Today, we take it for granted that the Sun produces energy via nuclear fusion. However, this realization only came about in the early 1900′s and wasn’t confirmed until several decades later. Prior to that, several other methods of energy production had been proposed. These ranged from burning coal to a constant bombardment of comets and meteors to slow contraction. Each of these methods seemed initially plausible, but when astronomers of the time worked out how long each one could sustain such a brightness, they came up against an unlikely opponent: Charles Darwin.