An Etymological Dictionary of Astronomy and Astrophysics

Two prominent → absorption lines, at 3968.5 Å and 3933.7 Å respectively, in the spectra of stars like the → Sun and cooler due to → singly ionized → calcium (Ca II). The strength of H and K lines can be an indication of considerable magnetic activity in the → chromosphere of these stars. The Ca II H and K lines are also common in some kinds of → eruptive variable stars. These lines are not seen in → hot stars, and start to become visible in → A-type stars.

See also: H and K, letters of alphabet, conventionally chosen; → line.

Atomic or → neutral hydrogen.

See also: From H, abbreviation of hydrogen + I “one” in Roman number system, nomenclature convention representing neutral atoms.

A region of neutral (atomic) hydrogen in interstellar space.
At least 95 percent of interstellar hydrogen is H I. It emits radio waves that are
21 cm long.

See also: → H I; → region

Ionized → hydrogen, that is a proton nucleus that has lost its unique electron.

See also: From H, abbreviation of hydrogen + II “two” in Roman number system, nomenclature convention representing singly ionized atoms.

A low-mass and → metal-poor galaxy (1/30-1/3 Zsun), experiencing strong episodes of → star formation, characterized by the presence of bright → emission lines on a faint → blue continuum. The fact that H II galaxies are metal poor and very blue objects seems to suggest that they are young. Nevertheless, several studies
show the existence of an → old stellar population underlying the present → star burst in most of these galaxies. This fact indicates that these objects are not young systems forming their first generation of stars. Same as → blue compact dwarf galaxy.

Spectroscopically, H II galaxies are essentially identical to the → giant H II regions found in nearby → irregular and → late-type galaxies. The correlation among structural parameters (→ H-beta → luminosity, → velocity dispersion, → linewidths) and between these parameters and the → chemical composition favors the interpretation of H II galaxies as giant H II regions in distant → dwarf irregular galaxies similar to the ones found nearby. Some examples of H II galaxies are: I Zw 18, SBS 0335-052, II Zw 33, UM 408.

See also: → H II; → galaxy.

A type of → emission nebulae composed of very hot gas (about 10⁴ K), mainly ionized hydrogen, created by the ultraviolet radiation of → massive stars. H II regions originate when O or early-type stars, born in → giant molecular clouds, start heating up the cold gas, causing it to become → ionized and “glow”. The effective temperatures of the → exciting stars are in the range 3 x 10⁴ to 5 x 10⁴ K, and throughout the nebula hydrogen is ionized. Helium is → singly ionized, and other elements are mostly singly or → doubly ionized. Typical densities in the H II region are of the order 10 to 10² cm^-3, ranging as high as 10⁴ cm^-3. Internal motions occur in the gas with velocities of order 10 km s^-1. The spectra of H II regions are mainly composed of strong → H I→ recombination lines and → forbidden lines such as [O III], [O II], [N II]. See also → ionization-bounded H II region;
→ density-bounded H II region; → compact H II region; → ultracompact H II region.

See also: → H II; → region

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.

See also: → H II; → region; → luminosity.

The → Balmer series spectral line of hydrogen which results from → atomic transition between the → energy levels 2 and 3. It has a wavelength of 656.4 nm and falls in the red region of the visible spectrum.

See also: H, symbol of → hydrogen; alpha (α), the first letter of Gk. alphabet.

The → Balmer series spectral line of hydrogen which results from → atomic transition between the → energy levels 2 and 4. It has a wavelength of 486.1 nm and falls in the → blue region of the → visible spectrum.

See also: H, symbol of → hydrogen; beta (β), the second letter of Gk. alphabet.

Same as → Hertzsprung-Russell diagram.

See also: Short for → Hertzsprung-Russell diagram.

A unit used to express the strength of average → far ultraviolet (FUV) intensity in the → interstellar radiation field. It is equal to 1.2 × 10^-4 erg cm^-2 s^-1 sr^-1

= 1.6 × 10^-3 cm^-2 s^-1 = 10⁸ photons cm^-2 s^-1.

See also: Named after Harm Habing, a pioneer in this field (Habing, H. J., 1968, Bull. Astr. Netherlands 19, 421).

In → exobiology, having a → temperature range within which → liquid water can exist on the surface of a → planet.

Etymology (EN): M.E., from O.Fr. habitation, from L. habitare “to live, dwell,” frequentative of habere “to have, to hold, possess,” from PIE base *ghrebh- “to seize, take, hold, have, give, receive” (cf. Mod.Pers. gereftan “to take, seize;” Mid.Pers. griftan; O.Pers./Av. grab- “to take, seize;”
Skt. grah-, grabh- “to seize, take,” graha “seizing, holding, perceiving;” M.L.G. grabben “to grab,” from P.Gmc. *grab, E. grab “to take or grasp suddenly”);
→ zone.

Etymology (PE): Zistpazir, from zist, → life,

• pazir “admitting, accepting, having,” → -able.

A zone around a → star where the → temperature would be in the range 0-100 °C to sustain → liquid water on the surface of rocky planets (or sufficiently large moons). Water is thought to be a necessary component to the → formation and evolution of Earth-type life. This zone
depends on the parent star’s luminosity and distance; it
will be farther from hotter stars. A more accurate definition of HZ needs to include other factors, such as orbital → eccentricity, heat sources other than stellar irradiation, and atmospheric properties. Same as → circumstellar habitable zone; → ecosphere.

See also: → habitable; → zone.

A type of intense dust storm that blows in the deserts of North Africa and Arabia, particularly severe in areas of drought.

See also: Haboob, from Ar. habub (هبوب) “a wind which blows hard and raises the dust.”

A blue-white → giant star of → spectral type B1 III with a visual magnitude of V = 0.61 lying in the constellation → Centaurus. It lies at a distance of 350 → light-years and is the eleventh brightest star of the night sky. Also called → Agena

See also: Hadar, from Ar. haZâr (حضار) “white camel.”

Any elementary particle which experiences the strong nuclear force. There are two sorts of hadrons: mesons, which have zero spin, and baryons, which have spin 1/2 or 3/2.

See also: Hadron, from Gk. hadr(os) “thick, bulky” + -on a suffix used in the names of subatomic particles (gluon, meson, neutron), quanta (photon, graviton), and other minimal entities or components (magneton).

The interval lasting until some 10^-5 seconds after the Big Bang when the Universe was dominated by radiation and its temperature was around 10¹⁵ kelvins. It is preceded by → Planck era and followed by → lepton era.

See also: → hadron; → era.

Of or related to → hadrons.

See also: → hadron; → -ic.

Ordinary matter composed of → hadrons.

See also: → hadronic; → matter.

An experiment performed in 1971 using four atomic → cesium clocks transported in jet airplanes eastward and westward around the Earth to verify the → time dilation predicted by the theory of → special relativity.

See also: J.C. Hafele and R. E. Keating, 1972, Science 177, 166; → experiment.

A transition metal found in zirconium ores. This silvery, ductile metal
is used in control rods for nuclear reactors and in tungsten filaments and electrodes. Symbol: Hf; Atomic mass: 178.49; Atomic number: 72; melting point 2230°C; boiling point 4602°C.

Etymology (EN): Hafnium, from N.L. Hafn(ia) “Copenhagen” + -ium. Hafnium was first observed by the French chemist Georges Urbain in 1911 in rare earth samples.
Subsequently, the Danish physicist Nils Bohr predicted hafnium’s properties using his theory of electronic configuration of the elements.

A dimensionless number characterizing the importance of → viscous force in a → forced flow.

Etymology (EN): named after the German hydraulic engineer Gotthilf H. L. Hagen (1797-1884); → number.

The interference fringes seen with thick plates near normal incidence.

See also: W. K. von Haidinger (1798-1871), Austrian mineralogist and geologist; → fringe.

A showery precipitation in the form of nearly spherical or irregular → pellets of ice having a diameter of up to 50 mm or more.
Hail is associated with → thunderstorm cells that have strong currents of rising air and relatively great → humidity content. Hail can only form in cumulonimbus clouds. Water droplets, after the formation,
freeze and begin to fall downward through the cloud, but the wind blows them back upward. As the droplets begin to fall back down again, they collect more water which also freezes, so the drop becomes bigger. Then the wind blows them back up again. This occurs several times, but eventually the frozen droplets become too big and heavy and fall as hail. See also → sleet.

Etymology (EN): From M.E. haghel, hayl; O.E. hægl, hagol; cf. O.H.G. hagal, Ger. Hagel “hail;” probably from PIE *haghlo- “pebble”;
cf. Gk. kakhlex “round pebble;” Pers. Lori hogela “(big) stone.”

Etymology (PE): Tagarg, from *takaraka, *tancaraka- “dense, condensed,” from Proto-Ir. base *tanc- “become narrow, dense, constrict,” cf. Pers. tanjidan, “to squeeze, → compress,” tang “narrow, constricted;” Shahmirzadi tāž/ti&#382d; Sariqoli tož/ti&#382d “to pull, drag;” Pashto tat “close, thick;” Skt. tanákti “it coagulates,” takrá- “buttermilk;” M.Irish techt “coagulated;” Lith. tánkus “thick;” PIE *tenk- “to twist together, become thick” (H. W. Bailey, 1979).

1. Any of the numerous fine filaments growing from the skin of humans or animals.
   

2. A mass or aggregate of hairs covering the human head. The mean diameter of human hair is about 100 μm (from 17 to 181 μm)
   

3. An outgrowth of the epidermis of a plant.
   → crosshairs; → no hair theorem; → Berenice’s Hair.

Etymology (EN): M.E. heer; O.E. hær; cf. O.H.G. har, Du. haar, Ger. Haar “hair;”
PIE base *kaisaro- “hair,” from *ker(s)- “to bristle;” cf. Skt. kesara- “hair, mane (of a horse or lion).”

Etymology (PE): Mu(y) “hair;” Mid.Pers. môy “hair.”
Gis, gisu, from Mid.Pers. ges, gesuk “hair, lock, tress,” Av. gaêsa- “curly hair,” gaêsav-, gaêθav- “curly, curly-haired,” cf. Gk. kaite “bushy hair, mane,” O.Ir. gaiset, PIE *ghaites “curly hair.”

1. In a cathode-ray tube, the glow surrounding a bright spot that appears on the fluorescent screen as the result of the screen’s light being reflected by the front and rear surfaces of the tube’s face.
   

2. The effect in which a halo appears around the image of a bright object recorded on a photographic film or plate. It is produced by the fan-like pattern of light reflected through the emulsion by the medium’s backing material.

Etymology (EN): Halation, from hal(o), → halo + -ation a combination of -ate and -ion, used to form nouns from stems in -ate.

Etymology (PE): Hâlegiri, from hâlé, → halo + giri, verbal noun of gereftan “to take, seize” (Mid.Pers. griftan, Av./O.Pers. grab- “to take, seize,” cf.
Skt. grah-, grabh- “to seize, take,” graha “seizing, holding, perceiving,” M.L.G. grabben “to grab,” from P.Gmc. *grab, E. grab “to take or grasp suddenly;” PIE base *ghrebh- “to seize”).

The leader and → follower spots have opposite polarities on either side of the equator. This reverses after the ~11 year
→ solar cycle. Also called Hale-Nicholson’s law.

See also: Named after George Ellery Hale (1868-1938), American astronomer;
→ law.

One of two equal or approximately equal parts of a divisible whole.

Etymology (EN): M.E., from O.E. h(e)alf “side, part,” from P.Gmc. *khalbas “something divided” (cf. M.Du. half, Ger. halb, Goth. halbs “half”).

Etymology (PE): Nim, nimé “half,” from Mid.Pers. nêm, nêmag “half;” Av. naēma- “half;” cf. Skt.. néma- “half.”

The moon when, at either quadrature, half its disk is illuminated.

See also: → half; → moon.

The length of time required for half of a given quantity of → radioactive material to → decay.

See also: → half; → life.

The angle between extreme points of the main lobe of an antenna pattern where the sensitivity of the antenna is half the value at the center of the lobe. This is the nominal resolving power of the antenna system.

See also: → half; → power;
→ beam; → width.

The thickness of material required to reduce the intensity of an → X-ray beam to one half of its initial value. The HVL is an indirect measure of the photon energies of a beam.

See also: → half; → value; → layer; → attenuation.

A plate of optical material whose thickness is such that the phase difference between the extraordinary and ordinary rays after passing through the place is exactly one-half cycle. It can serve to rotate the plane of polarization of a light beam.

See also: → half; → wave;
→ plate.

A retrograde irregular satellite of Neptune discovered in 2002. Also called Neptune IX. Halimede is about 62 kilometres in diameter.

See also: In Gk. mythology, one of the Nereids, the fifty daughters of Nereus and Doris.

The most famous comet orbiting the Sun once about every 75 years. The last time it appeared was in 1986, and it is predicted to return in 2061. Its earliest recorded sighting is traced back to 240 BC in China.
In 1705 Edmond Halley used Newton’s new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time. He found that the bright comets of 1531, 1607, and 1682 had almost the same orbits. He concluded that these appearances must belong to a single recurring comet, and predicted its return for 1758. Halley’s comet is the first known → periodic comet, hence its → designation 1P/Halley.

See also: Named after the English astronomer Edmond Halley (1656-1742),
who first computed its orbit and predicted its return in 1758; → comet.

1. Meteo.: Rings or arcs that seem to encircle the sun or moon and are the result of the refraction of light through the ice crystals that make up cirrus clouds.
   

2. → halo of galaxy.

Etymology (EN): Halo, from L. (acc.) halo, from Gk. halos “ring of light around the sun or moon.”

Etymology (PE): Hâlé, loanword from Ar.

The → probability distribution of the → number of galaxies
that a host → dark matter halo of a given mass contains. HOD is a powerful theoretical frame to populate dark matter halos with luminous galaxies. More specifically, it describes the bias between galaxies and dark matter by specifying (a) the probability P(N|M) that a halo of → virial mass M contains N galaxies of a particular class and (b) the relative spatial and velocity distributions of galaxies and dark matter within halos.

See also: → halo; → occupation; → distribution.

The diffuse, nearly spherical cloud of stars and → globular cluster s that surrounds a → spiral galaxy.

See also: → halo; → galaxy.

The → halo of galaxy belonging to our → Milky Way.

See also: → halo; → galaxy.

