Transcript: Chemical energy cannot power the Sun, so what is the energy source? Inspired by an idea by the German physicist Hermann von Helmholtz the English physicist Lord Kelvin explored the idea of gravitational contraction. In this mechanism the Sun is slowly shrinking and gravitational potential energy is being converted into heat energy which then radiates out into space. In his estimate the Sun might last a couple of hundred million years with this mechanism. It sounds like a long time, but by the mid-nineteenth century the debate about the age of the Sun began to collide with the debate about the age of the Earth. Most people assumed they formed at the same time. Charles Darwin’s theory of natural selection seemed to require many millions of years for the diversity of species to be achieved from simple origins. In the nineteenth century in England it was common to have scientific debates carried out in public for a public audience and scientists as well. Darwin had debated Wilberforce on the subject of natural selection and by general acclaim had won the debate. In 1871 Lord Kelvin debated Thomas Huxley who was standing in for Darwin on the issue of the age of the Sun and the age of the Earth. Darwin had estimated that the age of the Earth needed to be many hundreds of millions of years, perhaps billions of years to explain the diversity of species, but Kelvin said the Sun could be no older than half a billion years based on gravitational contraction. Darwin died without knowing whether the Earth could be old enough for his mechanism to work.