The atmosphere of the Sun has been observed in detail for some time.  We now have a pretty good idea of the nuclear processes which occur in its interior as well.  The Sun, like all other stable stars, produces its energy by means of thermonuclear fusion. The step-by-step process which occurs in the interior of nearly every star is called the proton-proton chain.  In a fusion reaction, hydrogen is heated and compressed to extreme temperatures and pressures. Under these conditions, energy is produced as mass is converted into energy.  The ultimate byproduct is helium.  This is the basis for hydrogen bombs. But that is a rather uncontrolled reaction. Scientists here on Earth are actively engaged in research that would allow us to utilize this very efficient method of producing energy. Recently, they seemed to have made significant progress.
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Carpe Caelum
Carpe Caelum Stellar Astronomy
Carpe Caelum Stellar Astronomy