06413_15 - O OB association An area in space in which young...

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O OB association An area in space in which young high-mass O and B type stars predominate. The stars emit strong ultraviolet radiation, which ionizes the surrounding hydrogen and forms emis- sion nebulae. Shock waves compress dust and gas, which may collapse under the force of gravity and begin the formation of more stars. objective The * lens or system of lenses nearest to the object being examined through an optical instrument. occipital condyle A single or paired bony knob that protrudes from the occip- ital bone of the skull and articulates with the Ü rst cervical vertebra (the * atlas). In humans there is a pair of occipital condyles, one on each side of the * fora- men magnum. Occipital condyles are ab- sent in most Ü sh, which cannot move their heads. occluded front See front . occlusion 1. The trapping of small pockets of liquid in a crystal during crys- tallization. 2. The absorption of a gas by a solid such that atoms or molecules of the gas occupy interstitial positions in the solid lattice. Palladium, for example, can occlude hydrogen. occultation The disappearance of a star, planet, or other celestial body be- hind the moon or another planet, while it is being observed. A solar * eclipse is a form of occultation. oceanic zone The region of the open sea beyond the edge of the continental shelf, where the depth is greater than 200 metres. Compare neritic zone . oceanography The study of the oceans. It includes the origin, structure, and form of the oceans, the nature of the sea Û oor and its sediments, the character- istics of the ocean waters (e.g. tides, salin- ity, and currents), and the types of Û ora and fauna living within the oceans. The effects of human intervention also form an important aspect of oceanography. ocean trench A deep narrow depres- sion in the ocean Û oor, often thousands of metres deep (the Mariana Trench in the Paci Ü c Ocean is 10 850 m deep). Trenches usually form near the edge of a continent where the tectonic plate carry- ing the ocean is being subducted beneath the continental plate ( see plate tec- tonics ). They are often associated with earthquakes and island arcs. ocellus A simple eye occurring in in- sects and other invertebrates. It typically consists of light-sensitive cells and a sin- gle cuticular lens. ochre A yellow or red mineral form of iron(III) oxide, Fe 2 O 3 , used as a pigment. octadecanoate See stearate . octadecanoic acid See stearic acid . octadecenoic acid A straight-chain un- saturated fatty acid with the formula C 17 H 33 COOH. Cis-octadec-9-enoic acid ( see oleic acid ) has the formula CH 3 (CH 2 ) 7 CH: CH(CH 2 ) 7 COOH. The glycerides of this acid are found in many natural fats and oils. octahedral See complex . octahydrate A crystalline hydrate that has eight moles of water per mole of compound.
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This note was uploaded on 05/29/2009 for the course CIVIL ENG 12345 taught by Professor Xhfxfh during the Spring '09 term at Amirkabir University of Technology.

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06413_15 - O OB association An area in space in which young...

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