The answer came from Einstein in 1906: E = mc2.

By the 1930's the concept of nuclear reactions became accepted.


Compare the mass difference between 4 hydrogen atoms and one helium atom:


6.6943 x 10-27 kg for 4 hydrogen atoms

6.6466 x 10-27 kg for 1 helium atom

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0.048 x 10-27 kg


This is only 0.7% of the mass, but if we take mass of the Sun times 0.007 times speed of light squared...


Total energy released = 1.26 x 1045 Joules


How long can the sun shine burning Hydrogen?


1.26 x 1045 Joules / 4 x 1026 Watts = 3.15 x 1018 seconds = 10 billion years!





Building Helium from Hydrogen takes several steps...