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Galaxy formation. Computer artwork of the stages in the evolution of a spiral galaxy (clockwise from top left). Clouds of gas created during the Big Bang gradually condense, resulting in the formation of stars. Mutual gravitational attraction and the rotation of the galaxy pulls these stars into a characteristic shape. The galaxy formed here is a spiral galaxy, which has a central bulge and a disc consisting of spiral arms. The first galaxies formed 600 million years after the Big Bang. (KEYSTONE/SCIENCE PHOTO LIBRARY/RUSSELL KIGHTLEY)