Some Uses of Spectral Classification in HR Diagrams

The MK spectrum-luminosity class diagram (S-LC diagram) between types 09 and G0 is a two-dimensional array containing approximately 100–150 information elements; each information element is a single cell denoted by a spectral type and a luminosity class. These individual, non-dimensional cells (i.e. AO III) can be considered as carriers of observational information toward the HR diagram. Some categories of peculiar spectra (for example, spectra resembling that of the “silicon star” θ Aur) can be attached to certain cells, and can be considered as local perturbations upon the underlying continuity of the spectrum-luminosity class sequences. By such a procedure, the S-LC diagram cells can be considered open-ended, in that progressive additions to the cells can be made indefinitely into the future. Such a procedure can incorporate successfully an unlimited variety of “normal” and “peculiar” categories of spectra - both those already known and those still to be discovered. A morphological system that is unitary thus comes into existence - a system of considerable astrophysical interest. The translation of the detailed language of this system into that of the classical HR diagram is a task for the future.