Transcript: Knowing what the Sun is made of does not tell us how it gets it energy. This was the subject of active debate throughout the nineteenth century. Around the middle of the nineteenth century, the only known energy source for the Sun was chemical energy, such as is obtained by burning fuel such as coal, or natural gas, or petroleum. Unfortunately it’s easy to show that this energy source is insufficient to explain the Sun’s radiation. We know how far away the Sun is and we know how much energy it emits. We know its size, and so we know how much matter it contains. If the Sun were composed of a chemical energy source, it could only last about ten thousand years at the energy rate that it’s emitting. We suspect and strongly knew even in the early nineteenth century that the Sun was substantially older than ten thousand years; therefore, simple chemical energy processes cannot power the sun.