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V

Variable Star - A star which changes in brightness.  The change may be regular or irregular.  A regular variable star may be caused by a binary star where the components pass in front of each other (an eclipsing variable star) or may indicate an instability in the star itself which can be used to determine the intrinsic brightness of the star, and hence the distance to the star (Cepheid and RR Lyrae variable are examples of this type).  Irregular variables may be caused by flares on the surface of the star or by energy being fed onto the surface of one star onto another, eventually building up a tremendous explosion (a nova).

Vignetting - Pronounced vin-yet-ing.  This is a darkening of the corners of an image (visually or photographically) caused by light falloff toward the edges of the field or by a restricting aperture preventing the field from being fully illuminated.

Visual Back - This is the adapter on the back of a Schmidt-Cassegrain or similar telescope that allows standard accessories to be attached to the scope.  It is normally what holds the diagonal and eyepiece onto the optical tube.