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

1) A wave that forms and propagates at the free → surface of a body of → fluid after that surface has been disturbed and the fluid particles have been displaced from their original positions. The motion of such waves is controlled by the restoring force of gravity rather than by the surface tension of the fluid.
2) Not to be confounded with → gravitational wave.

→ gravity; → wave.

The velocity at which the envelope of a → wave packet propagates, v_gr = dω/dk, at k₀ (the central value of k). The group velocity can be equal to, larger, or smaller than the → phase velocity.

→ group; → velocity.

The total number of → Lyman continuum photons emitted by an → H II region. It is usually derived using → radio continuum observations which are less affected by → interstellar extinction. The measured value is often a lower limit because of photon leakage from the H II region and absorption. See also → density-bounded H II region.

→ H II; → region; → luminosity.

The ratio of an amount of heat, dQ, transferred to a body in some process to the corresponding change in the temperature of the body: C = dQ/dT. The heat capacity depends upon the mass of the body, its chemical composition, thermodynamic state, and the kind of process employed to transfer the heat. The word "capacity" may be misleading because it suggests the essentially meaningless statement "the amount of heat a body can hold," whereas what is meant is the heat added per unit temperature rise. → specific heat.

→ heat; → capacity.

1) In particle physics, the projection of the spin of an elementary particle on the direction of momentum.
2) In fluid mechanics, → kinetic helicity.
3) In magnetohydrodynamics, → magnetic helicity.

From → helix + → -ity.

The passing on of physical or mental characteristics genetically from one generation to another (OxfordDictionaries.com).

M.E., from M.Fr. hérédité, from O.Fr. eredite "inheritance, legacy," from L. hereditatem (nominative hereditas) "heirship, inheritance," → heritage.

Rigandâšt, literally "possessing heritage," from rigan, → heritage, + dâšt past stem of dâštan "to have, hold, possess, maintain," → property.

The quality or state of being → heterogeneous. See also → homogeneity, → inhomogeneity.

Noun from → heterogeneous.

A high-precision echelle spectrograph built for exoplanet findings and installed on the ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. HARPS has discovered dozens of exoplanets, making it the most successful planet finder behind the Kepler space observatory. HARPS can detect movements as small as 0.97 m s^-1 (3.5 km h^-1), with an effective precision of the order of 30 cm s^-1, and a → resolving power of 120,000 (Mayor et al., 2003, ESO Messengar 114, 20).

→ high; → accuracy; → radial; → velocity; → planet; → search; → -er.

A population of neutral or partly ionized gas clouds in the → Galactic halo which are seen as high-altitude structures in the → atomic hydrogen  → 21 cm emission at high radial velocities (v_LSR > 100 km/sec). They have substantial neutral → column densities (> 10¹⁹ cm^-2) and their → metallicities range from 0.1 to about 1.0 times solar. The distances to the majority of them remain unknown. They may represent the continuing infall of matter onto the → Local Group. See also → compact high-velocity clouds.

→ high; → velocity; → cloud.

The condition for the stability of a → three-body system. Three-body systems exist widely in the → solar system and → extrasolar systems, including Sun-planet-moon systems, planets-star systems, and → triple star systems. This concept of stability was introduced by Hill (1878). He used the → Jacobi integral to construct bounds of motion for → conservative systems with time-independent → potentials, which was introduced to study the stability of the Moon in the Sun-Earth → restricted three-body problem. The stability is defined by the → zero-velocity surface based on the Jacobi integral. The concept of the Hill stability has been used by many researchers to study the stability of three-body systems. The studies include the Hill stability in the full → three-body problems, the hierarchical three body problems, and the restricted three body problems (See, e.g., S. Gong & J. Li, 2015, Astrophys Space Sci. 358,37).

Hill, G.W.: Researches in the lunar theory. Am. J. Math. 1(2), 129-147 (1878); → stability.

State or quality of having a uniform appearance or composition, being homogeneous

→ homogeneous + → -ity.

1) All human beings collectively; the human race; humankind.
2) The quality of being human; human nature.

→ human + → -ity.

Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with → relative humidity. → absolute humidity, → dew point, → mixing ratio, → specific humidity.

Humidity, from O.Fr. humide, from L. humidus "moist, wet," variant (by influence of humus "earth") of umidus, from umere "be moist."

Nam "humidity, moisture," 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."

A star whose velocity is so great that it will escape the → gravitational potential of our → Galaxy. Depending on the location and direction of motion, this criterion typically corresponds to a stellar velocity in the Galactic → rest frame larger than 400 km s^-1, and up to about 1200 km s^-1. The nature of the HVSs spans a wide range of types from → OB stars, to metal-poor → F-type stars and G/K dwarfs. While there is evidence from many late-type B HVSs in the → halo to originate from the Galactic → supermassive black hole (SMBH), other HVSs seem to originate from the → galactic disk. HVSs can obtain their large velocities from a number of different processes:
1) → Tidal disruption of → close binary stars by the central SMBH of the Milky Way. In this process one star is captured by the SMBH while the other is ejected at high speed via the → gravitational slingshot mechanism.
2) Exchange encounters in other dense stellar environments between hard binaries (→ hard binary) and → massive stars may cause stars to be ejected and escape our Galaxy.
3) Disruption of close binaries via → supernova explosions. The → runaway velocities of both ejected stars can reach large values when asymmetric supernovae are considered, i.e. when the newborn → neutron star receives a momentum kick at birth.
(see, e.g., T. M. Tauris, 2014, and references therein, arXiv:1412.0657).

→ hyper-; → velocity; → star.

1) Math.: An equation that is valid for all values of its variables. A mathematical relationship equating one quantity to another.
2) Logic: An assertion that two terms refer to the same thing.
3) Psychology: The character of persisting unchanged. The feeling that one knows who one really is.

From M.Fr. identité, from L.L. identitas "sameness," from ident-, combining form of L. idem "the same; at the same time; also; moreover," from id "it, that one" + demonstrative suffix -dem + -itas a suffix used to form abstract nouns expressing state or condition.

Idâni, from iduni, from Mid.Pers. êdônih "being this, being that, being so, the manner of being," from êdôn "thus, so," Mod.Pers. idun "this, in this manner, now;" O.Pers. aita- demonstrative pronoun "this;" Av. aēta- "this; this here; this now," aētaδ- (adv.) "here, there; then, thus; thereupon;" cf. Skt. etad "this," iti "thus, in this manner;" akin to L. idem, as above.

A basic rule in → group theory stating that there exists a unit group element e, called the identity, such that for any element a of the group a * e = e * a = a.

→ identity; → axiom.

In a mathematical system, an element which leaves unchanged any other element on which it operates. Thus 0 is the identity element for addition: a + 0 = a. And 1 is the identity element for multiplication: a . 1 = a.

→ identity; → element.

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

→ identity; → function.

In linear algebra, the simplest nontrivial diagonal matrix, an n-by-n square matrix with ones on the main diagonal and zeros elsewhere.

→ identity; → matrix.

An operator which takes a real number to the same real number.

→ identity; → operator.