An Encoding System to Represent Stellar Spectral Classes in Archival   Databases and Catalogs
  The data archives from space and ground-based telescopes present a vast opportunity for the astronomical community. We describe a classification encoding system for stellar spectra designed for archival databases that organizes the spectral data by "spectral classes." These classes are encoded into a digital format of the form TT.tt.LL.PPPP, where TT and tt refer to spectral type and subtype, LL to luminosity class, and PPPP to possible spectral peculiarities. Archive centers may wish to utilize this system to quantify classes of formerly arbitrary spectral classification strings found in classification catalogs corresponding to datasets of pointed spectroscopic observations in their holdings. The encoding system will also allow users to request archived data based on spectral class ranges, thereby streamlining an otherwise tedious data discovery process. Material in Appendix A is "normative" (part of the defined standard). Appendices B and C are "informative," meant to show how one data provider (MAST) has opted to handle some practical details. 
Myron A. Smith, Randall W. Thompson, Richard O. Gray, Christopher   Corbally, Inga Kamp
astro-ph.SR