ABSTRACT
A question that worried Lord Kelvin in the nineteenth century was the origin of the sun’s energy. Where did all that energy come from? If the sun were a large lump of coal, it would burn up very quickly. In fact, the best source of energy he knew of was gravitational energy. If the sun started very large and gradually collapsed on itself, the original potential energy would be converted into kinetic energy or heat. In this way, the sun could have been shining for only a few million years. at didn’t seem long enough for the evolution required by Charles Darwin. We now know from radioactive dating that the solar system is 4.5 billion years old.
