Before the discovery of hydrogen fusion in the early 20th century, astronomers proposed several theories to explain the Sun’s energy production. One of the earliest theories was the idea of combustion, where it was thought that the Sun might be burning fuel like coal or wood. However, this theory was quickly deemed insufficient, as calculations showed that the Sun would only last a few thousand years under such conditions, far less than the Earth’s age suggested by geological data.
In the 19th century, a more scientific explanation was put forward by the German physicist Hermann von Helmholtz and British scientist Lord Kelvin. They proposed the Kelvin-Helmholtz mechanism, which suggested that the Sun’s energy came from gravitational contraction. As the Sun shrinks, it converts gravitational potential energy into thermal energy, producing light and heat. This theory suggested the Sun could burn for tens of millions of years, which was a more plausible timeframe but still fell short of the geological and biological evidence suggesting a much older Earth.
It wasn’t until the advent of nuclear physics in the early 20th century that a more accurate explanation was developed. In the 1930s, scientists such as Hans Bethe and Carl von Weizsäcker provided the correct understanding of stellar energy production through nuclear fusion. They identified that the Sun’s energy comes from the fusion of hydrogen nuclei into helium, releasing vast amounts of energy in the process. This theory not only explained the Sun’s current energy production but also provided a timeline consistent with the age of the Earth and other solar system bodies.