An aster is a cellular structure shaped like a star, formed around each centrosome during mitosis in an animal cell.[1] Astral rays, composed of microtubules, radiate from the centrosphere and look like a cloud.
During mitosis, there are four stages of cell division: Prophase, Metaphase, Anaphase, and Telophase. During prophase, two aster-covered centrosomes migrate to opposite sides of the nucleus in preparation of mitotic spindle formation. During metaphase, the astral rays connect to the lined-up chromosomes at their centromere. Afterwards, during anaphase, the astral rays pull the chromosomes apart into its individual chromatids and pull them towards the centrosomes, located at opposite ends of the cell. This allows the cell to divide properly with each daughter cell containing full replicas of chromosomes. In some cells, the positions of the asters also determine the site of cell divisions.