An Etymological Dictionary of Astronomy and Astrophysics
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A unique Greek geared device, constructed around the end of the second century BC to display the movement of the Sun, the Moon, and possibly the planets around the Earth, and predict the dates of future eclipses. It measures about 32 by 16 by 10 cm and contains at least 30 interlocking gear-wheels, all of them having triangular teeth, from 15 to 223 in number. This device is one of the most stunning artefacts remained from antiquity, revealing an unexpected degree of technical creativity for the period. Nothing close to its technological sophistication appears again for well over a millennium, when astronomical clocks appear in the medieval Europe. It was discovered in 1901 in a sunken ship just off the coast of Antikythera, an island between Crete and the Greek mainland. Its significance and complexity were not understood until decades later. After lots of study involving several research fields, a copy of the device has recently been constructed. See, e.g., Freeth et al. 2006, Nature 444, 587.

Named after the Greek island in the Ionia Sea from which the fragments of the device were discovered in 1901 by sponge divers, who found a sunken Roman ship. Several pieces of evidence indicate that the Roman ship carrying the device wrecked sometime shortly after 85 BC. The ship also contained an enormous booty of bronzes, glassware, jewelry and pottery; → mechanism.

The mechanism that accounts for the → bistability jump.

→ bistability; → mechanism.

A mechanism aimed at explaining the → disruption of a → binary system. As one component loses mass dramatically, the resulting loss of → gravitational attraction changes the orbit of, or ejects completely, the → companion star.

Adriaan Blaauw (1914-2010), 1961, Bull. Astron. Inst. Netherlands 15, 265; → mechanism.

A mechanism, made possible by certain chance coincidences between → spectral lines of He II, O III and N III in some → planetary nebulae , that explains the presence with a high intensity of a selected group of O III and N III lines while all other lines of these elements are missing.

After I. S. Bowen who first discovered this mechanism in 1935; → fluorescence; → mechanism.

A → chemical reaction on solid surfaces in which one atom or molecule is → adsorbed on the catalyst surface, and another reacts directly from the gas phase. This type of mechanism may occur preferentially on very small → dust grains, where transient heating events prevent weakly bound species from remaining and in larger grains at high temperatures. Compare with the → Langmuir-Hinshelwood mechanism.

Proposed in 1938 by D. D. Eley (1914-2015), a British chemist and Professor of Physical Chemistry and E. K. Rideal (1890-1974), an English physical chemist.

A process which reinforces the → kappa mechanism in a → partial ionization zone. Because the temperature in the partial ionization zone is lower than in the adjacent stellar layers, heat tends to flow into the zone during compression, prompting further ionization.

γ, after the smaller ratio of → specific heats caused by the increased values of C_p and C_v; → mechanism.

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.

→ Higgs boson; → mechanism.

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.

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

A process based on the effects of → opacity (κ) that drives the → pulsations of many types of variable stars. Consider a layer of material within a star and suppose that it undergoes inward contraction. This inward motion tends to compress the layer and increase the density. Therefore the layer becomes more opaque (See also → partial ionization zone). If a certain amount of flux comes from the deeper layers it gets stuck in the high κ region. The energy accumulates and heat builds up beneath it. The pressure rises below the layer, pushing it outward. The layer expands as it moves outward, cools and becomes more transparent to radiation. Energy can now escape from below the layer, and pressure beneath the layer diminishes. The layer falls inward and the cycle repeats. The κ mechanism is believed to account for the pulsations of several star families, including → Delta Scuti stars, → Beta Cephei variables, → Cepheids, and → RR Lyrae stars (See Baker & Kippenhahn, 1962, Zeitschrift für Astrophysik 54, 114). Same as κ effect and → valve mechanism. See also → gamma mechanism.

κ, the Gk. letter which denotes opacity; → mechanism.

The heating of a body that contracts under its own gravity. For a large body like a planet or star, gravity tries to compress the body. This compression heats the core of the body, which results in internal energy which in turn is radiated as → thermal energy. In this way a star could be heated by its own weight.

William Thomson (Lord Kelvin) and Hermann von Helmholtz proposed that the sun derived its energy from the conversion of gravitational potential energy; → mechanism.

In the → three-body problem, the → perturbation of the orbit of a → secondary body by the garvity of a third body located at a distance much larger than the separation between the → primary body and the secondary. The secondary's orbit oscillates about a constant value involving a periodic exchange between the extreme values of its → inclination and orbital → eccentricity. The Kozai-Lidov mechanism results from the conservation of the quantity (1 - e²)^1/2.cos i for each component, where e is eccentricity and i is inclination. The total → angular momentum of the system remains constant while the angular momentum is exchanged betwwen the components. It has been suggested that the Kozai mechanism is responsible for the high eccentricities observed in the orbits of → extrasolar planets. If the parent star has a massive yet unseen substellar companion, orbiting at a great distance, and in an orbit highly inclined to the plane of the planets' orbits, the mechanism should induce high eccentricities into the orbits of the planets. Similarly, this mechanism may be responsible for the high eccentricities observed in the orbits of many → Kuiper Belt Objects such as 2003 UB313.

Named for the japanese Yoshihide Kozai (1962, Astronomical J. 67, 591), and the Russian Michael Lidov (1962, Planetary & Space Science 9, 719).

Suggested by Irving Langmuir (1881-1957) in 1921, who was awarded the Nobel Prize in Chemistry in 1932 for his work in surface chemistry. And further developed by Cyril Hinshelwood (1897-1967) in 1926, who received the Nobel Prize in Chemistry in 1956 for his researches into the mechanism of chemical reactions.

1) The structure or arrangement of parts of a machine or similar device, or of anything analogous.
2) The agency or means by which an effect is produced or a purpose is accomplished.

From Mod.L. mechanismus, from Gk. mekhane, → machine.

Sâzokâr, literally "making and working," from sâz "apparatus; (musical) instrument," from sâzidan, sâxtan "to build, make, fashion; to adapt, adjust, be fit" (from Mid.Pers. sâxtan, sâz-, Manichean Parthian s'c'dn "to prepare, to form;" Av. sak- "to understand, to mark," sâcaya- (causative) "to teach") + kâr "work," from kardan "to do, to make" (Mid.Pers. kardan; O.Pers./Av. kar- "to do, make, build;" Av. kərənaoiti "he makes;" 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").

A mechanism proposed by Eddington to explain → stellar pulsations. Same as the → kappa mechanism. In this analogy the stellar layer acts like a heat engine with radiation taking the role of stream. The expanding and contracting layer acts as the piston, and the opacity of the layer behaves as the valve mechanism (Eddington, 1917, The pulsation theory of → Cepheid variables, The Observatory 40, 290).

→ valve; → mechanism.