For a while the Sun could still shine without fusion. The Sun was originally formed when a large amount of matter fell toward a common center of gravity. As that matter compressed into a dense ball of gas, it grew very hot and began radiating heat and light—that is, it began to shine—even before nuclear fusion began to occur. If there were no nuclear fusion in the Sun, the collapse and compression of the Sun’s gases would continue to generate energy until all its material fell together and could collapse no further.

According to calculations first made by Lord William Thomson Kelvin (1824–1907) and Hermann von Helmholtz (1821–1894) in the late-nineteenth century, this kind of energy generation by collapsing gases would have allowed the Sun to shine at its current luminosity for millions of years. But the energy could not have lasted the 4.6 billion years that we know the Sun has been shining. Without nuclear fusion, the solar system would have gone dark long before life first appeared on Earth.