(Adj.) Pertaining to the use of diagrams, graphs, mathematical curves, or the like.
A particular crystalline form of → carbon occurring as a soft, black,
lustrous mineral. The carbon atoms in graphite are strongly bonded together in sheets.
Because the bonds between the sheets are weak, other atoms can easily fit between them,
causing graphite to be soft and slippery to the touch. Graphite conducts electricity
and is used in lead pencils and electrolytic anodes, as a lubricant, and as a
moderator in nuclear reactors. If graphite is subjected to high pressure, it
will be transformed into → diamond.
From Ger. Graphit, from Gk. graph(ein) "to write, draw," so called because it was used for pencils, → graph + -it a suffix of chemical compounds, equivalent to E. -ite.
javv-e xâkestari, havâsepher-e ~
Fr.: atmosphère grise
A simplifying assumption in the models of stellar atmosphere, according to which the absorption coefficient has the same value at all wavelengths.
Fr.: H-alpha (Hα)
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.
H, symbol of → hydrogen; alpha (α), the first letter of Gk. alphabet.
Fr.: phase de Hayashi
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: tKH = 107(M*/Msun)2/(R*/Rsun)3 yr.
1) An instrument for photographing the Sun, consisting of a camera and a
specially adapted telescope.
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.
nimsepehr (#), nimkoré (#)
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.
Biology: An individual, animal, or plant possessing both male and female reproductive organs.
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.
Biology: For an animal or plant, the condition of having both male and female reproductive tissue or organs.
Fr.: astrophysique des hautes énergies
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.
highly siderophile element (HSE)
bonpâr-e besyâr âhandust
Fr.: élément hautement sidérophile
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).
From Gk. sidero-, from sideros "iron" + → -phile.
Fr.: sphère de 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.
Named for George William Hill (1838-1914), an American astronomer who described this sphere of influence; → sphere.
Of, relating to, or produced using → holography; three-dimensional.
Fr.: réseau holographique
A → diffraction grating produced from a series of constructive → interference fringes. The fringes, whose intensities vary in a sinusoidal pattern, correspond to the grooves of the grating. They are recorded on a photosensitive substrate and subsequently treated using a chemical procedure. Since the grooves are created by the interference of light, such a grating is free from the random and periodic errors present in → ruled gratings.
A technique for making three-dimensional images by recording → interference patterns from a split → laser beam on a medium such as photographic film. One of the → coherent beams irradiates the object, the second beam illuminates a recording medium. The two beams produce an interference pattern, called → hologram, on the film. The hologram contains information on both → phase and → amplitude of the object. However, this information is in a coded form, and the image must be reconstructed. When the object is removed and the hologram is illuminated by the laser from the original direction, a 3-dimensional image of the object appears where the object was originally, as if it were not removed. The visible object seems so real that the observer can detect → parallax by changing the position of one's head.
From → holo- "whole" + → -graphy. By using the term holography, Dennis Gabor (1900-1979), the Hungarian-British electrical engineer and inventor, wanted to stress that the technique records complete information about a wave, both about its amplitude and its phase, in contrast to the usual photography in which only the distribution of the amplitude is recorded.
Fr.: sphères homocentriques
Concentric → spheres of Eudoxus.
Fr.: série de Humphreys
A series of → spectral lines in the → infrared spectrum of → neutral hydrogen emitted by electrons in → excited states transitioning to the level described by the → principal quantum number n = 6. It begins at 12368 nm (Hu α 12.37 microns) and has been traced to 3281.4 nm (3.28 microns).
Named after Curtis J. Humphreys (1898-1986), American physicist; → series.
Fr.: limite de Humphreys-Davidson
An empirical upper → luminosity boundary in the → H-R diagram. It consists of two sections, a sloping part and a horizontal part. The sloping part, which decreases with decreasing → effective temperature, corresponds roughly to the → Eddington limit. The horizontal part is the temperature-independent upper luminosity limit for late-type → hypergiants. It is thought that → massive stars above the Humphreys-Davidson limit encounter an → instability, possibly due to the opacity-modified Eddington limit, and experience high → mass loss episodes which prevent their evolution to cooler temperatures. → Luminous Blue Variable stars are examples of this high mass loss phase.
Named after Roberta M. Humphreys and Kris Davidson, who first dealt with this limit (1979, ApJ 232, 409); → limit.