How did scientists realize that energy from the sun couldnt be chemcial energy from the combustion of matter ?

Answer 1

Simply because combustion is a relatively cold process compared to nuclear fusion. If the sun were merely on fire the earth would be frozen over. A "burning" sun would transmit heat via infrared light and most of that would never make it to the earth's surface because of our atmosphere. The fusion of hydrogen atoms at the sun's core produces energy across the entire spectrum and this is what heats the earth.

Answer 2

The energy levels are far too high to be accounted for by combustion.

Answer 3

Because if it were, it would've burned out long long ago.