The Source of the Sun’s Energy

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The Source of the Sun’s Energy
At the beginning of the 20th century there was no convincing explanation for the enormous amount of energy radiated by the Sun. Although physics had made major advances during the previous century and many people thought that there was little of the physical sciences left to be discovered, they could not explain how the Sun could continue to release energy, apparently indefinitely. The law of energy conservation requires that there be an internal energy source equal to that radiated from the Sun’s surface. The only substantial sources of energy known at that time were wood and coal. Knowing the mass of the Sun and the rate at which it radiated energy, it was easy to show that if the Sun had started off as a solid lump of coal it would have burnt out in less than 2000 years. It was clear that this was much too short—the Sun had to be older than the Earth, and the Earth was known to be older than 2000 years—but just how old was the Earth?

Early in the 19th century, most geologists had believed that the Earth might be indefinitely old. This idea was disputed by the distinguished physicist William Thomson, who later became Lord Kelvin (Figure 3.1). His interest in this topic began in 1844 while he was still a Cambridge undergraduate.
It was a topic to which he returned repeatedly and that drew him into conflict with other scientists, such as John Tyndall, Thomas Huxley, and Charles Darwin. To evaluate the age of the Earth, Kelvin tried to calculate how long it had taken the planet to cool from an initial molten state to its current temperature. In 1862 he estimated the Earth to be 100 million years old.

 FIGURE 3.1 William Thomson, later Lord Kelvin (1824–1907). Kelvin was one of the pioneers
of modern physics, developing thermodynamics. He had a great interest in practical matters and
helped to lay the first transatlantic telegraph cable.




To the chagrin of the biologists, Kelvin’s calculations for the age of the Earth did not allow enough time for evolution to occur. Over the next four decades, geologists, paleontologists, evolutionary biologists, and physicists joined in a protracted debate about the age of the Earth. During this time, Kelvin revised his figure down to between 20 million and 40 million years. The geologists tried
to make quantitative estimates based on the time required for the deposition of rock formations or the time required to erode them, and they concluded that the Earth must be much older than Kelvin’s values. However, too many unknown factors were required for such calculations, and they were generally considered unreliable. In the first edition of his book, The Origin of Species, Charles Darwin calculated the age of the Earth to be 300 million years, based on the time estimated to erode the Weald, a valley between the North and South Downs in southern England. This was subjected to so much criticism that Darwin withdrew this argument from subsequent editions.


The discrepancy between the estimates was not resolved until the beginning of the 20th century, when Ernest Rutherford realized that radioactivity (discovered by Henri Becquerel in 1896, well after Kelvin had made his calculations) provides the Earth with an internal source of heat that slows down
the cooling. This process makes the Earth older than was originally envisaged; current estimates suggest that our planet is at least 4.6 billion years old. Radioactivity, as well as providing the additional source of heat, provides an accurate way of measuring the age of the Earth by comparing the amounts of radioactive minerals in the rocks. The age of the Earth put a lower limit on the age of the Sun and renewed the debate about the source of the Sun’s energy— What mechanism could sustain the Sun’s output for such a long period of time? It was not until the 1920s, when Eddington made his deduction that fusion of hydrogen was the most likely energy source, and later, when quantum theory was developed, that a consistent explanation became possible. Advertisement