Old stars with very low metallicities (→ metallicity) found in the → halo of the Galaxy. Also called → population II star.

See also: → halo; → population.

A faint, wide ring around → Jupiter that has the shape of a doughnut. It is about 22,800 km wide and about 20,000 km thick. This ring starts at 100,000 km from the center of Jupiter. The outer edge of the Halo merges into the → Main ring.

See also: → halo; → ring.

A member of a group of five chemical elements having closely related and similar
properties. The halogens are: fluorine, chlorine, iodine, bromine, and astatine. They make up Group 17 of the → periodic table and can be found on the left-hand side of the → noble gases.

See also: From Gk. halo- prefix from Gk. hals “salt” + → -gen.

The brightest star in the constellation → Aries. Hamal is a cool → giant of → spectral type K2 with a → luminosity about 55 times that of the Sun and lies about 65 light-years away.

Etymology (EN): Hamal, from Ar., shortened form of Ra’s al-Hamal (رأس‌الحمل), “the head of the sheep.”

One of a set of equations that describe the motion of a → dynamical system in terms of the → Hamiltonian function and the → generalized coordinates. For a → holonomic system with n degrees of freedom, Hamilton’s equations are expressed by: q^._i = ∂H/∂p_i and p^._i = - ∂H/∂q_i, i = 1, …, n.

See also: → Hamiltonian function; → equation.

Of all the possible paths along which a → dynamical system can move from one configuration to another within a specified time interval (consistent with any constraints), the actual path followed is that which minimizes the time integral of the → Lagrangian function. Hamilton’s principle is often mathematically expressed as δ∫Ldt = 0, where L is the Lagrangian function, the integral summed from t₁ to t₂, and δ denotes the virtual operator of Lagrangian dynamics and the → calculus of variations.

See also: → Hamiltonian function;
→ principle.

The study of → dynamical systems in terms of the → Hamilton’s equations.

See also: → Hamiltonian function; → dynamics.

A reformulation of classical mechanics that predicts the same outcomes as classical mechanics. → Hamiltonian dynamics.

See also: → Hamiltonian; → mechanics.

A function that describes the motion of a → dynamical system in terms of the → Lagrangian function, → generalized coordinates, → generalized momenta, and time. For a → holonomic system having n degrees of freedom, the Hamiltonian function is of the form: H = Σp_iq^._i - L(q_i,q^._i,t) (summed from i = 1 to n),
where L is the Lagrangian function. If L does not depend explicitly on time, the system is said to be → conservative and H is the total energy of the system. The Hamiltonian function plays a major role in the study of mechanical systems. Also called → Hamiltonian.

See also: Introduced in 1835 by the Irish mathematician and physicist William Rowan Hamilton (1805-1865); → function.

The dynamical operator in → quantum mechanics
that corresponds to the → Hamiltonian function in classical mechanics.

See also: → Hamiltonian function; → operator.

A small → village.

Etymology (EN): M.E. hamlet, hamelet, from O.Fr. hamelet “small village,” diminutive of O.Fr. hamel “village,” itself diminutive of ham “village;” of Germanic origin; cf. E. home, O.E. ham, Du. heem, Ger. Heim; cognate with → city.

Etymology (PE): Kalâ, from Tabari kalâ “village, borough.” Dozens of village names contain kalâ az suffix in Mâzandarân and Gilan.

1. The terminal part of the forelimb in humans and other primates.
   

2. A part serving the function of or resembling a hand.

Etymology (EN): M.E. O.E. hond, hand “hand; side; power;” cf. O.S., O.Fris., Du., Ger. hand, O.N. hönd, Goth. handus.

Etymology (PE): Dast “hand; strength; superiority;” Mid.Pers. dast; O.Pers. dasta-;
Av. zasta-; cf. Skt. hásta-; Gk. kheir; L. praesto “at hand;” Arm. jern “hand;” Lith. pa-žastis “arm-pit;” PIE *ghes-to-.

A scholarly book on a specific subject that is conveniently handled.

See also: → hand; → book.

1. A tendency to use one hand rather than the other.
   

2. The property of an object (as a molecule) of not being identical with its mirror image. Same as → chirality (Merriam-Webster.com).
   See also: → B-mode polarization, → E-mode polarization.

Etymology (EN): → hand + -ed + → -ness.

Etymology (PE): Dastâli, from dast, → hand, + -al, → -al, + noun suffix -i, on the model of → chirality.

The → polarization arising from line scattering in the presence of “weak” magnetic fields. The effect occurs when precession around magnetic field depolarizes and rotates polarization of the scattered light. The Hanle effect is sensitive to ~10³ times smaller field strengths than the → Zeeman effect. It is in particular used to measure the weak magnetic field of the solar → prominences, which is 10^-3 tesla and over 10^-2 tesla for the active prominences.

See also: Named for the German physicist Wilhelm Hanle (1901-1993), who published his his discovery in 1923 (Naturwissenschaften 11, 690); → effect.

Take place; occur; befall.

Etymology (EN): M.E. hap(pe)nen, from hap “luck, chance” + -en.

Etymology (PE): Fatidan, variant of oftâdan, fotâdan “to fall; to be fall, occur;” Sistani aft, aftid “to → fall.”

An → event or occurrence.

See also: Verbal noun of → happen; → -ing.

To disturb persistently; bother continually. → galaxy harassment.

Etymology (EN): From M.Fr. harasser “tire out, vex,” possibly from O.Fr. harer “set a dog on,” and perhaps blended with O.Fr. harier “to harry, draw, drag.”

Etymology (PE): Sotuhidan, infinitive from sotuh, → harassed.

Subject to → harassment.

Etymology (EN): P.p. of → harass.

Etymology (PE): Sotuh “afflicted, distressed, helpless,” from Mid.Pers. stô “distressed, defeated;” O.Pers. us-tav-, from us- “out, without,” ultimately from *ustau- “unable, weak,” from *us- “out,” → ex-, + *tau- “to be able,” → power.

The act or an instance of harassing. → galaxy harassment.

See also: Verbal noun of → harass.

Not soft; severe.
Physics: Having relatively high energy (of a beam of particles or photons); → hard X-rays; opposed to → soft.

Etymology (EN): Hard, from O.E. heard “solid, firm; severe, rigorous,” from P.Gmc. *kharthus (cf. Du. hard, O.H.G. harto “extremely, very,” Goth. hardus “hard”), from PIE *kratus “power, strength” (cf. Gk. kratos “strength,” kratys “strong”).

Etymology (PE): Saxt “hard, strong, firm, secure, solid, vehement, intense,” from Mid.Pers. saxt “hard, strong, severe;” Av. sak- “to understand or know a thing, to mark;” cf. Skt. śakta- “able, strong,” śaknoti “he is strong,” śiksati “he learns.”

In → stellar dynamics studies of → three-body encounters, a → binary system whose → binding energy far exceeds the → kinetic energy of the relative motion of an incoming third body. In such an encounter, a hard binary is likely to get harder and transfer energy to the incoming star, whereas a → soft binary is likely to be disrupted.

See also: → hard; → binary.

The front, bony part of the roof of the mouth. → soft palate.

See also: → hard; → palate.

The short wavelength, high energy end of the → electromagnetic spectrum. Hard X-rays are typically those with energies greater than around 10 keV. The dividing line between hard and → soft X-rays is not well defined and can depend on the context.

See also: → hard; → X-rays.

Any physical equipment. The physical equipment comprising a computer system; opposed to → software.

Etymology (EN): → hard + ware, from M.E., from O.E. waru, from P.Gmc. *waro (cf. Swed. vara, Dan. vare, M.Du. were, Du. waar, Ger. Ware “goods”).

Etymology (PE): Saxt-afzâr, from saxt, → hard + afzâr “instrument, means, tool,” from Mid.Pers. afzâr, abzâr, awzâr “instrument, means,” Proto-Iranian *abi-cāra- or *upa-cāra-, from cāra-, cf. Av. cārā- “instrument, device, means” (Mid.Pers. câr, cârag “means, remedy;” loaned into Arm. aucar, aucan “instrument, remedy;” Mod.Pers. câré “remedy, cure, help”), from kar- “to do, make, build;” kərənaoiti “he makes” (Pers. kardan, kard- “to do, to make”); cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make”).

(adj.) Of, pertaining to, or noting a series of oscillations in which each oscillation has a frequency that is an integral multiple of the same basic frequency.
(n.) A wave motion, superimposed on a fundamental wave, having a frequency which is an integral multiple of the fundamental frequency. → overtone.

Etymology (EN): From L. harmonicus, from Gk. harmonikos “harmonic, musical,” from harmonia “agreement, concord of sounds,” related to harmos “joint,” arariskein “to join together;” PIE base *ar- “to fit together.”

Etymology (PE): Hamâhang, “harmonious, concordant,” from ham- “together, with; same, equally, even” (Mid.Pers. ham-, like L. com- and Gk. syn- with neither of which it is cognate. O.Pers./Av. ham-; Skt. sam-; also O.Pers./Av. hama- “one and the same,” Skt. sama-; Gk. homos-; originally identical with PIE numeral *sam- “one,” from *som-) + âhang “melody, pitch, tune; harmony, concord,” from Proto-Iranian *āhang-, from prefix ā- + *hang-, from PIE base *seng^wh- “to sing, make an incantation;” cf. O.H.G. singan; Ger. singen; Goth. siggwan; Swed. sjunga; O.E. singan “to chant, sing, tell in song;” maybe cognate with
Gk. omphe “voice; oracle.”

A number whose reciprocal is the → arithmetic mean of the reciprocals of a set of numbers. Denoted by H, it may be written in the discrete case for n quantities x₁, …, x_n, as:

1/H = (1/n) Σ(1/x_i), summing from i = 1 to n.

For example, the harmonic mean between 3 and 4 is 24/7 (reciprocal of 3: 1/3, reciprocal of 4: 1/4, arithmetic mean between them 7/24). The harmonic mean applies more accurately to certain situations involving rates.

For example, if a car travels a certain distance at a speed speed 60 km/h and then the same distance again at a speed 40 km/h, then its average speed is the harmonic mean of 48 km/h, and its total travel time is the same as if it had traveled the whole distance at that average speed. However, if the car travels for a certain amount of time at a speed v and then the same amount of time at a speed u, then its average speed is the arithmetic mean of v and u, which in the above example is 50 km/h.

See also: → harmonic; → mean.

A motion that repeats itself in equal intervals of time (also called periodic motion).

See also: → harmonic; → motion.

Any oscillating particle in harmonic motion.

See also: → harmonic; → oscillator.

Math.: Any ordered set of numbers, the reciprocals of which have a constant difference between them. For example 1, ½, 1/3, ¼, …, 1/n. Also called → harmonic sequence.

See also: → harmonic; progression.

→ harmonic progression.

See also: → harmonic; → sequence.

Overtones whose frequencies are integral multiples of the → fundamental frequency. The fundamental frequency is the first harmonic.

See also: → harmonic; → series.

A type of galaxies characterized by strong emission in the blue and violet regions of the spectrum. They are often elliptical or lenticular.

See also: Named after the Mexican astronomer Guillermo Haro (1913-1988), who first compiled a sample of these objects; → galaxy.

A → polarimeter using the → spectrographic capabilities of the → High Accuracy Radial velocity Planet Searcher (HARPS) to measure the → Zeeman effect indicating the presence of a → magnetic field at the surface some stars. This combined instrument is installed at the ESO 3.6-m telescope at → La Silla Observatory (Chile) and covers the 3800-6900 Å wavelength region with an average → spectral resolution of 110,000 (Piskunov, et al., 2011, ESO Messenger 143, 7). HARPSpol is mainly used in research on → magnetic fields in stars. See also → magnetic star, → magnetic massive star, → magneto-asteroseismology

See also: → HARPS + -pol, from → polarimeter.

A number that is divisible by the sum of its digits.
For example, 18 is a Harshad number because 1 + 8 = 9 and 18 is divisible by 9 (18/9 = 2).
The simplest Harshad numbers are the two-digit Harshad numbers: 10, 12, 18, 20, 21, 24, 27, 30, 36, 40, 42, 45, 48, 50, 54, 60, 63, 70, 72, 80, 81, 84, 90. They are sometimes called Niven numbers.

See also: The name Harshad was given by Indian mathematician Dattaraya Kaprekar (1905-1986) who first studied these numbers. Harshad means “joy giver” in Sanskrit, from harṣa- “joy” and da “to give,” → datum.

A proposal regarding the initial state of the → Universe prior to the → Planck era. This → no boundary hypothesis assumes an imaginary time in that epoch.
In other words, there was no real time before the → Big Bang, and the Universe did not have a beginning. Moreover, this model treats the Universe like a quantum particle, in an attempt to encompass → quantum mechanics and → general relativity;
and attributes a → wave function to the Universe. The wave function has a large value for our own Universe, but small, non-zero values for an infinite number of other possible, parallel Universes.

See also: Hartle, J., Hawking, S., 1983, “Wave function of the Universe,” Physical Review D 28; → initial; → state.

A band in the → absorption spectrum of → ozone (O₃) extending in the → ultraviolet from 200 nm to 300 nm. It is stronger than the → Huggins band. See also: → Hartley band.

See also: W. N. Hartley, J. Chem. Soc. 39, 111 (1881).

A way of testing the quality of optical systems. In this method, incident rays from a point source are isolated by small holes in an opaque screen located close to the lens or mirror under test. Photographic plates are inserted into the beam within and beyond the focal region. The black dots on the exposed plates, which reveal differences of optical focus in the various zones of the lens or mirror, are analyzed to yield the objective’s figure. → Shack-Hartmann wavefront sensor.

See also: Named after the German astronomer Johannes Hartmann (1865-1936), who developed the method.
→ test.

A unit of energy used in atomic and molecular physics; symbol Ha or E_h. It is defined as: 1 Ha = m_ee⁴/(4ε₀²ħ),
where m_e is the mass of electron, e its charge, ε₀ the → permittivity of vacuum, and ħ → reduced Planck’s constant. Its value is 2 → rydbergs, or 4.3597 x 10^-18 → joule, or 27.213 → electron-volts.

See also: Named for the British physicist and mathematician Douglas R. Hartree (1897-1958).

