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occultation  The passage of an object of large angular size in front of a smaller object, such as the Moon in front of a distant star. 
Oort comet cloud The large spherical region around the Sun from which most "new" comets come; a reservoir of objects with aphelia at about 50,000 AU, or extending about a third of the way to the nearest other stars.
opacity Absorbing power; capacity to impede the passage of light.
open cluster  A comparatively loose or "open" cluster of stars, containing from a few dozen to a few thousand members, located in the spiral arms or disk of the Galaxy; sometimes referred to as a galactic cluster.
open universe A model of the universe in which gravity is not strong enough to bring the universe to a halt; it expands forever. In this model the geometry of spacetime is such that if you go in a straight line, you not only can never return to where you started, but even more space opens up than you would expect from Euclidean geometry.
optical in astronomy: relating to the visible-light band of the electromagnetic spectrum. Optical observations are those made mdth visible light.
optical double star Two stars at different distances that are seen nearly lined up in projection so that they appear close together, but that are not really gravitationally associated.
orbit The path of a body that is in revolution about another body or point.
oscillation  A periodic motion; in the case of the Sun, a periodic or quasi-periodic expansion and contraction of the whole Sun or some portion of it.
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