An Etymological Dictionary of Astronomy and Astrophysics
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A comparison light source containing the known spectral lines of helium and Argon.

→ 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.

From L. helix "spiral," from Gk. helix (genitive helikos), related to eilein "to turn, twist, roll."

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.

→ 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).

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.

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."

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.

→ 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.

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

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."

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.

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

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.

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.

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.

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.

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.

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.

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

A combining form meaning "seven."

From Gk. hepta "seven;" cognate with L. septem; Pers. haft, as below; Du. zeven, O.H.G. sibun, Ger. sieben, E. seven.

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.

→ 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.

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.

→ 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.