Fortunately, unlike more massive stars that have already reached this state, the Sun will require billions of years to reach this state. The star will continue to grow brighter, and when the burning approaches the surface, the Sun will enter the red giant phase, producing an enormous shell that may extend as far as Venus or even Earth. As the core hydrogen is used up, the nuclear burning will take place in a growing shell surrounding the exhausted core. The evolution of the Sun should continue on the same path as that taken by most stars. At least as important for the future of Earth is the fact that tidal friction will slow down Earth’s rotation until, in four billion years, its rotation will match that of the Moon, turning once in 30 of our present days. The increase in solar brightness can be expected to continue as the hydrogen in the core is depleted and the region of nuclear burning moves outward. The young Sun may also have been more massive, and thus more luminous, and would have lost its early mass through the solar wind. There were probably compensating thermostatic effects in the atmosphere of Earth, such as the greenhouse effect and cloudiness. This would produce an increase in Earth’s temperature, but no such effect appears in the fossil record. Model calculations conclude that the Sun becomes 10 percent brighter every billion years hence it must now be at least 40 percent brighter than at the time of planet formation. This raises the central temperature and increases the brightness. The helium remains there, where it absorbs radiation more readily than hydrogen.
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