An Etymological Dictionary of Astronomy and Astrophysics
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The discrepancy between the value of the → Hubble-Lemaitre constant inferred from a ΛCDM fit (→ Lambda cold dark matter model) to the → cosmic microwave background (CMB) and local measurements. The Universe appears to be expanding much faster now than predicted even with our latest understanding of its initial conditions and contents. Based on the → Hubble Space Telescope observations, the Hubble-Lemaitre constant is very recently estimated to be 74.03 km s^-1 Mpc^-1. This value indicates that the Universe is expanding at a rate about 9% faster than that implied by the → Planck satellite's observations of the → early Universe, which give a value for the Hubble constant of 67.4 km s^-1 Mpc^-1. For discussion, see D'Arcy Kenworthy et al. (2019, ApJ 875, 145).

→ Hubble-Lamaitre constant; → tension.

A cold ocean current that flows northward along the western side of South America, offshore Chile and Peru. Dominate weather in this area includes coastal fog and low clouds. The presence or lack of this current is a vital part of the weather pattern known as El Niño.

Named after the German naturalist and explorer Alexander von Humboldt (1769-1859). → current.

A series of → spectral lines in the → infrared spectrum of → neutral hydrogen emitted by electrons in → excited states transitioning to the level described by the → principal quantum number  n = 6. It begins at 12368 nm (Hu α 12.37 microns) and has been traced to 3281.4 nm (3.28 microns).

Named after Curtis J. Humphreys (1898-1986), American physicist; → series.

An empirical upper → luminosity boundary in the → H-R diagram. It consists of two sections, a sloping part and a horizontal part. The sloping part, which decreases with decreasing → effective temperature, corresponds roughly to the → Eddington limit. The horizontal part is the temperature-independent upper luminosity limit for late-type → hypergiants. It is thought that → massive stars above the Humphreys-Davidson limit encounter an → instability, possibly due to the opacity-modified Eddington limit, and experience high → mass loss episodes which prevent their evolution to cooler temperatures. → Luminous Blue Variable stars are examples of this high mass loss phase.

Named after Roberta M. Humphreys and Kris Davidson, who first dealt with this limit (1979, ApJ 232, 409); → limit.

The smallest three digit number in the decimal system and the smallest square of a two-digit number (10).

Hundred, from O.E. hundred "a counting of 100," from P.Gmc. *hunda- "hundred," as below, + *rath "reckoning, number."

Sad "hundred," from Mid.Pers. sad, sat, Av. sata- "hundred," satô.raocana- "with a hundred windows," satô.təmô.sata- "hundreds of hundred;" cf. Skt. śatá- "hundred;" Gk. hekaton; L. centum; Lith. simtas; P.Gmc. *hunda- "hundred" (Goth. hund; O.H.G. hunt); PIE *kmtom "hundred."

The inner bright part of the → Orion Nebula, from which most of the radiation is emitted. It is about 5' across corresponding to 0.7 pc (for a distance of 440 pc). See O'Dell (2001, ARAA 39, 99).

Named after the Dutch astronomer Christiaan → Huygens (1629-1695), who sketched the appearance of the Orion Nebula. His drawing, the first such known sketch, was published in Systema Saturnium in 1659. First named such by O. Gingerich (1982, Ann. NY Acad. Sci. 395, 308); → region.

A development of → Huygens' principle stating that every point on a → wavefront acts, at a given instant, as a source of outgoing secondary spherical waves. The secondary wavelets mutually interfere and the resulting net light amplitude at any position in the outgoing wavefront is the vector sum of the amplitudes of all the individual wavelets. Using this principle, Fresnel calculated with a high accuracy the distribution of light in → diffraction patterns. The Huygens-Fresnel principle was put on a firm theoretical basis by Kirchhoff and expressed as an integral derived from the → wave equation.

→ Huygens; → Fresnel diffraction; → principle.

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

→ hydro-; → sphere.

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.

In spectroscopy, the → splitting of a spectral line into a number of very thin components. It results from a small perturbation in the energy levels of atoms or molecules due to the magnetic dipole-dipole interaction arising from the interaction of the nuclear → magnetic moment with the → spin of the electron. It can be observed only at high spectral dispersion. → fine structure.

→ hyperfine; → structure.

