Darwin understood that his own theory demanded that the earth be hundreds of millions of years old. But due to limited physics of his day, scientists at that time calculated a very young age of the sun which made Darwin doubt his theory. In fact, due to these doubts, in later editions of “Origin of Species”, Darwin removed all mention of timescales. Below I will give a chronological outline of the story of how we have come to understand the Sun’s source of energy and its age. My main source and inspiration for this post is Frank Close’s book, “Neutrino“.
|2700 BC: Ancient Egyptians view the sun as a big ball of fire — no theory of its fuel yet. This celestial body they deified as “Ra“.|
|500 BC: Anaxagoras, apparently found a newly crashed hot meteorite and reasoned that, coming from the sky where the only hot thing was the sun, then it must be a piece of the sun and thus the sun was made of red-hot iron. This theory then held for more than two thousand years.|
|1850 AD: John Waterstone, a schoolteacher, does calculations showing that if the Sun’s heat comes from chemical energy (cooling iron or whatever), then the sun could only last 10,000 years. So looking for other candidates for sources of energy, he theorizes that the energy of rocks falling into the sun transferred their falling kinetic energy into heat. But he realized there were not enough material in the solar system to power the Sun, so, in 1853 he proposed that perhaps the Sun was falling in on itself and consequently releasing energy. Waterstone’s ideas would be picked up 10 years later by William Thomson.|
|1859: Charles Darwin publishes “On the Origin of Species“. His theory of evolution implied that the Earth had to be more like hundreds of millions of years old. Yet the known laws of physics could not explain how the Sun could have burned so long.|
|1860: William Thomson[aka, Lord Kelvin] explores Waterstone’s ideas further. First he explored the notion of material crashing into the Sun. But he sees that even of all the planets crashed into the Sun, it would only give it 3,000 years of life. Next he explored Waterstone’s notion of a collapsing sun’s kinetic energy, and calculates that even if the best scenario, the sun could be more than 100 million years old.|
|1860’s: Darwin writes the co-discoverer of natural selection, Alfred Russel Wallace, about Thomson’s work on the age of the sun and says they “have been for some time one of my sorest troubles.”|
|1896: Henri Becquereldiscovers radioactivity. 1898: Marie Skłodowska Curiediscovers Radium1903: William Wilsonshows that a few grams of Radium per cubic meter of the Sun would be enough to explain its power but quickly, solar spectrum experiments showed no radium on the Sun but instead, hydrogen and helium.|
|1905: Albert Einsteinpublished his theory of relativity and shows that mass, when converted to energy yields unbelievable energy. E=MC↑2.|
|1913: Earnest Rutherfordspeculates that H and He may react differently in the Sun because of its enormous temperatures and thus releasing energy in the nucleus of the atom. Thus the Sun was thought of as a nuclear furnace though the mechanism were a mystery, the source of power was now understood.
Gold Foil experiment exposing the structure of the atom.
|1920: Francis Aston discovered that helium had 1/120 less mass (thus given up as energy) than 4 atoms of hydrogen. Sir Arthur Eddingtonthen proposes that hydrogen in the Sun combines to form helium and looses mass and thus yields energy.|
|1939: Hans Bethe publishes his paper “Energy Production in Stars”. He discovered the CNO cycle to explain this but then realized that the Sun was not hot enough to support this reaction (other stars are hot enough, however). He then Bethe explored Eddington’s notion of hydrogen as the fuel and soon elaborated the P-P chain as the mechanism of our sun of converting mass into energy and thus allowing it enough fuel to last about 10 billion years. Darwin (1809-1882) was vindicated.||