An Etymological Dictionary of Astronomy and Astrophysics
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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⁺).

A term denoting the water portion of the Earth's surface.

→ hydro-; → sphere.

Of or pertaining to → hydrostatics.

→ hydro- + → static.

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.

1) The physical situation reached in a fluid when complete balance exists between the internal pressure at any point and the weight of the material above the point.
2) For a star, the balance between the inward → gravitational force and the outward gas pressure everywhere in the star. → hydrostatic equation. See also → hydrodynamic equilibrium.

→ hydrostatic; → equilibrium.

A model of the → Milky Way galaxy in which the → Galactic halo (composed of → gas, → magnetic fields, and → cosmic rays) is assumed to be in → hydrostatic equilibrium. Parker (1966) presented the first study of stability considerations between gas, magnetic fields and cosmic rays in an equilibrium configuration. He found that it is difficult to maintain a stable configuration due to magnetohydrodynamic self-attraction (→ Parker instability). Subsequent works taking into account turbulent motions showed that turbulent pressure can mitigate the influence of Parker instabilities. This enabled new attempts to find conditions under which a stable equilibrium configuration of the Galaxy could exist.

→ hydrostatic; → halo.

The term ρgz in the → Bernoulli equation. It is not pressure in a real sense, because its value depends on the reference level selected.

→ hydrostatic; → pressure.

A branch of physics that deals with the characteristics of → fluids at rest and especially with the pressure in a fluid or exerted by a fluid on an immersed body.

→ hydro- + → statics.

Geology: Relating to or caused by high temperature underground water or gas heated by natural processes.

→ hydro-; → thermal.

A diatomic ion containing one oxygen and one hydrogen atom with chemical formula OH^-.

→ hydr-; → oxide.

The univalent radical or group consisting of one hydrogen and one oxygen atom, forming a part of a molecule of a compound.

From → hydro- + ox(y)- a combining form meaning "sharp, acute, pointed, acid," used in the formation of compound words, from Gk, oxys "sharp, keen, acid" + -yl a suffix used in the names of chemical radicals, from Fr. -yle, from Gk. hyle "matter, substance;" → group.

The Male Water Snake. A minor constellation with three main stars, one of the 15 → circumpolar constellations in the southern hemisphere. It first appeared in Johann Bayer's Uranometria of 1603. It is often confused with → Hydra, the large constellation further north. It is also referred to as "male Hydra" or "little Hydra." Abbreviation: Hyi; genitive: Hydri.

From L., from Gk. hydros "water serpent."

Âbmâr "water snake," from âb "water" (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") + mâr "snake, serpent" (Mid.Pers. mâr "snake;" Av. mairya- "snake, serpent").

A combining form meaning "wet, moist, moisture," used in the formation of compound words: → hygrogram; → hygrograph; → hygrometer .

Hygro-, from Gk, combining form of hygros "wet, moist."

Nam "humidity, moisture" + -negâšt, → -gram. The first component nam, from Mid.Pers. nam, namb "moisture;" Av. napta- "moist," nabās-câ- "cloud," nabah- "sky;" cf. Skt. nábhas- "moisture, cloud, mist;" Gk. nephos "cloud, mass of clouds," nephele "cloud;" L. nebula "mist," nimbus "rainstorm, rain cloud;" O.H.G. nebul; Ger. Nebel "fog;" O.E. nifol "dark;" from PIE *nebh- "cloud, vapor, fog, moist, sky."

The graphical record made by a → hygrograph.

→ hygro- + → -gram.

An instrument that records the hygrometer's measure of water vapor.

→ hygro- + → -graph.

An instrument that measures the relative humidity of the air.

→ hygro- + → -meter.

A member of a class of equations of state used in some models concerned with the evolution of a hypothetical supermassive star. The supermassive star is assumed to consist of a → convective core, which obeys a → polytropic law, and an envelope, which contains most of the whole mass, and follows the hylotropic → equation of state. This equation is expressed by P = A ρ^4/3M^α, where A is a constant, ρ the density, M the mass, and α an index. First introduced by Mitchell C. Begelman (2010, MNRAS 402, 673).

From → hylotropic, on the model of → polytrope.

Physical chemistry: Describing a substance that is capable of undergoing a change in phase (as from a liquid to a gas), with no change in chemical composition. If the substance is hylotropic over a limited range of pressure and temperatures, it is a pure chemical substance. If it is hylotropic over all pressure and temperatures except the most extreme ones, it is a → chemical element. See also → hylotrope.

First suggested by Wilhelm Ostwald (1904, in Annalen der Naturphilosophie 3, 355), from Gk. hylo-, combining form of hyle "matter; wood," because of the dependence on the composition, + → -tropic.

Mâdegard, from mâdé, → matter, + gard, → -tropic.

Physical chemistry: The fact or condition of being → hylotropic.

→ hylotropic; → -tropy.

A prefix appearing in loanwords from Greek meaning:
1) "Over," like → super-, and sometimes implying:
2) "Excess higher than that denoted by super-;" e.g. → hypernova.

From Gk. hyper, preposition and adverb, "over, beyond, overmuch, above;" cognate with L. super- and Pers. abar-, as below.

1) Mid.Pers. abar; O.Pers. upariy "above; over, upon, according to;" Av. upairi "above, over," upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;" L. super-; O.H.G. ubir "over;" PIE base *uper "over."
2) hiper, loanword from Gk.