An Etymological Dictionary of Astronomy and Astrophysics

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.”