The Solar Furnace

Greek philosopher Anaximenes of Miletus believed the sun, like other stars, was a great ball of fire. His was an important insight, but not entirely accurate. The sun is not so simple. In terms of human experience, the sun is an unfailing source of energy. Where does all of that energy come from? In the nineteenth century, scientists knew of two possible sources: thermal heat (like a candle burning) and gravitational energy. The problem with thermal energy is that even the sun doesn’t have enough mass to produce energy the way a candle does—at least, not for billions of years. Calculations showed that the sun “burning” chemically, would last only a few thousand years.
While a sun that was a few thousand years old might have pleased some theologians at the time, there was a variety of evidence showing that the earth was much older.
So scientists turned their attention to gravitational energy, that is, the conversion of gravitational energy into heat. The theory went this way: As the sun condensed out of the solar nebula, its atoms fell inward and collided more frequently as they got more crowded. These higher velocities and collisions converted gravitational energy into heat. Gravitational energy could power the sun’s output at its current rate for about 100 million years.
But when it started to become clear that the earth was much older (geological evidence showed that it was at least 3.5 billion years old), scientists went back to the drawing board. The nineteenth century ended without an understanding of the source of energy in the sun.