What powers the Sun? Why does it have spots? How long will it sustain life on Earth? Efforts to answer these questions about the astrophysical object that is of greatest importance to humanity have produced many advances in physics and helped lay the main foundations of astrophysics. The three questions, however, remain to be answered. For example, the best models of the Sun's nuclear power predict a significantly higher neutrino flux than is observed. (See PHYSICS TODAY April, page 19.) We do not know what causes sunspots and other solar activity or even why the Sun emits x rays. Modeling how stars evolve leads to age estimates for some stars that are greater than recent estimates of the age of the universe.
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