I N S I D E T H I S I S S U E

Illustration by Brian G Segalby John N. BahcallWhat makes the sun shine? Howdoes the sun produce the vastamount of energy necessaryto support life on earth? These questionschallenged scientists for a hundred andfifty years, beginning in the middle of thenineteenth century. Theoretical physicistsbattled geologists and evolutionarybiologists in a heated controversy overwho had the correct answer.Why was there so much fuss aboutthis scientific puzzle? The nineteenthcenturyastronomer John HerschelSunshine makes life possible on earth.described eloquently the fundamentalrole of sunshine in all of human life inhis 1833 Treatise on Astronomy:The sun’s rays are the ultimatesource of almost every motionwhich takes place on the surfaceof the earth. By its heat areproduced all winds, ... By theirvivifying action vegetables areelaborated from inorganic matter,and become, in their turn, thesupport of animals and of man,and the sources of those greatdeposits of dynamical efficiencywhich are laid up for human usein our coal strata.In this essay, we shall review froman historical perspective the developmentof our understanding of how the sun (thenearest star) shines, beginning in thefollowing section with the nineteenthcenturycontroversy over the age of thesun. In later sections, we shall see howseemingly unrelated discoveries infundamental physics led to a theory ofnuclear energy generation in stars thatresolved the controversy over the age ofthe sun and explained the origin of solarradiation. In the section just before thesummary, we shall discuss how experimentsthat were designed to test the theory ofnuclear energy generation in stars revealeda new mystery, the Mystery of the MissingNeutrinos.I. The Age of the SunHow old is the sun? How does the sunshine? These questions are two sides ofthe same coin, as we shall see.The rate at which the sun is radiatingenergy is easily computed by using themeasured rate at which energy reachesthe earth’s surface and the distance betweenthe earth and the sun. The total energythat the sun has radiated away over itslifetime is approximately the product ofthe current rate at which energy is beingemitted, which is called the solar luminosity,times the age of the sun.The older the sun is, the greater thetotal amount of radiated solar energy.The greater the radiated energy, or thelarger the age of the sun, the more difficultit is to find an explanation of the sourceof solar energy.To better appreciate how difficultit is to find an explanation, let us considera specific illustration of the enormousrate at which the sun radiates energy.Suppose we put a cubic centimeter of iceoutside on a summer day in such a waythat all of the sunshine is absorbed bythe ice. Even at the great distance betweenthe earth and the sun, sunshine will meltthe ice cube in about 40 minutes. Sincethis would happen anywhere in space atthe earth’s distance from the sun, a hugespherical shell of ice centered on the sunand 300 million km (200 million miles)**Published with permission from Nobel e-Museum, http://www.nobel.se/, June 2000.December/ décembre 2000 JRASC219