An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

An atom made from an → antiproton and a → positron. In 2010 a research collaboration at CERN combined decelerated antiprotons with positrons to produce antihydrogen atoms. They managed 38 times to confine single antihydrogen atoms in a magnetic trap for more than 170 milliseconds (Andersen et al. 2010, Nature, 17 Nov.).

→ anti- + → hydrogen.

Same as → neutral hydrogen or → H I.

→ atomic; → hydrogen.

→ deuterium.

→ heavy; → hydrogen.

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.

Same as → H II region.

→ ionized; → hydrogen; → region.

A kind of → degenerate matter resulting from hydrogen gas when it is sufficiently compressed to undergo a phase change to liquid or solid state. Metallic hydrogen is thought to be present in compressed astronomical objects, such as the interiors of the solar system planets Jupiter and Saturn. Above the core of these planets (at a temperature of 10,000 degrees and a pressure of 3 million bars) the electrons are squeezed out of the hydrogen atoms and the fluid starts to conduct like a metal.

→ metallic; → hydrogen.

A molecule consisting of two hydrogen atoms (H₂) which is the most abundant molecule in the Universe. Molecular hydrogen plays a fundamental role in many astrophysical contexts. It is found in all regions where → self-shielding against the ultraviolet photons, responsible for its → photodissociation, is sufficiently large. Containing two identical hydrogen atoms, H₂ is highly symmetric. Due to this property, the molecule has no → dipole moment and all → rotation-vibrational transitions within the electronic → ground state are → quadrupolar with low → spontaneous emission  → Einstein coefficient values. The molecule exists in two almost independent states, namely → orthohydrogen and → parahydrogen. H₂ may be excited through several mechanisms, including: 1) → far ultraviolet (FUV) induced → optical pumping and → collisional excitation in → photodissociation regions (PDRs) associated with → star formation; 2) → hard X-rays penetrating and heating regions within → molecular clouds, which in turn excite H₂ via collisions with electrons or hydrogen atoms; and 3) collisional excitation of H₂ due to acceleration produced by interstellar → shock waves. H₂ is thought to be chiefly produced via surface reactions on → interstellar grains, but the exact formation mechanism is not fully understood.

→ molecular; → hydrogen.

Non-ionized → atomic hydrogen gas which constitutes an important component of the → interstellar medium, accounting for perhaps half its mass, even though its density is very low. Its radio emission → 21-centimeter line has made it possible to map the distribution of neutral hydrogen in the → spiral arms of our own Galaxy and other nearby galaxies.

→ neutral; → hydrogen.

Molecular hydrogen in which the nuclei (protons) of the two hydrogen atoms contained in the molecule have spins in the same direction. → parahydrogen

→ ortho-; → hydrogen.