Re: The Last Word
Na I 6160.753A is one of a very small number of neutral sodium lines that can be used for deriving an abundance of this element in solar-type and cooler stars. The D lines (5889, 5895 A) are immensely strong (they are the spectral lines that give flames in nearly all sources their strong orange color), meaning that most of the abundance information in the spectral line is submerged in favor of line-damping processes that tell you more about the thermal structure of the stellar atmosphere rather than the abundance of sodium. So the weak lines at 6154 and 6160 A are among the better choices remaining, although the former has a weak but annoying Cr I line blending with it that compromises its utility somewhat.
The abundance of sodium, it is found, turns out to vary in the red giants of globular clusters much like the CNO nuclides do, with some stars having elevated sodium and nitrogen abundances and lowered oxygen abundances relative to most of the others. It is well known (Hans Bethe won the Nobel Prize in physics for realizing this) that oxygen participates in the CNO cycle, which results in it being converted to nitrogen. Standard stellar structure models don't predict the mixing of such fully CNO-cycled material to the stellar surface, but it seems to happen in some stars. That sodium participates also was something of a surprise, and the only reaction cycle that works is with the isotopes of neon being cycled to sodium, which again is not predicted to be mixed to the surface of globular cluster giants, but it clearly must happen. Details of the mixing process are hard to work out, given that it's hydrodynamics in a nasty, turbulent regime.