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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. Subsequently restoring the weight or water to its earlier height requires energy, and upon completing the lift, this required energy continues to exist, again as gravitational potential energy. In the dynamics of orbits, decreasing an orbit's radius, bringing two astronomical objects into a smaller orbit, requires the gravitational potential energy to be "removed", i.e., converted to some other form of energy. This could be the kinetic energy given to yet another orbiting object that is kicked (accelerated), in an interaction of the orbiting object and the additional object. Or, in the presence of a gas, the orbiting object's gravitational energy could be converted to increased heat energy of the gas, through friction and compression, and such energy could ultimately be sent 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 than that maintained by fusion over any substantial length of time. 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.