The sun has a sustained nuclear fusion cycle in which two hydrogen nuclei are fused together to make one helium atom with an excess energy release. The energy released is equivalent to the mass lost plus the mass of the helium atom which is equivalent to the two original hydrogen nuclei.
The heat emitted by the sun is the result of a process similar to what happened to an atomic bomb. Namely the sun converts substances into sources of energy. This is different from the combustion process. Burning converts a substance into another form of substance. But if the substance is converted into a source of energy then only a small amount of substance is needed to produce a large amount of energy. An ounce of that substance can produce a number of energies that can dissolve more than a million tons of rock.
So, if the opinion of the natural scientist is true, then the sun still shines brightly because it constantly changes the substance into a source of energy. While, only one percent of the sun ball is enough to produce a source of energy to keep it hot for 150 thousand million years.