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and of helium and two decades beforeBethe’s calculations of nuclear fusionrates, Russell used well-knownobservations of stars and simple physicalreasoning to infer that the rate of the“unknown process” that supplies stellarenergy must increase rapidly withincreasing stellar temperature. Incredibly,he also correctly deduced that thisdependence of energy production ontemperature would lead to stars beingstable over very long periods of time.These insights are presented in thetext of a closely reasoned lecture thatcontains no equations.E. Rutherford, “The Radiation andEmanation of Radium,” Pt. II, Technics,Aug., 171, (1904) Collected Papers, I:650.C. Smith and M. N. Wise, Energy andEmpire: A biographical study of LordKelvin, (Cambridge: CambridgeUniversity Press), 1989. This book is astimulating and authoritative accountof Kelvin, his science, and his life.Chapters 15-17 deal with the age of thesun, the cooling of the earth, and theage of the earth.C. F. von Weizsäcker, “ÜberElementumwandlungen in Innern derSterne. II,” Physikalische Zeitschrift,39, 633 (1938). The CNO cycle isdescribed in the last paragraph ofSection 7.AppendixThe p—p chain reactionIn theoretical models of the sun, the p–pchain of nuclear reactions illustrated hereis the dominant source of energyproduction. Each reaction is labeled bya number in the upper left hand cornerof the box in which it is contained. Inreaction 1, two hydrogen nuclei ( 1 H,protons) are fused to produce a heavyhydrogen nucleus ( 2 H, a deuteron). Thisis the usual way nuclear burning getsstarted in the sun. On rare occasions, theprocess is started by reaction 2. Deuteronsproduced in reactions 1 and 2 fuse withprotons to produce a light element ofhelium ( 3 He). At this point, the p—p chainbreaks into three branches, whose relativefrequencies are indicated in the figure.The net result of this chain is the fusionof four protons into a single ordinaryhelium nucleus ( 4 He) with energy beingreleased to the star in accordance withEinstein’s equation. Particles called‘neutrinos’ (v) are emitted in these fusionprocesses. Their energies are shown inthe figure in units of millions of electronvolts (MeV). Reactions 2 and 4 were notdiscussed by Hans Bethe.The figure is adapted from J. N.Bahcall, Neutrinos from the Sun, ScientificAmerican, Volume 221, Number 1, July1969, pp. 28-37.The CNO cycleFor stars heavier than the sun, theoreticalmodels show that the CNO (carbonnitrogen-oxygen)cycle of nuclear fusionis the dominant source of energy generation.The cycle results in the fusion of fourhydrogen nuclei ( 1 H, protons) into a singlehelium nucleus ( 4 He, alpha particle), whichsupplies energy to the star in accordancewith Einstein’s equation. Ordinarycarbon, 12 C, serves as a catalyst in this setof reactions and is regenerated. Onlyrelatively low energy neutrinos (v) areproduced in this cycle.The figure is adapted from J. N.Bahcall, Neutrinos from the Sun, ScientificAmerican, Volume 221, Number 1, July1969, pp. 28-37.John Bahcall has been a Professor of NaturalSciences at the Institute for Advanced Study,Princeton, NJ since 1971. He is best knownfor his research work on solar neutrinos,quasars, stars in the Galaxy, and atomicprocesses. Bahcall is a former President ofthe American Astronomical Society and ledthe survey by US astronomers to set prioritiesfor astronomical research in the 1990’s.December/ décembre 2000 JRASC227