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In a star there are two main forces: expansion and gravity. In a blue star, when the force of gravity is stronger than the force of expansion the star collapses at its centre. Then, the pull of gravity from the centre of the star is so strong that it even swallows the light! This makes the star invisible: a black hole. Around each black hole there is an area called event horizon which emits radiation. There is a black hole at the centre of our galaxy.