Re: The Last Word
The observed solar spectrum at 6911 Angstroms is dominated by lines not of the gases in the Sun, but of gases in the Earth's atmosphere, in this case, O2. This is in what Frauenhofer called the B band, a pattern of strong spectral lines due to an electronic transition in the oxygen molecule; the bandhead is at 6867A, and the lines trail off from there for a hundred Angstroms or so to longer wavelengths. (The similar but stronger A band has its bandhead at 7593A.)
In the main part of the band, the lines are so strong that there is essentially zero transmitted light in the broad line core, and the lines of the isotopically substituted varieties of O2 (that is, rather than 16O-16O, 18O-16O and 17O-16O) are present and of appreciable strength; 18O is 1/500 as abundant as 16O, and 17O is 1/2600 as abundant as 16O. The strong 16O-16O lines obliterate the spectrum where they are present, which means this spectrum interval is generally avoided, if possible, when taking data on astronomical targets.