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First Understandings

Albert Einstein

What was the connection between Einstein´s equation and the energy source of the Sun? The answer was not obvious. Astronomers did their part by defining the constraints that observations of stars imposed on possible explanations for the generation of stellar energy. In 1919, Henry Norris Russell, a leading theoretical astronomer in the United States, summarized concisely the hints on the nature of the stellar energy source. Russell stressed that the most important clue was the high temperature in the interior of stars.

Francis William Aston discovered the key experimental piece of the puzzle in 1920. He made precise measurements of the masses of many different atoms, among them hydrogen and helium, and found that four hydrogen nuclei were heavier than a helium nucleus.

The importance of Aston´s measurements was recognized immediately by Sir Arthur Eddington, the British astrophysicist. Eddington argued in his 1920 presidential address to the British Association for the Advancement of Science that Aston´s measurement of the mass difference between four atoms of hydrogen and a helium atom meant that the sun could shine by converting hydrogen atoms into helium. This burning of hydrogen into helium would (according to E=mc2) release about 0.7% of the mass equivalent of the energy. In principle, this would allow the sun to shine for about 100 billion years.

Hans Bethe

In 1939, Hans Bethe described a quantitative theory explaining the fusion generation of energy in the stars (including our sun). The results of his calculations presented in a paper entitled "Energy Production in Stars", won him the Nobel prize for Physics in 1968. With the general theory for fusion reactions now understood, experimental efforts to control the release of fusion energy for net energy output could now be progressed and continue today.