3 What powers the Sun and stars?
As noted at the end of the previous section, an obvious question to ask is: What powers the Sun and indeed all stars? Given that the prodigious energy output of the Sun has been continuing at a relatively constant level for over four billion years, it must have a huge source of energy to sustain this for such a long period of time.
In Section 2, the energy of the Sun was compared to the output of a coal-fired power station. A quick calculation based on the mass of the Sun shows that chemical reactions, such as burning coal or hydrogen, would not be sufficient to maintain the current solar energy output for more than a few thousand years, and so cannot be the source of the Sun’s energy.
Another possibility considered in the past was whether the Sun might be powered by gravitational energy, emitting heat as it gradually contracted. This can also be discounted, as the energy available from this source would last no more than a few tens of millions of years – much less than the known age of the solar system.
The only source of energy powerful enough to maintain the Sun’s energy over billions of years is nuclear energy. As shown in Figure 4 earlier, the Sun is a giant nuclear fusion reactor, converting hydrogen into helium in its incredibly hot and dense core.