This is the region which is the most easily studied by astronomers and defines the 'surface' of the Sun itself at an optical depth near unity. Originally it was thought that the chemical abundance of this region was just the different strengths of the Fraunhofer Lines seen there. Then with the advent of quantum mechanics, it was discovered that the line strengths are more closely determined by the excitation patterns of these various atomic species. From a careful analysis of the hundreds of thousands of atomic lines detected in the Sun, astronomers have deduced the 'solar abundances' as follows:
Element................atoms Hydrogen 1,000,000 reference Helium 50,000 Carbon 350 Nitrogen 110 Oxygen 670 Neon 28 Sodium 2 Magnesium 34 Aluminum 3 Silicon 33 Potassium 0.3 Sulfur 16 Calcium 2 Iron 25 .................................