An Etymological Dictionary of Astronomy and Astrophysics
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Any of a class of compounds containing only → hydrogen and → carbon. Hydrocarbons are organic compounds found in coal, petroleum, natural gas, and plant life. They are used as fuels, solvents, and as raw materials for numerous products such as dyes, pesticides, and plastics. Petroleum is a mixture of several hydrocarbons.

→ hydro- + → carbon.

Fluid mechanics: A → partial differential equation which describes the motion of an element of fluid subjected to different forces such as pressure, gravity, and frictions.

→ hydrodynamic; → equation.

The attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine, which has a partial negative charge. The hydrogen then has the partial positive charge.

→ hydrogen; → bond.

A → nuclear reaction where hydrogen (H) nuclei combine to form helium (⁴He) nuclei. Same as the → proton-proton chain.

→ hydrogen; → fusion.

Chemistry: The → positively charged hydrogen atom, H⁺, formed by removal of the orbital electron. Same as → proton.

→ hydrogen; → ion.

The process of combining or exposing to → hydrogen.

The general name for the atomic hydrogen → cation H⁺.

→ hydro-; → -on.

A → water, → molecule with an additional hydrogen ion (H₃O⁺). Also called hydronium ion. Hydronium is an abundant molecular ion in the interstellar diffuse and dense molecular clouds (→ Sagittarius B2, → Orion molecular cloud OMC-1) as well as the plasma tails of → comets (→ Halley, → Hale-Bopp).

From hydr-, → hydro- + -onium a suffix used in the names of complex cations, extrcated from ammonium "ionized ammonia" (NH₄⁺).

The equation describing the → hydrostatic equilibrium in a star, expressed as: dP/dr = -GMρ/r², where P and M are the mass and pressure of a spherical shell with thickness dr at some distance r around the center of the star, ρ is the density of the gas, and G the → gravitational constant.

→ hydrostatic; → equation.

Any of the six functions sinh, cosh, tanh, coth, csch, and sech that are related to the → hyperbola in the same way the → trigonometric functions relate to the → circle. Many of the formulae satisfied by the hyperbolic functions are similar to corresponding formulae for the trigonometric functions, except for + and - signs. For example: cosh²x - sinh²x = 1. See also: → hyperbolic cosine, → hyperbolic sine. Hyperbolic functions were first introduced by the Swiss mathematician Johann Heinrich Lambert (1728-1777).

→ hyperbolic; → function.

An → atomic transition involving a → hyperfine structure.

→ hyperfine; → transition.

The sixteenth of → Saturn's known → natural satellites. It is shaped like a potato with dimensions of 410 x 260 x 220 km and has a bizarre porous, sponge-like appearance. Many of the sponge holes or craters have bright walls, which suggests an abundance of → water  → ice. The crater floors are mostly the areas of the lowest → albedo and greatest red coloration. This may be because the average temperature of roughly -180 °C might be close enough to a temperature that would cause → volatiles to → sublimate, leaving the darker materials accumulated on the crater floors. Hyperion is one of the largest bodies in the → Solar System known to be so irregular. Its density is so low that it might house a vast system of caverns inside. Hyperion rotates chaotically and revolves around Saturn at a mean distance of 1,481,100 km. It was discovered by two astronomers independently in 1848, the American William C. Bond (1789-1859) and the British William Lassell (1799-1880).

Hyperion, in Gk. mythology was the Titan god of light, one of the sons of Ouranos (Heaven) and Gaia (Earth), and the father of the lights of heaven, Eos the Dawn, Helios the Sun, and Selene the Moon.

An unstable elementary particles, belonging to the class called → baryons, which have greater mass than the neutron but very short lives (10^-8 to 10^-10 seconds).

From → hyper- + → -on a suffix used in the names of elementary particles (gluon; meson; neutron; graviton, and so on).

In aerodynamics, adjective used to describe a → sound speed in excess of Mach 5. See also → supersonic.

Hypersonic, from → hyper- + → sonic.

Meteo.: The process by which a layer of ice builds up on solid objects that are exposed to freezing precipitation or to supercooled fog or cloud droplets.

→ ice; → accretion.

1) An image; a representation.
2) A representation or picture of a sacred or sanctified Christian personage, traditionally used and venerated in the Eastern Church.
3) A person or thing regarded as a symbol of a belief, nation, community, or cultural movement.
4) Computers: A pictorial representation of a facility available on a computer system, that enables the facility to be activated by means of a screen cursor rather than by a textual instruction (TheFreeDictionary.com).

From Gk. eikon "likeness, image, portrait; a semblance;" in philosophy, "an image in the mind," related to eikenai "be like, look like," from PIE *weik- "to be like."

Zodiš, variant of Mid.Pers. uzdês "icon; image; idol," from uz-, → ex-, + dês, Av. daēs- "to show;" cognate with Gk. deiknumai "to show;" L. dicere "to utter, say, proclaim;" N.H.G. zeigen "to say;" O.E. têon "to annoince;" PIE *deik- "to show, point out; announce."

1) An act or instance of identifying; the state of being identified.
2) Psychology: The process of associating the self closely with other persons and assuming their characteristics or views unconsciously or partially so.
3) In psychological theories, a phase of recognition, the way in which the child's personality is formed by modeling it on a chosen parent.

Verbal noun of → identify.

Idâneš, verbal noun of idânidan→ identify.

Recognizing the lines in the spectrum of a star, nebula, galaxy, etc. using a calibration template.

→ identification; → line.

Math.: Any function f for which f(x) = x for all x in the domain of definition.

→ identity; → function.

Logic, Math.: An → expression indicating that two → statements so connected are → necessary and sufficient conditions for one another. The corresponding logical symbols usually used are: ↔, ⇔, ≡, and iff.

→ if; → and; → only.