THE IDEA THAT A BLACK HOLE could possibly exist came from an English rector, John Michell. In 1783 he calculated that the force of gravity exerted by a massive star could prevent light from escaping its surface. Michell's work was largely forgotten for 200 years. In 1971 astrophysicists noticed flickering x-rays coming from the constellation Cygnus, 6,000 light-years away: the radiation indicated that a black hole was apparently circling a star. As with any black hole, it formed as a star ran out of fuel and collapsed in on itself. If the sun were to somehow become a black hole, it would be less than three miles across, trapping light in the warped space that enfolds it. For Earth to become a black hole, it would be the size of a marble.