The sun maintains these internal conditions through a natural balance between two competing forces: gravity pulling inward and pressure pushing outward. The sun"s internal pressure precisely balances gravity at every point within it, thereby keeping the sun stable in size. Deep in the sun"s core, the pressure makes the gas hot and dense enough to sustain nuclear fusion. The energy released by fusion, in turn, heats the gas and maintains the pressure that keeps the sun in balance against the inward pull of gravity. The sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion. The core originally became hot through the release of energy by gravitational contraction, as gravity made the sun"s birth cloud contrast. The entire sun does not rotate at the same rate. In stead, the solar equator completes one rotation in about 25 days, and the rotation period increases with latitude to about.