A classification of stellar spectra published in the Henry Draper catalogue, which was prepared in the early twentieth century by E. C. Pickering and Miss Annie Canon. It is based on the characteristic lines and bands of the chemical elements.
The most important classes in order of decreasing temperatures are as follows: O, B, A, F, G, K, M.

See also: Harvard, named for John Harvard (1607-1638), the English colonist, principal benefactor of Harvard College, now Harvard University. → classification

1a) The gathering of a ripened → crop.

 1b) (Agriculture) the crop itself or the yield from it in a single
 growing season. <BR>

 2) To gather or reap (a ripened crop) from (the place where it has been growing)
 (TheFreeDictionary).

Etymology (EN): M.E. hervest, from O.E. hærfest “autumn;” cognate with
Du. herfst, Ger. Herbst “autumn”); PIE (s)kerp-
“to gather, pluck, harvest,” from (s)ker- “to cut”
(cf. Skt. krpāna- “sword, knife;”
Gk. karpos “fruit;” L. carpere “to cut, divide, pluck;” Av. karət- “to cut,” Mod.Pers. kârd “knife,” as below).

Etymology (PE): Xarman, ultimately from *xramana-, from *xram- “to thresh;” cf. Ormuri šraməd, Parâci khamör, Yidgha xurom, xuräm; Nuristâni Kati kram- “to thresh;” Skt. kram- “to stride out, to go” (H. W. Bailey, 1979).

The → full moon that appears closest in time to the → autumnal equinox.

See also: → harvest; → moon.

1. (of an egg) To open and produce a young animal; emerge from its egg.
   

   2a) The act or process of hatching.
   

   2b) A small opening in a wall that serves as a doorway or window. → dome hatch.

Etymology (EN): 1) M.E. hachen “to produce young from eggs by incubation,” probably from an unrecorded O.E *hæccan, of unknown origin, related to M.H.G., Ger. hecken “to mate” (used of birds).

2. M.E. hacche, O.E. hæcc “fence, half-gate;” cf. M.H.G. heck, Du. hek “gate, railing.”

Etymology (PE): 1) Lâcidan, from Tabari lâc “open, separated, wide aprat” (lâc hâytan “to split, to crack,” lâc bazoən “to split, to tear”), may be from Proto-Ir. *rauj “to break, burst;” cf. Av. (+*fra-) fra.uruxti- “destruction;” Khotanese *rrus- “to burst, break;” Baluchi ruj- “to break open;” Bartangi, Oroshori
ruj-/ruxt- “to dig;” Skt. roj- “to break, break open;” Pali luki- “part;” L. lugere “to mourn, grieve;” Armenian lucanem “I break up.”

2. Daricé, from dar, → door,

• -cé diminutive suffix.

The → component Aa of the → multiple star system  → Iota Orionis,. The name was approved in 2016 by the IAU Working Group on Star Names (WGSN).

See also: Hatsya, of unknown origin.

A → dwarf planet located beyond → Neptune’s orbit (→ trans-Neptunian object). Haumea is roughly the same size as → Pluto.

It spins on its axis once every four hours, making it the fastest spinning known large object in the → solar system. It has two known moons, called Hi’aka and Namaka.

Observations from multiple Earth-based observatories of Haumea passing in front of a distant star indicate the presence of a ring with a width of 70 km and radius of about 2,287 km. The ring is coplanar with both Haumea’s equator and the orbit of its satellite Hi’aka.

The → occultation by the main body indicates an oblong shape for Haumea
with axes of about 2,322 × 1,701 × 1,138 km. In other words, along one direction, Haumea is significantly longer than → Pluto, while in another direction it has an extent very similar to Pluto, while in the third direction is much smaller. Haumea’s orbit sometimes brings it closer to the Sun than Pluto, but usually Haumea is further (Ortiz et al., 2017, Nature 550, 219, doi:10.1038/nature24051).

See also: Named for the Hawaiian goddess of childbirth and fertility (temporary designation 2003 EL61). Its moons are named for daughters of Haumea.

The radiation produced by a → black hole when → quantum mechanical effects are taken into account. According to quantum physics, large fluctuations in the → vacuum energy occurs for brief moments of time. Thereby virtual particle-antiparticle pairs are created from vacuum and annihilated. If → pair production happens just outside the → event horizon of a black hole, as soon as these particles are formed they would both experience drastically different → gravitational attractions due to the sharp gradient of force close to the black hole. One particle will accelerate toward the black hole and its partner will escape into space. The black hole used some of its → gravitational energy to produce these two particles, so it loses some of its mass if a particle escapes. This gradual loss of mass over time means the black hole eventually evaporates out of existence. See also → Bekenstein formula, → Hawking temperature.

See also: Named after the British physicist Stephen Hawking (1942-2018), who provided the theoretical argument for the existence of the radiation in 1974; → radiation.

The temperature inferred for a → black hole based on the → Hawking radiation.

For a → Schwarzschild black hole, one has

T_H = ħc³/(8πGMk) where ħ is the → reduced Planck’s constant, c is the → speed of light, G is the → gravitational constant, M is the mass, and
k is → Boltzmann’s constant. The formula can approximately be written as: T_H≅ 6.2 x 10^-8 (Msun/M) K. Thus radiation from a solar mass black hole would be exceedingly cold, about 5 x 10⁷ times colder than the → cosmic microwave background. Larger black holes would be colder still. Moreover, smaller black holes would have higher temperatures. A → mini black hole of mass about 10¹⁵ g would have T_H≅ 10¹¹ K.

Etymology (EN): → Hawking radiation; → temperature.

A Japanese → asteroid sampling mission devoted to the study of → Ryugu. It was launched on December 3, 2014 and successfully arrived at the asteroid on June 27, 2018. The Hayabusa2 mission includes four rovers with various scientific instruments.

On September 21, 2018 the first two of these rovers, MINERVA-II robots, which hop around the surface of the asteroid, were released from Hayabusa2. This marked the first time a mission has completed a successful landing on a fast-moving asteroid body. This was followed later by the deployment of MASCOT (Mobile Asteroid Surface Scout), a lander developed by the German space agency DLR in partnership with the French Center for Spatial Studies (CNES). It carried four instruments and with its 16 h lifetime battery
collected data on the surface structure and mineralogical composition, the thermal behaviour and the magnetic properties of the asteroid. Hayabusa2 is expected to leave Ryugu with the collected samples in late 2019 and return to Earth in 2020.

See also: Hayabusa “peregrine falcon” in Japanese.

The region to the right the → Hayashi track, representing objects that cannot be in → hydrostatic equilibrium. Energy transport in these objects would take place with a → superadiabatic temperature gradient.

See also: → Hayashi track; → forbidden; → zone.

A period in the → pre-main sequence evolution of a low mass star during which the star has negligible nuclear energy production and low internal temperature. Hence energy transport inside the star takes place dominantly through → convection. The star contracts homologously and evolves in the → H-R diagram along the → hayashi track with decreasing → luminosity and nearly constant → effective temperature. The time
taken by a star of mass M to contract to radius R along a Hayashi track is of the order of the → Kelvin-Helmholtz time: t_KH = 10⁷(M/M_sun)²/(R/R_sun)³ yr.

See also: → Hayashi track; → phase.

The minimum → effective temperature
required for a → pre-main sequence star of given mass and radius to be in → hydrostatic equilibrium. This temperature delimits the boundary of the → Hayashi forbidden zone.

See also: → Hayashi track; → temperature.

The path on the → Hertzsprung-Russell diagram that is followed by a fully → convective  → pre-main sequence star to reach the → zero-age main sequence. Hayashi tracks for → low-mass stars are near vertical. At higher masses, stars become increasingly radiative as they contract and the Hayashi tracks are almost horizontal.

See also: Named after the Japanese astrophysicist Chushiro Hayashi (1920-2010), who published his paper in 1961 (PASJ 13, 450);
→ track.

1. A danger that one can foresee but cannot avoid.
   

2. Something causing unavoidable danger, peril, risk, or difficulty.
   

3. The absence or lack of predictability; chance; uncertainty (Dictionary.com).

Etymology (EN): M.E. hasard, from O.Fr. hasard, hasart “game of chance played with dice,” possibly from Sp. azar “an unfortunate card or throw at dice,” postulated to derive from Ar. az-zahr “the die,” but this etymology is controversial.

Etymology (PE): Âpé, from Av. au-pat-, “to fall down, off,” from pat- “to fall, fly;” Proto-Ir. *pat- “to fall; fly; rise;” related to Pers. oftâdan “to fall; to befall; to happen,” → fall. Pers. âfat “blight, pest, curse,” may belong to this family.

1. Full of risk; perilous; risky.
   

2. Dependent on chance (Dictionary.com).

See also: Adj. from → hazard.

A phenomenon where fine particles of → dust and/or → smoke suspended in the → atmosphere near Earth reduce the → visibility by → scattering light.

Etymology (EN): Maybe from M.E. *hase, O.E. hasu, variant of haswa “ashen, dusky.”

Etymology (PE): Nezm “mist, fog, vapor.”

An extremely → metal-poor and high-velocity F3-type → subgiant with → apparent visual magnitude 7.205 ± 0.02. It has other designations, among which BD-10 4149, GJ 1195, HIP 76976, and SAO 159459. HD 140283 is situated in the solar neighborhood at some 200 → light-years from the Earth (→ trigonometric parallax 17.15 ± 0.14 mas) in the constellation → Libra. Its color E(B - V) = 0.000 yields a visual → absolute magnitude  M_V = +3.377. Its → surface temperature is T_eff = 5777 K. Its iron and oxygen surface → chemical abundancees relative to hydrogen are [Fe/H] = -2.40 and [O/H] = -1.67, making it the most metal-poor star so far known (2021). Using precise observational data, an age of 14.46 ± 0.8 Gyr has been derived for this star. Within the errors, the age of HD 140283 does not conflict with the age of the → Universe, 13.77 ± 0.06 Gyr, obtained from the → cosmic microwave background radiation and the → Hubble constant. HD 140283 must have formed soon after the → Big Bang and is the oldest known star. This is why it is sometimes nicknamed the → Methuselah star (H. E. Bond et al., 2013, arxiv.1302.3180).

See also: The star’s name in the → Henry Draper system

A → hot star of → apparent visual magnitude 6.61 lying in the constellation → Orion. HD 43317 has a B3.5V → spectral type and has no detected binary companion. Its chemical surface abundances agree with the solar abundances, but with some co-rotating He abundance spots at the stellar surface.

The CoRoT satellite revealed that HD 43317 is a → hybrid pulsator of → Slowly Pulsating B star (SPB)/ → Beta Cephei type. Its → rotation period is 0.897673(4) days. Zeeman signatures in the Stokes V profiles of HD 43317 are clearly detected and rotationally modulated, which proves that this star exhibits an oblique magnetic field. The strength of the dipolar magnetic field is of the order of 1 kG to 1.5 kG (Buysschaert et al., 2017, A&A 605, A104).

See also: → HD number.

A remarkable → binary star system composed of → massive stars that
is associated with NGC 346, the largest → H II region

• OB star cluster in the → Small Magellanic Cloud. HD 5980 is a rather complex system because it consists of at least three stars: two stars form an → eclipsing binary with a period of 19.266 days, while the third component, an → O star, is detected by means of a set of absorption lines. Whether or not the third star is physically bound to the eclipsing binary remains currently unclear. HD 5980 underwent an
  → LBV-type event in August 1994. Before the LBV eruption, both components of the eclipsing binary already showed emission lines in their spectra and were thus classified as → Wolf-Rayet stars. However, as shown by the analysis of the spectra taken during and after the LBV event, at least the star that underwent the eruption was not a classical, helium-burning, Wolf-Rayet object, but rather a WNha star. This means a rather massive star with substantial amounts of hydrogen present in its outer layers. These WNha stars have → stellar wind properties that are intermediate between those of extreme → Of stars and classical → WN Wolf-Rayet stars.

See also: → Henry Draper system; → number.

A multiple → O-type star in the → Trumpler 14 cluster, which consists of at least three components. The main component, HD 93129A, is of spectral type O2 If*, a very rare hot star and the closest known O2 star (Walborn et al. 2002). It is one of the most luminous stars known.
HD 93129B, lying 2.7 arcsec apart, is an O3.5 V((f+)) type. Recent → HST observations have shown that component A is itself probably a double or → binary star with a separation of 0.055 arcsec.

See also: → HD number.

The core of the Galactic → giant H II region, → NGC 3603. It is a multiple object composed of several → massive stars with a collective → spectral type of around WN6+O5. One of the stars, → NGC 3603A-1, is a double-eclipsing binary with an orbital period of 3.77 days. The component masses are 116 Msun for the primary and 89 Msun for the secondary, respectively. The primary WN6ha component of A1 is the most massive star ever directly weighed. A second star, C, has newly been identified, which has been classified as an SB1 binary, and in which only the primary (WN6ha) component is visible. The third star, B, shows constant radial velocities over the observed time interval, and therefore is most likely not a binary. While the primary component of C might have a mass similar to or even greater than that of A1’s primary, it is possible that star B, be the most massive member in NGC 3603 and, therefore, the most massive main-sequence star known in the Galaxy (Schnurr et al., 2008, MNRAS, 389, L38).

See also: → HD number.

An identifying number assigned to the stars in the Henry Draper catalog. For example, the star Vega is HD 172167.

See also: → Henry Draper system; → number.

Any of the → spectral lines arising from → singly ionized helium in the atmosphere of → O-type and → Wolf-Rayet stars. He II lines are chiefly in absorption, but some of them, such as 4686 Å occur in emission in hotter stars. The presence of He II → absorption lines separates O types from → B-type stars.

A number of these He II lines belong to the → Pickering series involving transitions with → principal quantum numbern = 4 and higher.

Although the n = 3-4 (4686 Å) transition also belongs to ionized helium and often occurs in these hot stars, it does not belong to the Pickering series because it has a lower landing level quantum number (n = 3).
The same goes for n = 2-5 (4026 Å).

See also: → helium; → line.

An early → B-type star showing helium lines with abnormally large equivalent widths. The surface → chemical abundances of He-strong stars are influenced by the presence of a strong → magnetic field, resulting in a He overabundance that typically varies in strength over the stellar surface. Examples include HR 735, HD 184927, and CPD-62°2124.

See also: → helium; → strong; → line.

A → chemically peculiar star with very weak helium lines. Examples include 3 Sco, HD 176582, HD 217833, HR 2949, and HD 21699.

