Re: The Last Word
6437.698A is a line of singly ionized Europium. It's got some unfortunate blends to it so its not as nice a line as 6645.127 of the same species and multiplet. Europium is perhaps unique among the elements spectroscopically accessible in red giant stars in that all its isotopes are r-process isotopes; that is, the element is made more or less exclusively in core collapse supernovae. (Just about all the other elements heavier than iron that you can hope to observe are formed at least in part by the s-process, which happens in stable stars.) Consequently, europium is oddly important in studies of galactic chemical evolution, providing an independent handle on how much supernovae have contributed to the material that's in a given star.
(Sadly, my rep power skipped over the two forbidden lines of neutral oxygen, 6300 and 6363, the first of which is my choice for the most important single spectral line to get in a red giant's spectrum. Oh well.)