The Sun is powered by ordinary chemical burning.

Before the scale of stellar energy was understood, ordinary combustion was a natural analogy for the Sun. Fire was the familiar process capable of producing intense light and heat, so a chemically burning solar body did not initially seem absurd.

Quantitative physics destroyed the idea.

Once astronomers estimated the Sun’s size and luminosity, the available chemical energy was nowhere near enough. Even if the Sun were made of exceptionally combustible material, ordinary burning could keep it shining for only a tiny fraction of the geological age increasingly required for Earth.

Nineteenth-century physicists therefore moved to gravitational contraction as a more powerful source.

The final solution came from nuclear physics. In the core, hydrogen nuclei fuse into helium, and a small difference in mass appears as energy according to relativistic mass–energy equivalence.

That process can sustain the Sun for billions of years.

Chemical combustion still occurs in the ordinary matter around us, but the Sun is not a gigantic fire in that sense. Its energy source lies in nuclear reactions at temperatures and pressures unavailable to conventional chemistry.