Glossary -- O

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Occam's Razor

a way of approaching the development of a scientific model based on the belief that ``the best model is the simplest one---the one requiring the fewest assumptions and modifications in order to fit the observations'' (i.e., nature prefers the simplest most elegant solution).

objective

the primary optical element of a telescope, it gathers the electromagnetic radiation and does the initial focussing.

Olbers' Paradox

the problem that if the universe is infinite in size and age, then the night sky should everywhere be as bright as the Sun because no matter which direction you look, your line of sight will see a star or galaxy.

Oort Cloud

a large spherical cloud of billions to trillions of comets surrounding the Sun at distances between roughly 50,000 to 100,000 A.U. from the Sun. It has not been directly observed; its presence is inferred from the behavior and orbits of the long period comets.

opacity

a measure of a material's ability to absorb or block photons. A material with high opacity prevents most of the photons from passing through (the material is opaque).

open universe

a universe with too little matter (gravity) to stop the expansion (it has an ``open future'').

ozone

a type of oxygen molecule made of three oxygen atoms bound together (O₃). This molecule absorbs ultraviolet light.

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last update: 06 August 1999