An Etymological Dictionary of Astronomy and Astrophysics
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A combining form (hydr- before a vowel) originally meaning "water," but also "liquid, gas." In chemical nomenclature, often denotes a compound of hydrogen.

Gk. hydro-, combining form of hydor "water," cognate with Skt. udá- "water;" Khotanese ūtcā "water;" Hittite uātar; L. unda "wave;" O.C.S., Rus. voda; Lith. vanduo; P.Gmc. *watar (cf. Du. water; O.H.G. wazzar; Ger. Wasser; Goth. wato; O.E. wæter; E. water); from PIE base *wed- "water; wet."

Hidro-, loanword from Gk., as above. Âb- "water," from Mid.Pers. âb "water;" O. Pers. ap- "water;" Av. ap- "water;" cf. Skt. áp- "water;" Hitt. happa- "water;" PIE āp-, ab- "water, river;" cf. Gk. Apidanos, proper noun, a river in Thessalia; L. amnis "stream, river" (from *abnis); O.Ir. ab "river," O.Prus. ape "stream," Lith. upé "stream;" Latv. upe "brook."

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.

Same as → hydrogen cyanide.

→ hydro-; → cyano-; → acid.

Of or pertaining to → hydrodynamics.

→ hydro- + → dynamic.

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 state of a star when all its internal forces are in equilibrium. The main forces are gas pressure, radiation pressure due to thermonuclear fusion that tends to disrupt the star, and the opposing gravity. → hydrostatic equilibrium.

→ hydrodynamic; → equilibrium.

The branch of physics dealing with the motion, energy, and pressure of neutral → fluids.

→ hydro- + → dynamics.

The most abundant → chemical element in the Universe. Symbol H; → atomic number 1; → atomic weight 1.00794; → melting point -259.14°C; → boiling point -252.87°C. It was discovered by the English physicist Henry Cavendish in 1766, who called it the "inflammable air."
See also:
→ antihydrogen, → atomic hydrogen, → heavy hydrogen, → hydrogen bond, → hydrogen burning, → hydrogen coma, → hydrogen cyanide, → hydrogen fusion, → hydrogen ion, → hydrogen line, → hydrogen shell burning, → hydrogenate, → hydrogenation, → hydrogenize, → ionized hydrogen region, → metallic hydrogen, → molecular hydrogen, → neutral hydrogen, → orthohydrogen, → parahydrogen, → triatomic hydrogen molecular ion.

Hydrogen, from Fr. hydrogène, from Gk. hydro-, combining form of hydor "water" → hydro- + Fr. -gène "producing," → -gen; coined in 1787 by the French chemist Guyton de Morveau (1737-1816) because it forms water when exposed to oxygen.

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.

→ proton-proton chain.

→ hydrogen; → burning.

The cometary cloud of hydrogen, detectable in ultraviolet light, that is immensely bigger than even the huge visible coma it surrounds. It is produced by the dissociation of water into hydrogen and oxygen and by other processes set into motion by solar radiation and and the solar wind.

→ hydrogen, → coma.

A colorless or light blue liquid or gas, a triatomic cyanide, which is extremely flammable. HCN is an important industrial chemical and over a million tonnes are produced yearly in the world. It is produced industrially by reacting methane and ammonia in air at high temperature. A wide range of combustion processes produce HCN gas in the smoke or fumes. HCN is found naturally throughout the environment at low levels as it is released from volcanoes and certain plants and bacteria. Hydrogen cyanide is abundant in all kinds of astronomical environments, from dark clouds to star-forming regions and circumstellar envelopes. The first detection of interstellar HCN (at 88.6 GHz) and H¹³N (at 86.3 GHz) was reported by Buhl & Snyder (1971, ApJ 163, L47). Also called → hydrocyanic acid and → prussic acid.

→ hydrogen; → cyanide.

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.

An → emission or → absorption line in the spectra of various astronomical objects produced by the presence of hydrogen atoms in particular physical conditions.

→ hydrogen, → line.

A phase in the life of a star that has left the → main sequence. When no more hydrogen is available in the core, the core will start to contract as it is no longer releasing the necessary energy whose pressure supports the surrounding layers. As a result of this contraction, gravitational energy is converted into thermal energy and the temperature will rise. Therefore a shell of unprocessed material surrounding the original core will be heated sufficiently for hydrogen burning to start. During the evolution of → asymptotic giant branch stars hydrogen shell burning occurs alternatively with helium shell burning. → double shell burning.

→ hydrogen; → shell; → burning.

To undergo or cause to undergo a reaction with hydrogen. Same as → hydrogenize.

→ hydrogen; → -ate.

The process of combining or exposing to → hydrogen.

→ hydrogenate.

→ hydrogen; → -ize.

The study, measurement, and description of depths and currents in open seas, lakes, estuaries, and rivers.

→ hydro- + → -graphy.