26.1  The Sun (continued)

Energy From the Sun

The sun gives off tremendous amounts of energy in the form of electromagnetic radiation. But what is the source of this energy? Does the sun burn some type of fuel? If so, how long will it continue to burn before all its fuel is used up? Because the sun and Earth formed together, the answers to these questions can be used to estimate the age of Earth. As a result, scientists from many different fields began a debate that lasted more than 100 years.

In the late 1700s, the German philosopher Immanuel Kant wondered whether the sun produced its energy from an ordinary chemical reaction. He calculated that if it did, it could last only a few thousand years. In the mid-1800s, two physicists, Hermann von Helmholtz and William Thomson (better known as Lord Kelvin), proposed another idea. They hypothesized that gravity was causing the sun to shrink over time, converting its potential energy into thermal energy. According to their calculations, the sun was no more than 20 to 30 million years old.

Scottish geologist Charles Lyell disagreed. Lyell argued that geological processes on Earth had taken hundreds of millions of years. Therefore, Earth and the sun must be far older than other scientists had thought. Evidence from various branches of science eventually led scientists to conclude that the sun is actually about 4.6 billion years old.

How could the sun produce energy for so long? Clearly, the hypotheses about chemical reactions and the sun shrinking were incorrect, as they could not account for the actual age of the sun. It wasn't until the early 1900s that scientists discovered the real source of the sun's energy—nuclear fusion in its central region. Here the temperature and pressure are high enough for fusion to take place. In the process of fusion, as shown in Figure 2, less massive nuclei combine into more massive nuclei, releasing enormous amounts of energy.   The sun's energy is produced in its central region by the fusion of hydrogen nuclei into helium nuclei.

Figure 2  Nuclear fusion within the sun's central region converts mass into energy. The green particles in the diagram are protons and the purple particles are neutrons.