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 (see the Solar Neutrino Problem). 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.
In a “Catholic Magazine and Review” from 1889, known as The Month, there is a good record of the development of the problem faced in an article titled “The Age of the Sun and Darwinism”. It begins with a review of the recently discovered Law of Conservation of Energy in which they establish that a method of generation must be established and that this question is necessarily entangled with the age of the Sun and also, life on Earth. Without a constant generation of energy, the Sun would quickly cool and this was known to be unlikely due to archaeological evidences which hinted that the Sun’s output had been constant for at least 4,000 years.
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While burning coal seemed a good candidate since coal power was just coming into fashion at the time, scientists had calculated that even burning in pure oxygen, the Sun could only last ~6,000 years. The article feared that this may signal “the end of supplies of heat and light to our globe would be very near indeed” since religious scholars held the age of the Earth to be some “4000 years of chronological time before the Christian era, and 1800 since”.
The bombardment hypothesis was also examined explaining that the transference of kinetic energy can increase temperatures citing examples of bullets striking metal surfaces or hammers heating anvils. But again, calculations hinted that this too was wrong. The rate with which the Sun would have to accumulate mass was extremely high. So much so that it would lead to the “derangement of the whole mechanism of the heavens.” The result would be that the period of the year over the past ~6,000 years would have shortened by six weeks and that the Earth too would be constantly bombarded by meteors (although some especially strong meteor showers at that time lent some credence to this).
The only strong candidate left was that of gravitational contraction proposed by Sir William Thomson (later Lord Kelvin) and Hermann von Helmholtz in a series of papers they began publishing in 1854. But in 1859, Darwin published the Origin of Species in which he required an age of at least two billion years. Thomson’s and Helmholtz’s hypothesis could only support an age of some tens of millions of years. Thus astronomy and biology were brought head to head. Darwin was fully aware of this problem. In a letter to a friend, he wrote that, “Thomson’s views of the recent age of the world have been for some time one of my sorest troubles”.
To back the astronomers was the developing field of spectroscopy in which they determined that the sun and other stars bared a strong similarity to that of nebulae. These nebulae could contract under their own gravity and as such, provided a natural establishment for the formation of stars, leading gracefully into the contraction hypothesis. Although not mentioned in the article, Darwin did have some support from geologists like Charles Lyell who studied the formation of mountain ranges and also posited an older Earth.
Some astronomers attempted to add other methods in addition to gravitational contraction (such as tidal friction) to extend the age of the solar system, but none could reach the age required by Darwin. Similarly, some biologists worked to speed up evolutionary processes by positing separate events of abiogenesis to shave off some of the required time for diversification of various kingdoms. But these too could not rectify the problem.
Ultimately, the article throws its weight in the camp of the doomed astronomers. Interestingly, much of the same rhetoric in use by anti-evolutionists today can be found in the article. They state, “it is not surprising to find men of science, who not only have not the slightest doubt about the truth of their own pet theories, but are ready to lay down the law in the realms of philosophy and theology, in science which with, to judge from their immoderate assertions, their acquaintance is of the most remote? Such language is to be expected from the camp-followers in the army of science, who assurance is generally inversely proportional to their knowledge, for many of those in a word who affect to popularize the doctrine of Natural Selection.”
In time, Darwin would win the battle as astronomers would realize that gravitational contraction was just the match that lit the fuse of fusion. However, we must ask whether scientists would have been as quickly able to accept the proposition of stellar fusion had Darwin not pointed out the fundamental contradiction in ages?