**black holes**could exist. As an expert on celestial Mechanics, he knew how the planets and moons move around the sun. In his book, Exposition of the System of the world, published in 1795, Laplace made an amazing prediction. Laplace realized that if a star were massive enough, its

**escape velocity**would be greater than the speed of

**light**. So he calculated how big a star would have to be, if it had the same overall density as Earth, to have an escape velocity equal to the speed of

**light**. He calculated that the star would have to be 250 times the diameter of the Sun. Laplace predicted that a star of this size would have such an enormous gravitational pull that light particles would have such an enormous gravitational pull that light particles would never leave its surface. The star would be invisible.

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