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Gravitational potential energy is the potential energy inherent in two massive objects being some distance from each other, given their gravitational attraction. If one object is much larger than the other, e.g., Earth and a baseball, the energy is thought of as a property of the baseball given its position within Earth's gravitational field. A weight-driven clock uses the weight's gravitational potential energy to operate the clock and a water wheel uses the gravitational potential energy of water at a higher level to operate a mill or other machinery. Restoring the weight or the water to its height requires energy, which, upon completing the lift, continues to exist as the gravitational potential energy. In the dynamics of orbits, decreasing an orbit's radius, bringing two astronomical objects closer together requires some place for the gravitational potential energy to go. Kicking (accelerating) a third interacting object might take away the energy as the kicked object's kinetic energy. Or, in the presence of a gas, heating the gas through friction and compression may absorb the energy as heat and ultimately send it away through thermal emission.
During some phases when not within their main sequence, stars generate their heat from gravitational potential energy, gravity shrinking the star so the mass is brought closer together, the energy converted to the heat (the Kelvin-Helmholtz mechanism). Gravitational potential energy in strong-field gravity can be extreme, with even more power over any substantial length of time than possible with fusion. Material falling into supermassive black holes is considered the only possible source of energy for the observed electromagnetic radiation (EMR) of quasars, the brightest astronomical objects in the universe other than brief flashes.