The phenomenon exhibited by a body (especially a ferromagnetic or imperfectly elastic material) in reacting to changes in the forces, especially magnetic forces, affecting it.
In ferromagnetic materials, the lag in the change in the magnetic induction B behind the change in the intensity of the external magnetizing field, due to the dependence of B on its previous values (past history).

Hysteresis, from Gk. hysteresis "being behind or late," from hystere-, stem of hysterein "to come late, lag behind" + -sis a suffix forming abstract nouns of action.

Pasmând "lagging behind," from pas "behind" (Mid.Pers. pas "behind, before, after;" O.Pers. pasā "after;" Av. pasca "behind (of space); then, afterward (of time);" cf. Skt. pazca "behind, after, later," L. post "behind, in the rear; after, afterward;" O.C.S. po "behind, after;" Lith. pas "at, by;" PIE *pos-, *posko-) + mând stem of mândan "to remain; to be fatigued," mân "house, family" (Mid.pers. mândan "to remain, stay;" O.Pers. mān- "to remain, dwell;" Av. man- "to remain, dwell; to wait;" cf. Gk. menein "to remain;" L. manere "to stay, remain, abide," mansio "a staying, a remaining, night quarters, station" (Fr. maison, ménage; E. manor, mansion, permanent); PIE *men- "to remain, wait for."

A closed curve showing the change in magnetic induction of a ferromagnetic body to which an external field is applied as the intensity of this field is varied from +H_s to -H_s and back again, where H_s is the magnetic field intensity corresponding to saturation.

→ hysteresis; → loop.

Dissipation of energy which occurs, due to magnetic hysteresis, when the magnetic material is subject to changes of magnetization.

→ hysteresis; → loss.

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.

The minimum temperature to which a fuel must be heated in order to initiate self sustained combustion independent of another heat source.

→ ignition; → temperature.

To set aside, e.g. → ignorable coordinate.

From Fr. ignorer, from L. ignorare "not to know, disregard," from ignarus "not knowing, unaware," from in- "not," → un-, + gnarus "aware, acquainted with," related to gnoscere "to → know".

Nâdidé engâštan literally "supposed not seen," from nâdidé "not seen" (from nâ- "not," → un- + didé "seen," p.p. of didan "to see," → sight) + engâštan "to suppose" (→ hypothesis).

The separation between two detached but adjacent points in an image.

→ image; → resolution.

The process by virtue of which the original image can be created by removing the blurring and the noise that occur during image formation.

→ image; → restoration.

The method used to detect very brief flashes of → Cherenkov radiation generated by the → cascade shower of → relativistic charged particles produced when a very high-energy → gamma ray (in the range 50 GeV to 50 TeV) strikes the atmosphere at a typical altitude of 10 km. Owing to this technique, it possible to discriminate cosmic gamma rays from the cosmic ray background and to determine their energy and source direction. More specifically, the incoming gamma-ray photon undergoes → pair production in the vicinity of the nucleus of an atmospheric molecule. The electron-positron pairs produced are of extremely high energy and immediately radiate in a → bremsstrahlung process. This radiation is itself extremely energetic, with many of the photons undergoing further pair production. A cascade of charged particles ensues which, due to its extreme energy, produces a flash of Cherenkov radiation lasting between 5 and 20 nano-seconds. The total area on the ground illuminated by this flash corresponds to many hundreds of square meters, which is why the effective area of IACT telescopes should be large.

→ imaging; → atmospheric; → Cherenkov; → technique.

Latin verb meaning "to command, rule, reign."

L. imperare "to command, give orders, exercise authority," from → in- "into, in" + parare "to make ready, supply, order," related to parire "produce, give birth to," from PIE root *pere- "to produce, procure;" cf. Skt. prthukah "child, calf, young of an animal;" Gk. poris "calf, bull;" Czech spratek "brat, premature calf;" Lith. periu, pereti "to brood;" O.H.G. farro, Ger. Farre, Du. varre "bull," O.E. fearr "bull;" see below for possible Iranian cognates.

Parmâtidan, from BMP plm'(d)y "to command, order," Sogd. framat- "to command," variants of farmudan, farmâyidan "to command, to order," ultimately from prefixed Proto-Ir. *fra-maH-, from *maH- "to measure," → experiment.