The He-weak stars do not form a homogeneous group. Some of them display intense Si, or Ti and Sr lines, and are considered a hot extension of the magnetic → Ap/Bp stars. Others show overabundances of P and Ga, typically noted for → HgMn stars.

The star HD 139160 belongs to the non-magnetic subgroup of He-weak stars.

See also: → helium; → weak; → line.

1. That part of anything that forms or is situated at the top, summit, or upper end.
   

2. The → nucleus and surrounding → coma of a → comet.

Etymology (EN): Head, from O.E. heafod “top of the body,” also “chief person” (cf. O.S. hobid; Goth. haubiþ Ger. Haupt “head”), from PIE *kauput- “head;” cf. Skt. kaput-, kapala- “skull;” L. caput “head;” Pers. dialect Lori: kapu “head,” kapulek “skull, middle of the head;” Kurd. kapol “skull;” Pashto kaparay “skull.”

Etymology (PE): Sar “head,” soru, sorun “horn”
(karnâ “a trumpet-like wind instrument,” variant sornâ “a wind instrument”);
Mid.Pers. sar “head,” sru “horn;” Av. sarah- “head,” srū- “horn, nail;” cf. Skt. śiras- “head, chief;” Gk. kara “head,” karena “head, top,” keras “horn;”
L. cornu “horn,” cerebrum “brain;”
P.Gmc. *khurnaz (Ger. Horn, Du. horen; cognate with E. horn, as above, from PIE *ker- “head, horn;”
O.E. horn “horn of an animal,” also “wind instrument;”
E. horn); PIE base *ker- “head, horn, top, summit.”

A member of the class of radio galaxies (→ radio galaxy) that have a strong radio emission coming from a bright “head” and a more diffuse emission from a “tail.” They are often found in clusters.

See also: → head; → tail; → galaxy.

1. A hollow muscular organ that pumps the blood through the circulatory system by rhythmic contraction and dilation. In vertebrates there may be up to four chambers (as in humans), with two atria and two ventricles.
   

   2. The heart regarded as the centre of a person’s thoughts and emotions, especially love or compassion (OxfordDictionaries.com).

Etymology (EN): M.E. herte, from O.E. heorte “heart breast, soul, spirit, will, desire; courage; mind, intellect;” cf. O.Saxon herta, O.Frisian herte, O.Norse hjarta, Du. hart, O.H.G. herza, Ger. Herz; cognate with Pers. del, as below; PIE root *kerd- “heart.”

Etymology (PE): Del “heart” (Pashtu z’rrah, zyah; Baluci zirde “heart, mind, soul;” Kurd. zar; Sogd. žyâwar); Mid.Pers. dil; Av. zərəd-; cf. Skt. hrd-; Gk. kardia; L. cor “heart” (Fr. cœur; Sp. corazon, It. cuore); Russ. serdtse; Arm. sirt; E. heart, as above.

Energy possessed by a substance in the form of kinetic energy of atomic or molecular translation, rotation, or vibration.

Etymology (EN): Heat, from O.E. hætu, hæto, from P.Gmc. *khaitin- “heat,” from *khaitaz “hot” (cf. O.N. hiti, Ger. hitze “heat,” Goth. heito “fever”).

Etymology (PE): Garmâ “heat, warmth,” from Mid.Pers. garmâg; O.Pers./Av. garəma- “hot, warm;” cf. Skt. gharmah “heat;” Gk. thermos “warm;” L. formus “warm,” fornax “oven;” P.Gmc. *warmaz; O.E. wearm; E. warm; O.H.G., Ger. warm; PIE *ghworm-/*ghwerm- “warm.”

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.

See also: → heat; → capacity.

A type of → heat transfer by means of molecular agitation within a material without any motion of the material as a whole.

See also: → heat; → conduction.

A type of → heat transfer involving mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it.

See also: → heat; → convection.

Assuming that the Universe is a thermodynamically → isolated system, a state of absolute uniformity in the Universe in which all temperature differences would reduce to zero and no energy will be available for use, according to the → second law of thermodynamics. In that condition of maximum → entropy, the Universe would be in a state of unchanging death.
First introduced by the German physicist Hermann von Helmholtz (1821-1894) in 1854, on the basis of William Thomson’s (1824-1907) idea.

See also: → heat; → death; → Universe.

The amount of heat energy required to transform an amount of a substance from the liquid phase to the gas phase. → molar heat of vaporization.

See also: → heat; → vaporization.

A structure that protects against excessive heat, especially that which
covers the vulnerable surfaces of a → spacecraft and protects it when re-entering the Earth’s atmosphere.

See also: → heat; → shield.

The spontaneous transportation of heat through matter, from a region of higher temperature to a region of lower temperature.

See also: → heat; → transfer.

Meteorology: A period of several successive days of abnormally hot and usually humid weather occurring in summer.

Etymology (EN): → heat; → wave.

Etymology (PE): Celle-ye tâbestân literally “the fortieth of summer,” i.e. “midsummer,” from cellé pertaining to “forty (days),” from cel, cehel, → forty, + tâbestân, → summer.

1. The process whereby a system’s temperature increases. → warming.
   

2. A device or system for supplying heat, especially central heating, to a building; the heat supplied.

See also: → heat; → -ing.

The sky or Universe as seen from the Earth; the firmament. Often used in the plural.

Etymology (EN): From M.E. heven, O.E. heofon, possibly from P.Gmc. *khemina- (cf. M.L.G. heben, O.N. himinn, Goth. himins, Du. hemel, Ger. Himmel “heaven, sky”); PIE base *kem-/*kam- “to cover.”

Etymology (PE): Âsmân, from Mid.Pers. âsmân “sky, heaven;” O.Pers. asman- “heaven;” Av. asman- “stone, sling-stone; heaven;” cf. Skt. áśman- “stone, rock, thunderbolt;” Gk. akmon “heaven, meteor, anvil;” Akmon was the father of Ouranos (Uranus), god of sky; Lith. akmuo “stone;” Rus. kamen; PIE base *akmon- “stone, sky.”
The link between the “stone” and “sky” concepts indicates that the sky had once been conceived as a stone vault by prehistoric Indo-Europeans.

→ astronomical object.

See also: → heaven; → -ly; → body.

→ Kennelly-Heaviside layer.

See also: English physicist Oliver Heaviside (1850-1925).

Of great weight; of great amount, quantity.

Etymology (EN): M.E. hevi; O.E. hefig, from P.Gmc. *khabigas (cf. O.N. hebig).

Etymology (PE): Sangin “heavy, weighty; stony, like stone, hard,” from sang “stone, rock” (Mid.Pers. sang; O.Pers. aθanga-; Av. asenga- “stone” (related to Mod.Pers. âsmân “sky” → heaven); PIE *aken-) + -in adj. suffix.

In astrophysics, any → chemical element heavier than → helium. Such elements are also inappropriately referred to as “→ metals.”

See also: → heavy; → element.

→ deuterium.

See also: → heavy; → hydrogen.

Water in which the hydrogen is replaced by → deuterium. Deuterium Oxide (D₂O).

See also: → heavy; → water.

A → lunisolar calendar used by Jews for religious purposes. The year consists of 12 months alternating between 29 and 30 days, making a year of 354 days.
In order to conform to the → solar year, a → leap month is included every third year. A month
begins the day the new moon is first seen. The years are counted from the time of “creation,” believed by Jewish theologians to have occurred in the year 3761 B.C. Also called → Jewish calendar.

Etymology (EN): Hebrew, from O.E., from O.Fr. Ebreu, from L. Hebraeus, from Gk. Hebraios, from Aramaic ‘ebhrai, corresponding to Heb. ‘ibhri “an Israelite,” literally “one from the other side,” in reference to the River Euphrates, or perhaps simply denoting “immigrant;” from ‘ebher “region on the other or opposite side;” → calendar.

Etymology (PE): Gâhšomâr, → calendar; yahud→ Jewish calendar.

A prefix meaning hundred (10²) used in the formation of compound words.

Etymology (EN): From Fr., from Gk. hekaton “hundred.”

Etymology (PE): Hekto-, loanword from Fr., as above.

Distance upward from a given level to a fixed point.

Etymology (EN): M.E., from O.E. hiehthu; → high + -th a suffix forming nouns of action (e.g., birth) or abstract nouns denoting quality or condition (depth; length; warmth).

Etymology (PE): Bolandi, bolandâ “height,” noun forms from boland “high,” variants bâlâ “up, above, high, elevated, height,” borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz),
Lori dialect berg “hill, mountain;” Mid.Pers. buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high.”
Farâzâ, noun of farâz “above, up, upon, on the top, aloft,” from Mid.Pers. farâz, farâc “forward, prominent, distinguished;” Av. frānk- (adj.) “turned toward the front,” fraca (adv.) “forward, forth,” fraš (adv.) “forward, forth; before;” Proto-Iranian *frānk-.

A diffuse bright region surrounding the shadow that an observer’s head casts on an irregular surface. It can be best observed on dewy reeds or grass. This phenomenon is reminiscent of the → glory, but without its color and regular structure.

Etymology (EN): Heiligenschein, Ger., literally “saint’s shining light,” from heiligen, from heilig “holy, sacred” (P.Gmc. *khailagas; M.H.G. heilec; O.H.G. heilag; Goth. hailag; O.N. heilagr; O.E. halig; E. holy) + Schein “glow, shine” (M.H.G. schinen, O.H.G. skinan,
P.Gmc. *skinanan; E. shine; cf. Mod.Pers. sâyé “shadow;” Mid.Pers. sâyak “shadow;” Av. a-saya- “throwing no shadow;” Skt. chāya- “shadow;” Gk. skia “shade;” Rus. sijat’ “to shine;” PIE base *skai- “bright”).

Etymology (PE): Sepant foruq, from sepant “holy” (Mid.Pers. spand “holy,” Spandarmat “Holy Thought; 5-th day of the month; 12-th month of the year;”
from Av. spənta- “holy; beneficent,” spəntô.mainyav- “coming from or belonging to the holy spirit,” spəntô.təma- “holiest”)

• foruq “light, brightness” (related to rôšan “light; bright, luminous;” ruz “day,” afruxtan “to light, kindle;” Mid.Pers. payrog “light, brightness,” rošn light; bright," rôc “day;” O.Pers. raucah-; Av. raocana- “bright, shining, radiant,” raocah- “light, luminous; daylight;”
  cf. Skt. rocaná- “bright, shining, roka- “brightness, light;” Gk. leukos “white, clear;” L. lux “light” (also lumen, luna; E. light, Ger. Licht, and Fr. lumière;
  PIE base *leuk- “light, brightness”).

A person who inherits or has a right of inheritance in the property of another following the latter’s death (Dictionary.com).

See also: → heritage.

The uncertainty in the measurement of the position and momentum of an elementary particle. The more precisely one quantity is known, the less certain the precision of the other. A similarly linked pair of quantities is the time and energy content in a volume of space.

See also: Named after Werner Heisenberg (1901-1976), the German physicist who in 1927 derived the uncertainty principle. In 1932 he was awarded the Nobel Prize in Physics; uncertainty, from → un- “not” + → certainty; → principle.

Of or near the → Sun, especially rising and setting with the Sun. → heliacal rising, → heliacal setting.

Etymology (EN): Heliacal “pertaining to the sun,” from Gk. heliakos “of the sun,” from helios, → sun; cognate with Pers. hur, as below.

Etymology (PE): Hurâné “sunlike,” since the star rises in the morning like the Sun, from hur “sun”, variant xor;
Mid.Pers. xvar “sun;” Av. hû-, hvar- “sun;” Skt.
surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun;
PIE *sawel- “sun” + -âné similarity suffix.

The first appearance of a star following a period of invisibility due to its conjunction with the Sun. → heliacal rising of Sirius.

See also: → heliacal; → rising.

The first rising of → Sirius at dawn shortly before → sunrise. The heliacal rising of Sirius played a significant role in ancient Egypt by heralding the annual flooding of the Nile. The event took place some 70 days after the star had been seen for the last time in the western horizon at sunset.

The heliacal rising of Sirius and its association with the rebirth of the Nile was so important that it marked the start of the Egyptian calendar year. At the time, the heliacal rising occurred in early July, as seen from the ancient capital of Memphis. But due to the → precession of the equinoxes the star now reappears in early August in Egypt. The date depends on the latitude (assuming transparent skies), being later for higher latitudes. For latitude 48° it occurs on about August 19.

See also: → heliacal; → rising; → Sirius.

The last visible setting of a star below the western horizon just after sunset entering into a conjunction with the Sun.

See also: → heliacal; → setting.

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.

See also: From → helix + → -ity.

A combining form of Gk. helios “sun.”

See also: Helio-, combining form of from Gk. helios “sun;” cognate with
Persian hur “sun”, variant xor and xoršid (Av. hvarə-xšaēta- “shining sun”);
Mid.Pers. xvar “sun;” Av. hû-, hvar- “sun;” Skt.
surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun;
PIE *sawel- “sun.”

Having or representing the Sun as a center. → heliocentric cosmology, → heliocentric gravitational constant, → heliocentric Julian Day,
→ heliocentric parallax,
→ heliocentric system.

See also: → helio- + → center +
→ -ic.

A model of the Universe in which the Sun was centrally located.

See also: → heliocentric; → cosmology.

A parameter representing the product of the → gravitational constant by the → solar mass. It is 13.27 x 10¹⁹ m³ s^-2.

See also: → heliocentric; → gravitational; → constant.

The → Julian Date referenced to the center of the → Sun. Since the Earth revolves around the Sun, and since light travels at a finite speed, observations of a given object taken at different positions in the Earth’s orbit are not equivalent, and so a correction for Earth’s orbit around the Sun is required. Left uncorrected, the time of an observational event measured by Earth clocks will vary by 16.6 minute over the course of a year. If not properly accounted for, this can lead to a spurious signal in a → periodogram.

See also: → heliocentric; → Julian Date.

The parallax of a celestial body when viewed from two points in the Earth’s orbit around the Sun. More specifically, the angular difference in a celestial object’s position as seen from the center of the Sun and the center of the Earth. Also called → annual parallax.

See also: → heliocentric; → parallax.

A system in which the Sun is assumed to lie at its central point while the Earth and other bodies revolve around it.

See also: → heliocentric; → system.

