|The Sun, Edvard Munch, 1916|
So, we have the nuclear fusion idea, and this shows that the sun can last a few billion years. Secularists extrapolate backward and work solar evolution from the failed Big Bang concept, saying that the sun is billions of years old already. A problem with that idea is that projecting into the past is not justified; just because it's built to ride long and hard doesn't mean it's been doing that for a long time already. There's another problem: the sun is very stable, which is an indicator that it was created comparatively recently.
The 19th century saw the first scientific explanation for the sun’s energy. William Thomson (Lord Kelvin) and Hermann von Helmholtz proposed that the sun derived its energy from the conversion of gravitational potential energy. This process (now called the Kelvin-Helmholtz mechanism) would cause the sun slowly to shrink, but the shrinkage would be so gradual as to be virtually undetectable. The Kelvin-Helmholtz mechanism is a viable model, and astronomers think that all stars derive at least some of their energy from this mechanism at some stages. However, scientists generally rejected the Kelvin-Helmholtz model toward the end of the 19th century, because it could power the sun for “at most” 30 million years. At that time, many scientists were committed to gradual geological and biological evolution, processes that required much more time than the Kelvin-Helmholtz mechanism would allow.To read the rest, click on "Is the Sun Shrinking?"
If the conversion of gravitational potential energy does not power the sun, then what does? Early in the 20th century, astronomers began to suggest some sort of nuclear power source for the sun. Eventually, increased knowledge of nuclear physics revealed that the fusion of hydrogen into helium in the solar core is the most likely source of the sun’s energy. The sun contains abundant hydrogen for fuel, and from physics we know that the conditions in the solar core are sufficient to sustain the fusion of hydrogen into helium. If this is the source of the sun’s energy, it could power the sun for nearly 10 billion years.