Astronomers tell us that over the course of its lifetime the Sun has burned ever more brightly. Not only have they been able to make a solid case for this increase in luminosity, but they are also able to tell us by how much. During the last 4.5 billion years the Sun’s luminosity has increased by about 25 percent. As there is no palaeo-luminosity proxy, this result is based on how the Sun produces energy.

The Sun’s energy is produced by the fusion of four hydrogen nuclei to form one helium nucleus. As the helium nucleus weighs a tiny bit...

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