A record made by a → heliograph.

See also: → helio-; + → -gram.

1. An instrument for photographing the Sun, consisting of a camera and a specially adapted telescope.
   

2. A simple device using a mirror to reflect sunlight to a distant observer. By moving the mirror, flashes of light can be used to send coded messages. The heliograph was a highly effective instrument for instantaneous optical communication over 80km or more in the 19th century.
   

3. Meteo.: An instrument which records the duration of sunshine and gives a quantitative measure of the amount of sunshine.

See also: → helio-; + → -graph.

An instrument used to measure the angular separation of two stars that are too far apart to be included in the field of view of an ordinary telescope.  The instrument was originally designed for measuring the variation of the
Sun’s diameter at different seasons of the year.

See also: → helio- + → -meter.

The edge of the solar system where the pressure of the → solar wind balances that of the → interstellar medium plasma. In other words, the surface boundary that separates the → heliosphere from interstellar space. It is
estimated to be situated 100 → astronomical units or more from the Sun. A → bow shock likely forms as the interstellar medium wind approaches the heliosphere and is deflected around the heliosphere, forcing it into a teardrop-shaped structure with a long, comet-like tail.

Etymology (EN): From → helio- + pause “break, cessation, stop,” from M.Fr. pause, from L. pausa “a halt, stop, cessation,” from Gk. pausis “stopping, ceasing,” from pauein “to stop, to cause to cease.”

Etymology (PE): Hurmarz, from hur, → helio-, + marz “frontier, border, boundary,” → frontier.

The branch of astrophysics that investigates the interior structure of the Sun by studying its surface wave oscillations. See also → asteroseismology and → stellar pulsation. The surface of the Sun vibrates much like a bell.
A piano has 88 keys or musical tones, whereas the Sun has millions of notes.
These vibrations are the result of internal pressure waves that reflect off the → photosphere and repeatedly cross the solar interior. They are detected through the → Doppler shift of absorption lines formed in the photosphere.
Because these vibrations make the solar surface move up and down, analysis of the surface patterns is used to study conditions far below the Sun’s surface. The mean period of the vibrations is about five minutes, which corresponds to a speed of 0.5 km s^-1 or a frequency of about 3 mHz.
See also → p mode.

Etymology (EN): From → helio- + → seismology.

The region located between the → termination shock and the → heliopause where the turbulent and hot → solar wind is compressed as it passes outward against the interstellar wind.

Etymology (EN): Heliosheath, from → helio- + sheath, from O.E. sceað, scæð; cf. M.Du. schede, Du. schede, O.H.G. skaida, Ger. Scheide “scabbard.”

Etymology (PE): Hurniyâm, from hur- “sun,” → helio-,

• niyâm “sheath,” from Proto-Iranian *nigāma-, from ni- “down; into,” → ni- (PIE),
• gāma- “to go, to come” (Av. gam- “to come; to go,” jamaiti “goes;” O.Pers. gam- “to come; to go;” Mod./Mid.Pers. gâm
  “step, pace,” âmadan “to come;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE root *gwem- “to go, come”); cf. Skt. nigamá- “insertion, incorporation.”

The vast, three-dimensional region of space around the Sun filled with the → solar wind and the remnant of the → solar magnetic field
carried in it. It is bounded by the → heliopause, which is estimated to be 100 → astronomical units or more from the Sun. The radius of the heliosphere is expected to vary with the → solar cycle. The heliosphere may be very elongated owing to the presence of an interstellar wind of neutral hydrogen flowing from the direction of the Galactic center.

Etymology (EN): From → helio- + → sphere.

An instrument consisting of a mirror moved by clockwork for tracking the movement of the Sun and reflecting the sunlight into a stationary solar telescope. A heliostat is similar to a → coelostat.

Etymology (EN): Heliostat, from → helio- + -stat
prefix denoting something that stabilizes, keeps, fixes, from -stata, from Gk. -states “one that causes to stand,” or statos “standing,” from *sta- “to stand.”

Etymology (PE): Hurdâštâr, from hur-, → helio- + dâštâr “holder, maintainer,” from dâštan “to hold, maintain; to have; to possess;” Mid.Pers. dâštan; O.Pers./Av. dar- “to hold, keep back, maintain, keep in mind;” cf.
Skt. dhr-, dharma- “law;” Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE base *dher- “to hold, support.”

Chemical element; symbol He; atomic number 2; atomic weight 4.0026; melting point below -272°C at 26 atmospheres pressure; boiling point -268.934°C at 1 atmosphere pressure.

See also: Helium, from Gk. helios “sun;” cognate with
Persian hur “sun”, variant xor;
Mid.Pers. xvar “sun;” Av. hû-, hvar- “sun;” Skt.
surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun;
PIE *sawel- “sun.”
The element was discovered by spectroscopy during a solar eclipse in the Sun’s chromosphere in 1868 by the French astronomer Pierre-Jules-Cesar Janssen (1824-1907).

The relative amount of helium with respect to another → chemical species, usually → hydrogen, in an astronomical object.

See also: → helium; → abundance.

The stage in the evolution of a star, after the exhaustion of hydrogen, when
the star produces its energy by the fusion of helium into carbon and oxygen.

See also: → helium; → burning.

The sudden onset of → helium burning in the core of an → intermediate-mass star that has exhausted its hydrogen and has become a → red giant. With a → degenerate core, the temperature increases but the pressure does not. Therefore, the core cannot expand and cool, so the temperature continues to rise. When it approaches 100,000,000 K, helium will begin to fuse into carbon in the → triple alpha process. The helium flash ends the giant star’s ascent of the → red giant branch. However, the violent ignition of helium in the core does not increase the star’s luminosity. On the contrary, the energy released in the helium flash expands and cools the core and ultimately results in a reduction in the energy output. On the → H-R diagram the star moves down
from red giant branch to the → horizontal branch, a stable state with steady helium burning in the core.

See also: → helium; → flash.

1. The normal component of → liquid helium (⁴He) existing between the superfluid transition point (→ lambda point about 2.17 K) at 1 atmosphere of pressure and its boiling point of 4.2 K.
   

2. In stellar and interstellar plasma spectroscopy, neutral helium.

See also: → helium.

1. A → superfluid, colder component of → liquid helium (⁴He), occurring when → helium I is cooled below the → lambda point.
   

2. 2. In stellar and interstellar plasma spectroscopy, ionized helium.

See also: → helium.

A stage in the evolution of an → asymptotic giant branch star, when all the helium in the core is fused into carbon and oxygen. No more fusion takes place in the core, and as a result the core contracts. The core contraction generates a sufficient temperature for fusing the surrounding layers of helium. Since helium shell burning is unstable, it causes → helium shell flashes.

See also: → helium; → shell; → burning.

A violent outburst of energy that occurs periodically in an
→ asymptotic giant branch star. It occurs when helium is being burnt in a thin shell surrounding the inner dense core of carbon and oxygen. → Helium shell burning is unstable, producing energy mainly in short intense flashes. The shell flash causes considerable expansion of the star followed by collapse, thus setting up deep convection. As a consequence, the → convective zone in the outer part of the star goes deeper and may → dredge-up carbon to the surface. See also → late thermal pulse; → very late thermal pulse; → AGB final thermal pulse.

See also: → helium; → shell; → flash.

An → evolved star which has lost most or all of its hydrogen-rich envelope, leaving just a core of helium.

See also: → helium; → star.

A wavelength calibration of astronomical spectra using a helium-argon light source.

See also: → helium; → argon;
→ calibration.

A comparison light source containing the known spectral lines of helium and Argon.

See also: → helium; → argon;
→ lamp.

A curve which lies on a cylinder or cone, so that its angle to a plane perpendicular to the axis is constant.

Etymology (EN): From L. helix “spiral,” from Gk. helix (genitive helikos), related to eilein “to turn, twist, roll.”

Etymology (PE): Picâr “that which twists,” from
pic, present stem of picidan “to twist, entwine, coil” + -ar agent noun suffix (on the model of parastâr, padidâr, dustâr, xâstâr).

A large and bright → planetary nebula in the constellation → Aquarius. Its apparent diameter is about half the size of the full Moon, corresponding to about 2.5 → light-years for a distance of about 700 light-years. It is the nearest bright planetary nebulae to Earth and one of the most spectacular examples of such objects. The Helix Nebula possibly consists of at least two separate disks with outer rings and filaments. The brighter inner disk seems to be expanding at about 100,000 km/h and to have taken about 12,000 years to form. High-resolution observations of the inner edge of the Helix’s main ring have revealed thousands of cometary knots of gas with faint tails extending away from the central star. The knots have masses similar to the Earth, but are typically the size of our Solar system. The comet-like shape of the knots results from the steady evaporation of gas from the knots, produced by the strong winds and ultraviolet radiation from the central star of the nebula.The origin of the knots is currently not well understood.

See also: → helix; → nebula.

One of the largest identified → impact craters both on → Mars and within the → Solar System. Hellas spans more than 2000 km across in the → southern hemisphere, a region that is much more heavily cratered and higher in average elevation than the northern hemisphere. The depth of Hellas from its bottom to its inner rim is more than 4 km. In comparison, the depth of the Grand Canyon in the United States is roughly 1.6 km, that is 2.5 times smaller! The western part of the Hellas basin contains the lowest point on Mars, about 8.2 km below the Mars datum or Martian “sea level.” The formation of the impact structure is believed to have taken place in the early Noachian epoch, between 3.9 and 4.6 billion years ago (Planetary Science Institute webpage).

See also: Hellas refers to the classical name for Greece; → basin.

1. Any of various protective head coverings worn by soldiers, policemen, firemen, etc.
   

2. Something resembling a helmet in form or position. → helmet streamer.

Etymology (EN): From M.Fr. helmet, diminutive of helme “helmet,” from Frank. *helm (cf. O.H.G. helm “helmet”);
PIE base *kel- “to cover, to hide;” cf. Av. sar- “shelter;” Laki šârd “hidden, hiddenly,” šârden “to hide;” Kurd. šâr-, šârdinawa “to hide;” Skt. śárman- “cover, protection, refuge;” L. celare “to conceal;” Goth. huljan “to cover, conceal;” O.H.G. helan “to hide.”

Etymology (PE): Xud “helmet,” from O.Pers. xaudā- “hat, cap,” tigra-xauda- “wearing the pointed cap” (as is shown in the sculpture of Skunkha the Scythian at Behistan); Av. xaoδa- “hat, cap, helmet;” Ossetic xodä; Arm. (borrowed) xoir “headband.”

A large-scale → coronal feature with apparent → cusp, seen during a → solar eclipse. They usually arise from → sunspots and → active regions, so at the base of a helmet streamer one will often find a → prominence.  They form magnetic loops that connect the sunspots and suspend material above the surface of the Sun.  The magnetic field lines trap the material to form the streamers. The action of the → solar wind is at the origin of the peak feature.

See also: → helmet; → streamer.

Of a system, the quantity whose decrease gives the maximum amount of external work which is performed when any physical or chemical process is carried out reversibly at constant temperature. It is defined by F = U - TS, where U is the → internal energy, T the → absolute temperature, and S the final → entropy.

See also: After the German physicist and physician Hermann Ludwig Ferdinand von Helmholtz (1821-1894), who made important contributions to the thermodynamics of gaseous systems; → free; → energy.

A → decomposition theorem, whereby a continuous → vector field, F, can be broken down into the sum of a → gradient and a → curl term:
F = -∇φ + ∇ xA, where φ is called the → scalar potential and A the → vector potential.

See also: → Helmholtz free energy; → theorem.

A systematic trend in the motion of some → Galactic halo→ old stars thought to be a relic of the → merging of a dwarf satellite galaxy devoured by our Milky Way. Using kinematic data from the → Hipparcos satellite, Helmi et al. (1999, Nature 402, 53) found two halo star streams which share a common progenitor: a single coherent object disrupted during or soon after the Milky Way’s formation, and which probably resembled the Fornax and Sagittarius dwarf spheroidal galaxies.

See also: See also Helmi & White 1999, MNRAS 307, 495; → stream.

A service in an organization or computer system where users are directed for technical support or assistance.

Etymology (EN): M.E., O.E. help (m.), helpe (f.) “assistance, succor,” helpan “o help” (cf. O.N. hjialp, hjalpa, M.Du., Du. hulp, helpen, O.H.G. helfa, helfan, Ger. Hilfe, helfen); PIE base *kelb- “to help” (cf. Lith. selpiu “to support, help”); desk, from M.E. deske; M.L. desca, descus “table to write on,” from L. discus “quoit, platter, dish,” from Gk. diskos “disk, dish.”

Etymology (PE): Yâigâh, from yâri “help, assistance,” → gravity assist,

• -gâh “place; time,” → station.

A mineral that is often found in meteorites. It is an oxide of iron (Fe₂O₃) that is similar to magnetite. It does not attract a magnet. When it is rubbed against an object harder than itself, it leaves a reddish-brown stain. Hematite is also sometimes called bloodstone.

See also: From M.Fr. hematite, from L. hæmatites, from Gk. haimatites lithos “bloodlike stone,” from haima (genitive haimatos) “blood”

• -ites, → -ite, + lithos “stone.”

A defect of the eyes in which sight is normal in the night or in a dim light but is abnormally poor or wholly absent in the day or in a bright light. Also called day blindness. Opposite of → nyctalopia

Etymology (EN): From N.L., from Gk hemeralop- (stem of hemeralops having such a condition, from hemer(a) “day” + al(aos) “blind”

• -ops having such an appearance) + -ia a noun suffix.

Etymology (PE): Ruzkuri, from ruz, → day, + kuri “blindness,” from kur, → blind.

Half of a sphere bounded by a great circle, especially one of the halves into which the earth or the celestial sphere is divided.

Etymology (EN): From L. hemisphærium, from Gk. hemisphairion, from hemi- “half,” (from PIE base *semi-; cf. Skt. sami, L. semi-, O.H.G. sami- “half,” and O.E. sam-) + sphaira, → sphere.

Etymology (PE): From nim-, → half + sepehr, koré,
→ sphere.

A catalog of stars in which every star is classified by its stellar spectrum. This system is named for the astronomer Henry Draper, but was cataloged by Annie J. Cannon (225,300 stars), and later extended by Margaret W. Mayall.

