Towards a Multi-Dimensional Stellar Classification System

Abstract

Practically all the MK spectral types known today are the result of visual inspection of photographic spectra. This includes the monumental work by Nancy Houk. Two questions related to the MK system remained unanswered all along, namely 1) How accurate are the luminosities and intrinsic colors derived from these types? And 2) Can the classification be done on the basis of measured spectroscopic criteria such as equivalent widths? A third question must also be raised, namely 3) Should we not rather classify in a system with more than two dimensions? With the Parallax Catalogue of van Altena et al. (1995) which is just coming out, and the HIPPARCOS Catalogue which will be available soon such problems can be analyzed anew. Also now techniques are available to produce measured spectroscopic criteria in large quantities. The first steps in this direction will be discussed here.