An Etymological Dictionary of Astronomy and Astrophysics
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A theoretical limit of approximately 6 × 10¹⁹  → electron-volts for the energy of → cosmic rays above which they would lose energy in their interaction with the → cosmic microwave radiation background photons. Cosmic ray protons with these energies produce → pions on blackbody photons via the Δ resonance according to: γ_CMB + p → p + π⁰, or γ_CMB + p → n + π⁺, thereby losing a large fraction of their energy. These interactions would reduce the energy of the cosmic rays to below the GZK limit. Due to this phenomenon, → Ultra-high-energy cosmic rays are absorbed within about 50 Mpc.

Named after Kenneth Greisen (1966), Physical Review Letters 16, 748 and Georgiy Zatsepin & Vadim Kuzmin (1966), Journal of Experimental and Theoretical Physics Letters 4, 78; → limit.

An open-source (adiabatic/non-adiabatic) pulsation code that calculates the oscillation frequencies of an input stellar model. The code is based on a "Magnus Multiple Shooting" scheme, which is a numerical technique to solve boundary value problems. This is done by subdividing the possible solution space into sub-intervals and solving the pulsation equation in these sub-intervals, assuming continuity conditions. GYRE is integrated into → Modules for Experiments in Stellar Astrophysics (MESA)'s → asteroseismologic module (Townsend & Teitler, 2013, MNRAS 435, 3406).

GYRE, of unknown origin.

The frequency with which an electron or other charged particle executes spiral gyrations in moving across a magnetic field.

→ gyro-; → frequency.

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.

→ H I; → region

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.

→ 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.

→ H II; → region; → luminosity.

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

→ 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").

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").

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.

→ harmonic; progression.

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.

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

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.

→ Hawking radiation; → temperature.

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.

→ Hayashi track; → temperature.

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.

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.

Gâhšomâr, → calendar; yahud→ Jewish calendar.

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.

From → helio- + → sphere.

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.

→ 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.

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.

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

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

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.

From nim-, → half + sepehr, koré, → sphere.

→ inheritable.

→ 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).

Of or relating to → heredity.