See also: Henry Draper (1837-1882), an American pioneer of astronomical spectroscopy who established the observing techniques and program for the work that would bear his name when published, seven years after his early death; → system.

A powerful numerical technique to solve the stellar structure equations where the star is sub-divided in a finite number of grid cells for which the local conditions are evaluated and computed from the surface inwards to the center by utilizing a Newton-Raphson solver. Relevant physical quantities are either defined at the cell boundaries or as mean values over the complete cell.

See also: Henyey, L. G.; Forbes, J. E.; Gould, N. L., 1964, ApJ 139, 306; → method.

A nearly horizontal path on the → Hertzsprung-Russell diagram that a → pre-main sequence star of small mass follows in an early stage of evolution after leaving the → Hayashi track and before reaching the → main sequence. During this stage the pre-main sequence star remains almost wholly in radiative equilibrium.

See also: After Louis George Henyey (1910-1970), American astronomer. Henyey et al. (1955, PASP 67, 154).

A combining form meaning “seven.”

Etymology (EN): From Gk. hepta “seven;” cognate with L. septem; Pers. haft, as below; Du. zeven, O.H.G. sibun, Ger. sieben, E. seven.

Etymology (PE): Haft-, from haft “seven;” Mid.Pers. haft; Av. hapta; cf. Skt. sapta; Gk. hepta, L. septem; PIE *septm.

A → polygon with seven → angles and seven → sides.

See also: → hepta-; → -gon.

A young → A-type or → B-type star showing → emission lines in its spectrum. Herbig AeBe stars are → pre-main sequence stars of
→ intermediate mass (→ intermediate-mass star). They are often called the higher mass counterparts of → T Tauri stars.

See also: Named after George H. Herbig (1920-2013), who first classified them (Herbig 1960, ApJS 4, 337); → A star; → B star; e indicating
→ emission.

A small patch of → nebulosity in a → star-forming region, created when fast-moving → jets of material (with speeds up to about 1000 km per sec) from a newborn star collide with the → interstellar medium.

See also: → Herbig AeBe star; Guillermo Haro (1913-1988), who first in 1940s studied these objects in detail and recognized that they were a by-product of the star formation process; → object.

An ancient → constellation (right ascension about 17h, declination 30° north), one of the largest in the sky, which is located between → Lyra and → Corona Borealis. It is traditionally depicted as the hero Hercules in a kneeling position. There are no very bright stars in Hercules, the brightest one is → Rasalgethi, a variable → red supergiant of magnitude about 3.5. Abbreviation: Her; Genitive: Herculis.
See also: → Hercules cluster.

Etymology (EN): L. Hercules, from Gk. Heracles “glory of Hera,” the most popular hero of Gk. mythology, son of Zeus and the woman Alcmena, who the god seduced in the shape of her husband Amphitryon, king of Thebes.

Etymology (PE): Herâkles, as above; Herkul, from Fr. Hercule, as above; Arabicized name of the constellation:

(الجاثی، الجاثی علی‌رکبتیه) “the kneeling one,” râqes (راقص) “the dancing one.”
Bar zânu nešasté “the kneeling one,” Pers. descriptive rendering of the Gk. mythological figure, by the famous 11-th century astronomer Biruni.

A small, irregular → cluster of galaxies with fewer than 100 galaxies in its core.
It has no strongly dominant central galaxy and is notable for the high proportion of spirals. It lies some 500 million → light-years away in the constellation → Hercules; also known as Abell 2151.

See also: → Hercules; → cluster.

→ inheritable.

See also: → inherit; → -able.

1. Passing, or capable of passing, naturally from parent to offspring through the genes: Blue eyes are hereditary in our family.
   

2. Of or relating to inheritance or heredity (Dictionary.com).

See also: Of or relating to → heredity.

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

Etymology (EN): M.E., from M.Fr. hérédité, from O.Fr. eredite “inheritance, legacy,” from L. hereditatem (nominative hereditas) “heirship, inheritance,” → heritage.

Etymology (PE): Rigandâšt, literally “possessing heritage,” from rigan, → heritage, + dâšt past stem of dâštan “to have, hold, possess, maintain,” → property.

→ inheritable.

See also: → inherit; → -able.

1. Something inherited at birth, such as personal characteristics, status, and possessions.
   

2. Anything that has been transmitted from the past or handed down by tradition (Dictionary.com).

Etymology (EN): M.E. from M.Fr., from O.Fr. iritage, eritage, heritage “heir; inheritance, ancestral estate, heirloom,” from heriter “inherit,” from L.L. hereditare, ultimately from L. heres (genitive heredis) “heir, heiress,” from PIE root *ghe- “to be empty, left behind” (related Gk. word khera “widow”).

Etymology (PE): Rigan from rig “left, abandoned” (in mordé rig “heritage, effects of a dead person, anything hereditary, heirloom”) + noun suffix -an (as in rowzan, rowšan, suzan, rasan, zaqan, hâvan, etc.); ultimately from Proto-Ir. *raic- “to leave, abandon;” cf. Av. raēc- “to leave, let;” Mid.Pers. (+ *pati-) phryz-, Mod.Pers. parhêz, parhiz “to keep away from, abstain, avoid;” Khotanese (+ *fra-) hars- “to be left, remain;” Mod.Pers. rištan “to set at liberty, absolve;” Mid.Pers. (+ *ui-) wirēz-, Mod.Pers. gurēz, goriz, gurēxtan, gorixtan “to flee, run away;” Gk. leipein “to leave;” L. linquere “to leave;” PIE *leik^w- “to leave, let” (Cheung 2006).

Biology: An individual, animal, or plant possessing both male and female reproductive organs.

Etymology (EN): From L. hermaphroditus, from Gk. hermaphroditos the mythical son of Hermes and Aphrodite who merged bodies with a naiad and thereafter possessed both male and female qualities.

Etymology (PE): Narmâde, literally “male-female,” → male, → female.

Biology: For an animal or plant, the condition of having both male and female reproductive tissue or organs.

See also: → hermaphrodite; → -ism.

Of or related to hermeneutics, interpretative; explanatory. Also hermeneutical.

See also: → hermeneutics.

The science or art of → interpretation. Originally the term was limited to the interpretation of the Scriptures, but since the nineteenth century it has developed into a general theory of human understanding through the work of Friedrich Schleiermacher (1768-1834), Wilhelm Dilthey (1833-1911), and others. The comprehension of any written text requires hermeneutics. Many different hermeneutic theorists have proposed many different methodologies.

Etymology (EN): From Gk. hermeneutikos “interpreting,” from hermeneutes “interpreter,” from hermeneuein “to interpret,” of unknown origin. It was formerly thought to derive from Hermes, the tutelary divinity of speech, writing, and eloquence.

Etymology (PE): Âzand-pardâzik, from âzand, → interpretation,

• pardâz, present stem of pardâxtan “to accomplish, bring to perfection; to care,” → theoretician, + -ik, → ics.

Someone who interprets literary or scriptural texts.

See also: Agent noun from → hermeneutics

Math.: The Hermitian conjugate of an m by n matrix A is the n by m matrix A^* obtained from A by taking the → transpose and then taking the complex conjugate of each entry. Also called adjoint matrix, conjugate transpose. → Hermitian operator.

See also: Hermitian, named in honor of the Fr. mathematician Charles Hermite (1822-1901), who made important contributions to number theory, quadratic forms, invariant theory, orthogonal polynomials, elliptic functions, and algebra. One of his students was Henri Poincaré; → conjugate.

An operator A that satisfies the relation A = A, where
A is the adjoint of A. → Hermitian conjugate.

See also: → Hermitian conjugate; → operator.

Sir William Herschel (1738-1822), German-born English astronomer, the discoverer of the → infrared radiation and planet → Uranus.
→ Herschelian telescope, → Herschel Satellite

A European Space Agency (ESA) mission to perform imaging photometry and spectroscopy in the → far infrared and → submillimeter regions of the electromagnetic spectrum, covering approximately the 55-672 µm range. In fact Herschel is the first space facility dedicated to these wavelength ranges. It carries a 3.5 m diameter passively cooled mirror. The science payload complement - two cameras/medium resolution spectrometers (PACS and SPIRE) and a very high resolution → superheterodyne spectrometer (HIFI) - are housed in a superfluid helium cryostat. Herschel was launched on 14 May 2009, together with the → Planck Satellite. Its observing position lies at the L2 → Lagrangian point, some 1.5 million km from Earth. Herschel is designed, among other things, to study the formation of galaxies in the early Universe, and to investigate the formation of stars and their interaction with the → interstellar medium.

See also: → Herschel; → satellite.

A → reflecting telescope in which the → primary mirror is tilted so that light is focused near one side of the open end of the tube. The → eyepiece then picks up this light directly, avoiding light loss from reflection by a → secondary mirror. The drawback is → astigmatism, unless the → focal ratio is large. Herschel used this design in his giant 48-inch instrument.

See also: → Herschel; → telescope.

The SI unit of frequency, defined as a frequency of 1 cycle per second.

See also: After Heinrich Rudolf Hertz (1857-1894), the German physicist, who made several important contributions to the study of electromagnetism.

A laboratory experiment carried out by Heinrich Hertz in 1888 to generate and detect
→ electromagnetic waves for the first time. It involved a
high voltage power source, consisting of two → capacitors,
each provided with a conducting rod. The rods were separated by a small → spark gap and connected to an → induction coil. When the electrodes were raised to a sufficiently high → potential difference, a spark passed across the gap,
and an oscillating discharge took place. A group of waves with a wavelength of a few meters were emitted at each discharge. A wire loop provided with a detecting spark gap, held away from the oscillating sparks, produced sparks upon arrival of the oscillating electric and magnetic fields.

See also: → hertz (Hz); → experiment.

→ frequency to wavelength conversion.

See also: → hertz; → meter; → conversion.

An electrical system used for the production of → electromagnetic waves. It consists of two equal → capacitors connected to two electrodes with a → spark gap between the electrodes. The system is connected to an → induction coil. When the induction coil is activated, electromagnetic waves are generated across the spark gap. See also → Hertz experiment.

See also: → hertz (Hz); → oscillator.

A region of the → Hertzsprung-Russell diagram, between the → main sequence and the
→ giant branch, occupied by very few stars. It corresponds to a very short period in stellar evolution.

See also: Named after the Danish astronomer Ejnar Hertzsprung (1873-1967), who first noticed this phenomenon; → gap

A display of stellar properties using a plot of
→ effective temperature (or instead → color or → spectral type) along the abscissa versus
→ luminosity (or → absolute magnitude). The temperature is plotted in the inverse direction, with high temperatures on the left and low temperatures on the right. On the diagram the majority of stars are concentrated in a diagonal strip running from upper left to lower right, i.e. from high temperature-high luminosity → massive stars to low temperature-low luminosity → low-mass stars. This feature is known as the → main sequence. This is the locus of stars burning hydrogen in their cores (→ proton-proton chain). The lower edge of this strip, known as the → zero age main sequence (ZAMS), designates the positions
where stars of different mass first begin to burn hydrogen in their cores. Well below the main sequence there is a group of stars that, despite being very hot, are so small that their luminosity is very small as a consequence. These are the class of → white dwarfs. These objects represent old and very evolved stars that have shed their outer layers to reveal a very small but extremely hot inner core. They are no longer generating energy but are merely emitting light as they cool (→ white dwarf cooling track). Stars with high luminosities but relatively low temperatures occupy a wide region above the main sequence. The majority of them have used up all the hydrogen in their cores and have expanded and cooled as a result of internal readjustment. Called → red giants, they are still burning helium in their cores (→ helium burning, → carbon burning). There are also stars with very high luminosities, resulting from their enormous outputs of energy, because they are burning their fuel at a prodigious rate. These are the → supergiants. They can be hot or cool, hence blue or red in color. Same as → H-R diagram.

See also:
→ asymptotic giant branch, → blue horizontal branch star, → extreme horizontal branch star, → field horizontal branch star, → Hayashi track, → horizontal branch, → post-asymptotic giant branch star, → red giant branch, → supra-horizontal branch star, → zero age horizontal branch star, → Humphreys-Davidson limit.

See also: Named after the Danish Ejnar Hertzsprung (1873-1967) and the American Henry Norris Russell (1877-1957). However,
the first H-R diagram was published not by Hertzpurung neither Russell, but by a PhD student of Karl Schwarzschild at Göttingen. The student was Hans Rosenberg (1879-1940), who in 1910 published the diagram for stars in the → Pleiades (Astronomische Nachrichten, Vol. 186 (4445), p. 71, 1910). Although Hertzpurung had a very preliminary diagram in 1908, his first proper diagram was published in 1911. Likewise, Russell published his version only in 1915 with the better and more numerous data then available (Nielsen, A.V., 1969, Centaurus 9, 219; Valls-Gabaud, D., 2002, Observed HR diagrams and stellar evolution, ASP Conf. Proceedings, Vol. 274. Edited by Thibault Lejeune and João Fernandes);
→ diagram.

The Martian geologic era after the Noachian Era which lasted from about 3500 million to 2500 million years ago. During this period Martian climate began to change to drier, dustier conditions. Water that flowed on the Martian surface during the Noachian Era may have frozen as underground ice deposits, and most river channels probably experienced their final flow episodes during this era. → Noachian era; → Amazonian era.

See also: Named after the Martian plains of Hesperis; → era.

An → evening star, especially the planet Venus in its appearance as the evening star.

Etymology (EN): M.E., from L., from Gk. hesperos “evening, western;” → west.

Etymology (PE): Setâre-ye šâmgâh “evening star,” from setâré→ star + šâmgâh “evening,” from šâm “evening, evening meal” + gâh “time.” The first component, šâm, from Mid.Pers. šâm “evening meal, supper,” from Av. xšāfnya- “evening meal,” from Av. xšap-, xšapā-, xšapan-, xšafn- “night” (O.Pers. xšap- “night,” Mid.Pers. šap, Mod.Pers. šab “night”); cf. Skt. ksap- “nigh, darkness;” Hittite ispant- “night.” The second component gâh “time,” Mid.Pers. gâh, gâs “time,”
O.Pers. gāθu-, Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come.”

→ High Energy Stereoscopic System (H.E.S.S.).

See also: → H.E.S.S.; → collaboration.

A diagram showing the relative density of occurrence of stars at various → color-magnitude positions of the → Hertzsprung-Russell diagram for a given → galaxy.

