How energy is produced in the Sun and other stars was a big puzzle when Hans Bethe was a teenager in the early 1920s. When Arthur Eddington wrote his long Encyclopedia Britannica article on stars in 1911, he could report only on energy release from gravitational contraction, a clearly inadequate source. By 1920, however, Francis Aston had shown experimentally that the mass of the helium atom is slightly less than that of four hydrogen atoms.
Modern measurements tell us that the alpha particle’s mass is less than four times the proton mass by 26 MeV, about 7%. Soon after Aston’s discovery, Eddington suggested that the Sun’s energy source might be the conversion of hydrogen to helium. If that were so, he argued, the Sun could shine for a very long time.
But no one in the 1920s knew how this putative fusion process might work. In the following decade, however,...
