A star-shaped structure, composed of microtubules, which surrounds the centrioles during mitosis in animals. The aster forms during prophase. The spindle fibers arise gradually between the two asters (at opposite ends of the cell) until they attach to the kinetochores of the centromeres of each chromosome. Asters are not found in vascular plant cells.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.