Solar Fuel

The sun is using up its fuel supply at an increasing rate, will gradually get hotter during the next ten billion years. By that time the earth's surface temperature will be lifted to about 750DEG Fahrenheit, hot enough to boil away the oceans, char organic matter, and melt tin, lead and zinc. Then the last of the sun's hydrogen atoms will be converted into helium. With no more fuel on hand, the sun will cool and fade.

Cornell's brilliant, Alsatian-born astrophysicist Hans Albrecht Bethe thus elaborated his already famed theory of the source of stellar energy (TIME, Feb. 27, 1939) in a lecture under the auspices of Sigma Xi, national science honor society, now published.*

His theory is the only one that accounts for the presumed age of the sun (some two billion years) and also for its enormous steady production of energy. Says Theorist Bethe: "At the rate of one cent per kilowatt hour we should have to pay a billion billion dollars to keep the sun going for a single second."

Ordinary chemical reactions are totally inadequate to explain stellar energy. Even if the sun were composed entirely of coal (carbon) and the right amount of oxygen to burn it, the energy of that combustion could supply the sun's heat for a mere 2,500 years. Helmholtz' old theory that the energy comes from contraction of the sun's mass (in effect from the falling inward of all its matter) is also inadequate because it would explain only 30 million years of sunniness. Even radioactivity, the spontaneous disintegration of atoms such as uranium and radium, will not answer: the sun would have to be composed entirely of uranium, and it is not.

But when four hydrogen atoms are combined into one helium atom, as is possible at the sun's center temperature of 20,000,000DEG Centigrade, there is a loss of 0.0286 units of atomic weight. It is this mass which is converted into energy, according to Einstein's relativity formulas. On this principle, for each gram of the sun's hydrogen there would be about 55,000 kilowatt hours of available energy.

(A gram of coal burning yields about 1/100th of a kilowatt hour.) This is ample to explain the sun's energy, and the supply is big enough to last 30 billion years at its present rate of consumption.

The formation of helium from hydrogen is theoretical. It has not been done in the laboratory. It is also complex, involves six steps in which carbon atoms participate but are finally released unchanged. But it is the only theory which accounts for the sun. Further, it accounts for the energy of all the stars except the relatively cool "red giants." A star, in keeping with Bethe's theories on the sun, apparently gets hotter and brighter, "behaves very foolishly" toward the end, uses up the last of its fuel supply in a burst of glory and a "brilliant death."

* Vol. Ill of Science in Progress (a collection of Sigma Xi lectures); Yale University Press;

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