See also: Named after R. Hess who originated it in 1924: “Die Verteilungsfunktion der absoluten Helligkeiten in ihrer Abhängigkeit vom Spektrum”. Probleme der Astronomie. Festschrift fur Hugo v. Seeliger. Springer, Berlin. p. 265; → diagram.

Prefix denoting “other, different.”

Etymology (EN): From Gk. heteros “the other (of two), another, different.”

Etymology (PE): Degar “another, other;” from Mid.Pers. dit, ditikar “the other, the second;” O.Pers. duvitiya- “second,” Av. daibitya-, bitya- “second;” Skt. dvitiya- “second,” PIE *duitiio- “second.”

1. Denoting a device or method of combining two → electromagnetic waves of different → frequency (a locally generated wave and an incoming wave)
   in a → nonlinear device to produce two frequencies which are
   equal to the → sum and → difference of the first two. The phenomenon is the counterpart of → beats produced by → sound waves. For example, heterodyning a 100-kHz and a 10-kHz signal will produce a 110-KHz and a 90-kHz signal. See also → homodyne.
   

2. The term heterodyne is often loosely used instead of
   → superheterodyne in the radio frequency field.

See also: Heterodyne, from → hetero- + -dyne, from Gk. dynamics→ dynamics; → receiver.

An → interferometer using a technique that involves introducing a small → frequency shift between the optical frequencies of the two interfering light beams. This results in an intensity modulation at the → beat frequency of the two beams for any given point of the → interference pattern. A convenient way of introducing such a frequency shift is by means of an acousto-optic modulator.

See also: → heterodyne; → interferometer.

→ superheterodyne receiver.

See also: → heterodyne; → receiver.

→ superheterodyne technique.

See also: → heterodyne; → technique.

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

See also: Noun from → heterogeneous.

1. Composed of parts of different kinds; having widely dissimilar elements or constituents. See also → homogeneous, → inhomogeneous.
   

2. Chemistry: A mixture that does not have uniform composition and properties throughout; composed of different substances or the same substance in different phases.

See also: → hetero- + -genous, → homogeneous.

Based on chemical composition, the atmosphere is divided into two broad layers: the → homosphere and the heterosphere. The heterosphere has heterogeneous chemical composition, with layered structure, of nitrogen, oxygen, helium, and hydrogen, respectively.
The heterosphere begins from about 90 km from the Earth’s surface and extends to space.

See also: → hetero-; → shere.

Methodology, Math.: Pertaining to a method of analyzing outcome through comparison to previously recognized patterns in the absence of an → algorithm for formal proof.

Etymology (EN): From L. heuristicus (from Gk. heuretikos “inventive,” related to heuriskein “to find,” from heur-) + -isticus, → -ic.

Etymology (PE): Yâftik, from yâft past tense of yâftan, yâb- “to → find” + -ik, → -ic.

A prefix meaning → six. → hexagon.

See also: → six.

A six-sided → polygon.

See also: → hexa-; → -gon;.

A → chemically peculiar star of late → B-types. The most distinctive features of HgMn stars are extreme atmospheric overabundance of Hg (up to 5 dex) and of Mn (up to 3 dex). The origin of abundance anomalies observed in late B-type stars with HgMn peculiarity is still poorly understood. More than two thirds of the HgMn stars are known to belong to spectroscopic binaries with a preference of orbital periods ranging from 3 to 20 days (Hurbig et al., 2012, arXiv:1208.2910).

See also: Hg, → mercury; Mn, → manganese; → star. mercury-manganese

A list of 100 compact groups of galaxies that were identified by a systematic search of the → Palomar Observatory Sky Survey red prints. Each group contains four or more galaxies, has an estimated mean surface brightness brighter than 26.0 magnitude per arcsec² and satisfies an isolation criterion.

See also: Hickson, Paul, 1982, ApJ 255, 382; → compact; → group.

Being out of sight; concealed.

Etymology (EN): From M.E., from O.E. hydan, from W.Gmc. *khuthjanan, from PIE keudh- (cf. Gk. keuthein “to hide, conceal”), from base (s)keu- “to cover, conceal.”

Etymology (PE): Penhân “hidden,” from Mid.Pers. pad nihân, from pad “to, at, for, in”
(from O.Pers. paity; Av. paiti “to, toward, in, at;” cf.
Skt. práti; Gk. poti) + nihân “concealment, secrecy, hiding place” (Mod.Pers. nahân), from Proto-Iranian *ni-dāna-, from ni- “down; into,” → ni- (PIE),

• dā- “to put; to establish; to give” (dadâiti “he gives;” cf. Skt. dadâti “he gives;” Gk. didomi “I give;” L. do “I give;” PIE base *do- “to give”).

Same as → missing mass, → non-luminous matter, or → dark matter.

See also: → hidden; → mass.

A theory based on the hypothesis that the discrepancies with respect to classical reality found in → quantum mechanics stem from our lack of knowledge about the observed system (→ EPR paradox). According to this hypothesis, the system should be described by additional quantum parameters, of still unknown nature, but different from position, velocity, spin, etc. The hidden variable theory has been ruled out by the violation of → Bell’s inequality for all theories with local property, as suggested by the → Aspect experiment.

See also: → hidden; → variable.

Of, belonging to, or characteristic of a hierarchy. → hierarchical clustering;
→ hierarchical cosmology; → hierarchical multiple system; → hierarchical structure formation.

See also: → hierarchy; → -al.

A model in which a system of self-gravitating particles will gradually aggregate into larger and larger gravitationally bound groups and clusters.

See also: → hierarchical; → clustering.

A cosmology characterized by clustering of galaxy clusters in increasingly larger systems.

See also: → hierarchical; → cosmology.

A → multiple star system in which the stars can be divided into two groups, each of which traverses a larger orbit around the system’s center of mass. Each of these smaller groups must also be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, and so on. Hierarchical multiple systems have long-term dynamical stability.

See also: → hierarchical; → multiple; → system.

A cosmological → structure formation model in which the smallest gravitationally bound structures (→ quasars and galaxies) form first, followed by → groups, → galaxy clusters, and → superclusters of galaxies.

See also: → hierarchical; → structure; → formation.

A triple star system in which the (inner) binary is orbited by a third body in a much wider orbit. → hierarchical multiple system.

See also: → hierarchical; → stellar; → system.

A system in which the components are organized in increasingly larger structures.

Etymology (EN): From O.Fr. ierarchie, from M.L. hierarchia “ranked division of angels,” from Gk. hierarchia “rule of a high priest,” from hierarches “high priest, leader of sacred rites,” from ta hiera “the sacred rites” (neut. pl. of hieros “sacred”) + archein “to lead, rule.”

Etymology (PE): Pâygân, from pâyé “step, rank, degree,” from pây, pâ “foot, step,” from
Mid.Pers. pâd, pây; Av. pad- “foot” (cf. Skt. pat; Gk. pos, gen. podos; L. pes, gen. pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + -gân suffix forming plural entities, from Mid.Pers. -gânag, -gâna, from Proto-Iranian *kāna-ka-.

A hypothetical, neutral → elementary particle which plays a key role in the → standard model
of → particle physics. This massive particle, whose mass is estimated to be about 125 GeV (→ giga → electron-volts)
and a zero → spin, carries the → Higgs field. In the current version of the → electroweak theory,
→ W boson and → Z boson and all the fundamental constituents (→ quarks and → leptons) get their masses by interacting with the Higgs boson.
The Higgs boson is produced by the fusion of two → gluons via a triangular loop of virtual top quarks. In the decay process, a loop of virtual top quarks allows the Higgs boson to decay into two photons. The particle’s discovery was announced by → CERN in July 2012.

See also: Named after the Scottish physicist Peter Ware Higgs (1929-), one of the researchers who theorized the existence of this particle in 1964. In fact three groups of physicists almost simultaneously published their results on this subject:
François Englert and Robert Brout in August 1964; Peter Higgs in October 1964;
and Gerald Guralnik, Carl Hagen, and Tom Kibble in November 1964; → boson.

A → scalar field supposed to be responsible for the genesis of → inertial mass. According to the → standard model of → particle physics, the Higgs field appeared
10^-36 to 10^-12 seconds after the → Big Bang, during the → electroweak epoch, when the temperature dropped below a critical threshold. The Higgs field permeates all space, and through its interaction with the fundamental particles it provides those particles with a mass. Any particle that does not interact with the Higgs field, such as the → photon,
will be mass-less.

See also: → Higgs boson; → field.

In the → standard model of → particle physics, a mechanism postulated to endow mass to
→ elementary particles. Simply put, a background field, called the → Higgs field, becomes locally distorted whenever a particle moves through it. The distortion generates the particle’s mass.

See also: → Higgs boson; → mechanism.

1. Situated above the ground or exceeding the common degree or measure.
   

2. Exceeding the common degree or measure; strong; intense.
   

3. Meteo.: An area of high pressure, referring to a maximum of atmospheric pressure. Same as → anticyclone (Fr. haute pression).

Etymology (EN): M.E. heigh, variants hegh, hey, heh; O.E. heh, heah “of great height, lofty, tall,” (cf. Du. hoog, O.H.G. hoh, Ger. hoch, Goth. hauhs “high;” also Ger. Hügel “hill”); from PIE *koukos “hill.”

Etymology (PE): Boland “high,” variants bâlâ “up, above, high, elevated, height,” borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz),
Lori dialect berg “hill, mountain;” Mid.Pers. buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high.”

Meh “great, large” (Mid.Pers. meh, mas, Av. maz-, masan-, mazant- “great, important,” mazan- “greatness, majesty,” mazišta- “greatest,” cf. Skt. mah-, mahant-, Gk. megas, L. magnus; PIE *meg- “great”).
Por “much, very, too much; full” (Mid.Pers. purr “full;” O.Pers. paru- “much, many;” Av. parav-, pauru-, pouru-, from
par- “to fill;” PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru-; Gk. polus;
O.E. full).

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).

See also: → high; → accuracy; → radial; → velocity; → planet; → search; → -er.

An array of → IACT telescopes for studying
cosmic → gamma rays in the 100 GeV to 100 TeV energy range.
The HESS observatory is located in Namibia, southern Africa, at an altitude of 1800 m, and the project is an international collaboration of more than 100 scientists from nine countries. In its Phase I, HESS used four telescopes each consisting of a light collector with a diameter of 13 m and a focal length of 15 m placed at the corners of a square 120 m apart. Each telescope is segmented into 380 round mirror facets of 60 cm diameter and uses a camera consisting of 960 closely packed → photomultiplier tubes. The first of the telescopes went into operation in Summer 2002. Phase II includes a fifth
telescope, called Large Cherenkov Telescope (LCT), of 27 m diameter, located in the centre of the initial array. This upgrade lowers the triggering threshold of the HESS array to about 20 GeV, thus broadening the energy window in which gamma-ray astronomy can be done, opening up more opportunities in astrophysical research (see, e.g., Bernlöhr et al. 2003, Astroparticle Physics 20, 111).

See also: H.E.S.S., short for High Energy Stereoscopic System, is also intended to pay homage to Victor F. Hess (1883-1964), an Austrian-American physicist who received the Nobel Prize in Physics in 1936 for his discovery of → cosmic rays.

The latitude belt roughly between 60 and 90 degrees North and South. Also referred to as the polar region.

See also: → high; → latitude.

A galaxy or quasar having a → redshift larger than about 0.8, corresponding to a → look-back time half the present age of the Universe. The qualifier “high” is, however, relative and depends on context and authors’ assessment.

See also: → high; → redshift;
→ object.

The state of the → tide when at its highest level.

Etymology (EN): → high; → tide.

Etymology (PE): Owpiš, from Persian Gulf dialects, literally “forward water,” from ow, variant of âb, → water, + piš “→ forward.”
Madd, loan from Ar.

Also known as → high tide.

See also: → high; → water.

A branch of astrophysics that deals with objects emitting highly energetic radiation, such as X-ray astronomy, gamma-ray astronomy, and extreme ultraviolet astronomy, as well as neutrinos and cosmic rays.

See also: → high; → energy; → astrophysics.

Cosmic rays which typically have energies in the range 10¹⁵ to 10²⁰ electron volts. For the most part, they are protons and other atomic nuclei, and come from distant cosmos, perhaps even from outside our own Galaxy.

See also: → high; → energy; → cosmic; → ray.

A neutrino produced in high-energy particle collisions, such as those occurring when → cosmic rays strike atoms
in the Earth’s → atmosphere. Their energy range expands from a few → MeVs up to tenths of a → peta- (P) → electron-volts.

See also: → high; → energy; → neutrino.

A rare class of → H II regions in the → Magellanic Clouds. In contrast to the typical H II regions of the Magellanic Clouds, which are extended structures (sizes of several arc minutes corresponding to more than 50 pc, powered by a large number of exciting stars), HEBs are very dense and small regions (~ 4" to 10" in diameter corresponding to ~ 1-3 pc). They have a higher degree of → excitation ([O III] 5007Å /Hβ) with respect to the typical H II regions, and are, in general, heavily affected by local → dust. They are powered by a relatively smaller number of → massive stars.

See also: → high; → excitation; → blob.

A star whose mass exceeds 8 solar masses. Same as → massive star. → intermediate-mass star; → low-mass star.

See also: → high; → mass; → star.

A member of one of the two main classes of → X-ray binary systems where one of the components is a neutron star or a black hole and the other one a → massive star. HMXBs emit relatively → hard X-rays and usually show regular pulsations, no X-ray bursts, and often X-ray eclipses. Their X-ray luminosity is much larger than their optical luminosity. In our Galaxy HMXBs are found predominantly in the → spiral arms and within the → Galactic disk in young → stellar populations less than 10⁷ years old. One of the most famous HMXB is Cygnus X-1 which was the first stellar-mass black hole discovered. See also: → low-mass X-ray binary.

See also: → high; → mass; → X-ray; → binary.

A laser beam with the output power in the range 10¹²-10¹⁵
watts/cm², capable of depositing kilo-joule order energies during nano-second time intervals in small volumes (about 1 mm³). High power lasers, which can produce temperatures of 10-50 million degrees and pressures of 10-100 million bars, are used to simulate astrophysical conditions in laboratories.

Etymology (EN): → high; → power;, → laser.

Etymology (PE): leyzer, → laser; por “much, many, full,” → full; tavân, → power.

