja “went,” Lith. eiti “to go;” Rus. idti “to go.”

Etymology (PE): Yon, from Fr., from Gk., as above.

The thin glowing streamers in a comet’s ion tail.

See also: → ion; → ray.

Of a comet, same as → gas tail.

See also: → gas; → tail.

Of or pertaining to ions; occurring in the form of ions.

Etymology (EN): From → ion + → -ic.

A quantity, pertaining to an ion of a chemical element, expressing the relative number of the ion with respect to that of hydrogen.

See also: → ionic; → abundance.

A molecule that consists of the ions of the chemical elements that make up the molecule.

See also: → ionic; → molecule.

The process by which ions are produced, typically occurring by interaction with electromagnetic radiation (“photoionization”), or by collisions with atoms or electrons (“collisional ionization”).

See also: Verbal noun of → ionize.

A quantity used in studies of → emission nebulae to convert the → ionic abundance of a given chemical element to its total → elemental abundance. The elemental abundance of an element relative to hydrogen is given by the sum of abundances of all its ions. In practice, not all the ionization stages are observed.
One must therefore correct for unobserved stages
using ICFs. A common way to do this was to rely on → ionization potential considerations. However, → photoionization models show that such simple relations do not necessarily hold. Hence, ICFs based on grids of photoionization models are more reliable. Nevertheless here also care should be taken for several reasons: the atomic physics is not well known yet, the ionization structure of a nebula depends on the spectral energy distribution of the stellar radiation field, which differs from one model to another, and the density structure of real nebulae is more complicated than that of idealized models (see, e.g., Stasińska, 2002, astro-ph/0207500, and references therein).

See also: → ionization; → correction; → factor.

Same as → ionization potential.

See also: → ionization; → energy.

An abrupt discontinuity between an H II region and the molecular cloud in which it has formed. In this transition region interstellar gas changes from a mostly neutral state to a mostly ionized state.

See also: → ionization; → front.

A ratio representing the number of ionizing photons to the number of electrons in a nebular emitting region.

See also: → ionization; → parameter.

The energy required to remove an electron from an isolated atom or molecule. The ionization potential for hydrogen is 13.6 eV, which corresponds to an ultraviolet ionizing photon with a wavelength of 912 A. Also called → ionization energy.

See also: → ionization; → potential.

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.

See also: → 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.

See also: → ionization; → bounded; → H II region.

To change into ions. Verbal form of → ionization.

See also: From → ion + → -ize.

Converted into ions.

See also: P.p. of → ionize.

A gas composed partially or totally of → ions.

See also: → ionized; → gas.

Same as → H II region.

See also: → ionized; → hydrogen; → region.

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

See also: → ionized; → nebula.

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

See also: 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.

See also: → 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.

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