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Big Bang and the Formation of Stars and Galaxies – Fact Sheet The universe started about 13 billion years ago The whole universe started out the size of an orange At the start the temperature is 1,000 billion degrees centigrade As the Universe expanded, it cooled down and hydrogen atoms are formed Hydrogen gas clouds are drawn together through the force of gravity. In the centre of the cloud it becomes incredibly hot until a nuclear explosion sends out heat and light and a star is born. A hundred billion stars form together and a galaxy is created. In the centre of some galaxies is a black hole. Everything nearby gets pulled into a black hole; stars, dust clouds, even light – it all gets sucked in and disappears.