An observation that provides a particularly narrow, peaked image of a point source. → point spread function.

See also: → high; → resolution;
→ observation.

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.

See also: → high; → velocity; → cloud.

A mountainous or elevated region; → plateau.

Etymology (EN): → high; → land.

Etymology (PE): Kuhsâr “mountainous, hilly area,” from kuh, → mountain, + -sâr suffix denoting profusion, abundance, variant -zâr, → catastrophe.

A → chemical element that is → geochemically characterized as having a strong → affinity to partition into → metals relative to → silicates.

The highly siderophile elements, → ruthenium (Ru), → rhodium (Rh), → palladium (Pd), → rhenium (Re), → osmium (Os), → iridium (Ir), → platinum (Pt), and → gold (Au), are of interest to planetary scientists because they give insights into the early history of → accretion and → differentiation. HSEs prefer to reside in the metal of planetary cores. Therefore, the HSEs found in planetary → mantles are considered to be overabundant relative to their known preferences for metal over silicate. Therefore, it has been inferred that processes other than → equilibrium partitioning have been responsible for establishing the abundances of → mantle siderophiles. A detailed understanding of the absolute → concentrations and relative abundances of the HSEs may therefore give important insights into the earliest history of a planet (Jones et al., 2003, Chemical Geology 196, 21).

Etymology (EN): From Gk. sidero-, from sideros “iron” + → -phile.

Etymology (PE): Âhandust, from âhan, → iron, + -dust, → -phile.

1. A long walk or march for recreational activity, military training, or the like.
   

   2. To walk or march a great distance, especially through rural areas, for pleasure, exercise, military training, or the like (Dictionary.com).

Etymology (EN): From E. dialectal hyke “to walk vigorously,” maybe a Northern form of hitch “to move or draw (something) with a jerk,” of unknown origin.

Etymology (PE): Vaniž, from Sangesari wəniž-/wəništ “to walk about, go round;” cf. Shughni näγ-, Roshani niγ-, naγên- “to turn round;” Book Pahlavi/Zoroastrian Mid.Pers. nâz-, nâž- “to roll, turn;” Mid.Pers. nâys- “be proud, delicate.”

A generalization of Euclidean space in a way that extends methods of vector algebra from the two- and three-dimensional spaces to infinite-dimensional spaces.
Multi-dimensional space in which the eigenfunctions of quantum mechanics are represented by orthogonal unit vectors.

See also: Named after the German mathematician David Hilbert (1862-1943), recognized as one of the most influential mathematicians of the 19th and early 20th centuries for his numerous contributions to various areas of mathematics; → space.

The asteroids found on the outer edge of the main asteroid belt in a 2:3 orbital resonance with Jupiter. The group is not an asteroid family since the members are not physically related. The group
consists of asteroids with semi-major axes between 3.70 AU and 4.20 AU, eccentricities less than 0.30, and inclinations less than 20°. It is dominated by D- and P-type asteroids.

See also: Named for the prototype 153 Hilda, discovered by
Johann Palisa (1848-1925) on November 2, 1875, and named Hilda after a daughter of his teacher, the astronomer Theodor von Oppolzer (1841-1886); → asteroid.

A natural elevation of the earth’s surface, smaller than a mountain.

Etymology (EN): M.E.; O.E. hyll, from P.Gmc. *khulnis (cf. M.Du. hille, Low Ger. hull “hill,” O.N. hallr “stone,” Goth. hallus “rock,” O.E. holm “rising land, island”), from PIE base *kel- “to rise, to be prominent” (cf. Skt. kuta- “summit, peak;” Mod.Pers. kutal, kotal high hill, the skirts of a hill;" Tabari dialect keti “hill; top of the head;
L. collis “hill,” culmen “top, summit,” cellere “raise,” celsus “high;” Gk. kolonos “hill,” kolophon “summit;” Lith. kalnas “mountain,” kalnelis “hill”).

Etymology (PE): Tappé “hill.”

The spherical region around a → secondary in which the secondary’s gravity is more important for the motion of a particle about the secondary than the tidal influence of the → primary. The radius is described by the formula: r = a (m/3M)^1/3, where, in the case of the Earth, a is the semi-major axis of the orbit around the Sun, m is the mass of Earth, and M is the mass of the Sun. The Hill sphere for the Earth has a radius of 0.01
→ astronomical units (AU).
Therefore the Moon, lying at a distance of 0.0025 AU, is well within the Hill sphere of the Earth.

See also: Named for George William Hill (1838-1914), an American astronomer who described this sphere of influence; → sphere.

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).

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

A process in which a → close encounter between a → tightly bound binary star system and a → supermassive black hole causes one binary component to become bound to the black hole and the other to be ejected at very high velocities, up to 4,000 km s^-1. → hypervelocity star.

See also: Hills, J. G, “Hyper-velocity and tidal stars from binaries disrupted by a massive Galactic black hole,” Nature 331, 687; → mechanism.

The tenth of Jupiter’s known satellites, 186 km in diameter revolving at a mean distance of 11,480,000 km from Jupiter. Discovered in 1904 by the Argentine-American astronomer Charles Dillon Perrine (1867-1951).

See also: Himalia was a nymph of the island of Rhodes. She was seduced by the god Zeus (Jupiter).

Same as → Indian numeral system.

See also: → numeral; → system.

Etymology (EN): O.E. hype “hip,” akin to Du. heup, O.H.G. huf, Ger. Hüfte, Swed. höft, Goth. hups “hip,” of uncertain origin.

Etymology (PE): Šanj (Dehxodâ) “hip, buttock, thigh, haunch,” of unknown origin.

A → European Space Agency satellite, which was launched in August 1989 and operated until March 1993. It was the first space mission devoted to → astrometry with an unprecedented degree of accuracy. The telescope on Hipparcos had a main mirror of diameter 29 cm. Calculations from observations by the main instrument
generated the Hipparcos Catalogue of 118,218 stars charted with the
highest precision (published in 1997) containing positions, distances, → parallaxes, and → proper motions. An auxiliary star mapper pinpointed many more stars with lesser but still unprecedented accuracy, in the Tycho Catalogue of 1,058,332 stars. The Tycho 2 Catalogue, completed in 2000, brings the total to 2,539,913 stars, and includes 99% of all stars down to magnitude 11.
→ Gaia.

See also: Hipparcos, acronym for → High  → Precision  → Parallax  → Collecting → Satellite, chosen for its similarity to the name of the Greek astronomer Hipparchus of Nicaea (c. 190-125 BC), one of the most influential astronomers of antiquity, who compiled an extensive star catalogue in which he gave the positions of over 1,000 stars and also classified them according to their magnitude (on a scale of 1 to 6, brightest to faintest). Ptolemy later incorporated this information into his → Almagest. In addition, he discovered the → precession of the equinoxes.

The smallest known moon orbiting the planet → Neptune, discovered in 2013. Hippocamp has an estimated diameter of only about 34 km and orbits close to → Proteus, the outer and the second largest of Neptune’s moons. The orbital → semi-major axes of the two moons differ by only 10%. Hippocamp is probably an ancient fragment of Proteus.
Billions of years ago a comet collision would have chipped off a chunk of Proteus. Images from the Voyager 2 space probe from 1989 show a large → impact crater on Proteus, whose size compares with Hippocamp’s (Showalter et al., 2019, Nature 566, 350).

See also: Formerly known as S/2004 N 1, Hippocamp is named after the sea creatures in Greek and Roman mythology. The mythological Hippocampus possesses the upper body of a horse and the lower body of a fish. The Roman god Neptune would drive a sea-chariot pulled by Hippocampi.

A curve described by the → polar equation  r² = 4b (a - b sin²θ), where a and b are positive constants. For appropriate
values of a and b, the curve looks like the infinity symbol, ∞. See also → spheres of Eudoxus.

See also: Hippopede, literally “a horse’s foot,” denoting a “horse fetter (hobble),” from Gk. hippos, → horse, + -pede variant of -ped, combining form of pos,→ foot.

A type of graphical representation, used in statistics, in which frequency distributions are illustrated by rectangles.

Etymology (EN): Histogram, from Gk. histo-, a combining form meaning “tissue,”
from histos “mast, loom, beam, warp, web,” literally “that which causes to stand,” from histasthai “to stand,” from PIE *sta- “to stand” (cf. Pers. ist-, istâdan “to stand;” O.Pers./Av. sta- “to stand, stand still; set;”
Skt. sthâ- “to stand;” L. stare “to stand;” Lith. statau “place;” Goth. standan); → -gram.

Etymology (PE): Nemudâr, → diagram + sotuni “column-like,” from sotun “column,” from Mid.Pers. stun, from O.Pers. stênâ “column,” Av. stuna-, Skt. sthuna- “column.”

Of, pertaining to, treating, or characteristic of → history or past events (Dictionary.com). → historical supernova.

See also: → history; → -al.

A supernova event recorded in the course of history before the invention of the telescope. The well recorded supernovae of this small group are
SN 185, SN 1006, SN 1054 (→ Crab Nebula), SN 1181, SN 1572 (→ Tycho’s star), and SN 1604 (→ Kepler’s star).

See also: → historical; → supernova.

1. The branch of knowledge dealing with past events.
   

2. The record and explanation of past events and times, especially in connection with a particular people, country, period, person, etc. See: → star formation history, → historical supernova.

Etymology (EN): History, from M.E. histoire, historie, from O.Fr. estoire, histoire, from L. historia “narrative, tale, story,” from Gk. historia “a learning or knowing by inquiry, record, account,” from historein “to inquire,” from histor “one who knows or sees, wise man, " from PIE *wid-tor-, from base *weid- “to know; to see;” cf. Pers. bin- “to see” (present stem of didan);
Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
Skt. veda “I know.” Related to Gk. idein “to see,” and to eidenai “to know,” → idea.

Etymology (PE): Târix, from Ar., itself, according to Abu Rayhân Biruni (973-1048, in Athar al-Baqqiya), loan from Mid.Pers. mâhrôz “date,” first Arabicized as murux, from which the infinitive taurix, and then târix.

Another name for → frost.

The world’s largest meteorite found in 1920, near Grootfontein, Namibia. It was discovered by Jacobus H. Brits while ploughing one of his fields with an ox. The meteorite is tabular in shape and measures 2.95 x 2.84 m; it has an average thickness of about 1 m (1.22 m maximum and 0.75 m minimum). The Hoba meteorite weighs about 65-70 tons. Its chemical composition is 82.4 % iron, 16.4 % nickel, 0.8 % Cobalt, and traces of other metals. No crater is present around the site of the meteorite, probably because it fell at a lower rate of speed than expected. The flat shape of the object may be responsible for its low velocity at impact.

See also: Named after Hoba West, the farm it was discovered; → meteorite.

An → orbital maneuver using two timed engine impulses to move a spacecraft between two coplanar circular orbits. It is performed through an elliptic orbit which is tangent to both circles at their periapses (→ periapsis).

See also: → Hohmann transfer orbit.

An elliptical orbit that is the most economical path for a spacecraft to take from one planet to another. In the case of Earth-Mars travel, the desired orbit’s → perihelion will be at the distance of Earth’s orbit, and the → aphelion will be at the distance of Mars’ orbit. The portion of the solar orbit that takes the spacecraft from Earth to Mars is called its trajectory. Earth and Mars align properly for a Hohmann transfer once every 26 months. → Hohmann transfer.

See also: Named after Walter Hohmann (1880-1945), German engineer, who developed basic principles and created advanced tools necessary for the conquest of space. In 1925 he published The Attainability of the Heavenly Bodies in which he described the mathematical principles that govern space vehicle motion, in particular spacecraft transfer between two orbits.

1. General: An opening through something; an area where something is missing; a serious discrepancy.
   

2. Astro.: → black hole; → coronal hole.
   

3. Electronics: The absence of an electron in the valency structure of a crystalline semiconductor, behaving like a positive charge carrier.

Etymology (EN): O.E. hol “orifice, hollow place,” from P.Gmc. *khulaz (cf. O.H.G. hol, M.Du. hool, Ger. hohl “hollow”), from PIE base *kel- “to cover, conceal.” → cell.

Etymology (PE): Surâx “hole,” from Mid.Pers. sûlâk “whole, aperture,” Av. sūra- “hole;” cf. Gk. koilos “hollow,” L. cava “cave,” cavus “hollow;” PIE base keuə- “to swell; vault, hole.”
Câlé, câl “hole,” from câh “a well, pit,” from Mid.Pers. câh “a well;” Av. cāt- “a well,” from kan- “to dig,” uskən- “to dig out” (O.Pers. kan- “to dig,” akaniya- “it was dug;” Mod.Pers. kandan “to dig”); cf. Skt. khan- “to dig,” khanati “he digs,” kha- “cavity, hollow, cave, aperture.”

The injection of holes in a semiconductor which can be produced by application of a sharp conducting point in contact with an n-type semiconductor.

See also: → hole; → injection.

1. A day fixed by law or custom on which ordinary business is suspended in commemoration of some event or in honor of some person.
   
2. Any day of exemption from work.
   
3. (in plural form) A period of cessation from work or one of recreation; vacation.

Etymology (EN): O.E. haligdæg, from halig “holy,” → heiligenschein,

• dæg, → day.

Etymology (PE): 1) Sepantruz, from sepant “holy,” → heiligenschein, + ruz, → day.

2. Âsudruz, from âsud, âsudan “to rest, repose,” from Mid.Pers. âsutan, Av. ā- + saē- (saii-) “to lie down, go to sleep;”

• ruz, → day.
  

3. Âsudgân, from âsud, as before, + -gân suffix forming plurals.

A combining form meaning “complete, entire, total, whole,” used in the formation of compound words: → holonomic, → holography, holomorphic.

Etymology (EN): From M.E. holo-, from O.fr., from L. hol-, holo-, from Gk. holos “whole,” akin to Pers. har- “every, all, each, any,” as below.

Etymology (PE): Haru, from Mid.Pers. har(v) “all, each, every” (Mod.Pers. har “every, all, each, any”); O.Pers. haruva- “whole, all together;” Av. hauruua- “whole, at all, undamaged;” cf. Skt. sárva- “whole, all, every, undivided;” Gk. holos “whole, complete;” L. salvus “whole, safe, healthy,” sollus “whole, entire, unbroken;” PIE base *sol- “whole.”