Conversion of the measured continuum, of the equivalent widths and of the line profiles into physically significant parameters, such as temperature, gravity, turbulence and chemical abundances of elements is the domain of stellar atmosphere studies, for which a number of good textbooks exist, for example

The Observation and Analysis of Stellar Photospheres by D. Gray, second edition, 1992, Cambridge University Press and

Stellar Atmospheres by D. Mihalas, second edition, 1978, Freeman.

Analysis of stellar atmospheres falls outside the scope of this book. Nevertheless, some comments on the precision of the abundances, estimates of abundances and catalogs are necessary.

The precision of the abundances

Abundances are quoted in dex and usually they are given to tenths of a dex (see part two, chapter 2). It is thus often assumed that the precision of the abundances is 0.1 dex. Such a precision is, however, not attainable except in very special cases. A good illustration of the problems that are encountered is given in the proceedings of the workshop on ‘Elemental abundance analysis’ (Adelman and Lanz 1987). At this meeting the same initial data of two sharp-lined Ap stars were provided to several groups of astronomers. The abundances obtained by the different groups show that uncertainties still exist at the level of 0.1–0.2 dex, due to different physical parameters used, differences in computer codes and so on.

If this is true for stars for which the observational data were strictly the same, it is clear that, for stars observed by different observers with different equipment, using different sets of atomic data and different computer codes, precision can only decrease.