3.3. Relative Abundances
It is my impression that the study of relative abundances holds the promise for solving many of the problems of the chemical evolution of galaxies. Since different elements are formed in different processes corresponding to different stellar mass ranges, focussing on different elemental abundances can lead to the isolation of variables discussed above. Historically, there have been some very simple ideas and experiments along these lines, but the future promises a great degree of sophistication. For example, I expect that nucleocosmochronology will see a vast increase in diagnostic power in the next decade and comparison of r- and s- process elements will enjoy a greater range of applicability (e.g., Pagel & Tautvaisiene 1995, 1997).
Here I will try to stick to the simple basics. There are three important concepts to discuss when considering relative abundances. These are differences in nucleosynthetic origin (I take as an example primary versus secondary processes), time delays (or the breakdown of the IRA), and changing yields (e.g., metallicity dependent yields).