The MK Spectral classification system was founded by W.W. Morgan and P.C. Keenan in the year 1943, with the publication of the first photographic spectral classification atlas, An Atlas of Stellar Spectra (Morgan, Keenan & Kellman 1943). Since that time, the MK system has been extensively revised and refined by Morgan, Keenan and others. In the late 1970's, two important spectral atlases, summarizing the development of the MK system up until that time, were published. These atlases, the Revised MK Spectral Atlas for Stars Earlier than the Sun by Morgan, Abt & Tapscott (1978) and An Atlas of Spectra of the Cooler Stars: Types G,K,M,S and C, by Keenan & McNeil (1976), are the inspiration for this new digital spectral classification atlas. Indeed, some of the pages in this atlas are "digitized" versions of pages from those two atlases.
The MK spectral classification system is a natural, empirical system of spectral classification which uses in the classification process only the directly observable features in the spectrum. The MK system is defined by a set of standard stars, and classification on the system is carried out by the comparison of the program star with the standard stars, taking into account all of the features in the spectrum. The use of standards is vital because it maintains the autonomy of the system as well as ensuring that different observers will classify stars on the same system.
When the MK system was first defined, it was based on photographic spectra in the blue-violet part of the spectrum. This was done by necessity, as photographic emulsions in the 1940's were sensitive only to blue-violet light. However, it was a fortunate choice, as the blue-violet portion of the spectrum (essentially from the Ca II K-line to H) contains a high density of astrophysically important atomic lines and molecular bands, which allow accurate classification of the star in a two-dimensional temperature, luminosity grid. Classification systems can and have been set up in the red, IR and the ultraviolet. These should remain independent of the traditional MK system, as different parts of the spectrum can sample different levels in the atmosphere of the star.
The current spectral atlas should be considered an ongoing work which will likely be completed only in a number of years. The spectra in this atlas have been obtained with the Gray/Miller spectrograph on the 0.8 meter telescope of the Dark Sky Observatory, using a CCD detector. Two spectral resolutions have been used in this atlas. Most of the illustrations use spectra obtained with the 1200g/mm grating, which gives a spectral resolution of 1.8 Å/2 pixels and a spectral range of 3800 Å - 4600 Å, but some illustrations, especially those of the later-type stars (K, M, C and S) use spectra obtained with the 600g/mm grating. These have a resolution of 3.6 Å/2 pixels, and a spectral range of 3800 Å - 5600 Å. The higher resolution spectra are presented in a rectified intensity versus wavelength format, in which the spectral continuum has been normalized to unity. The 3.6 Å resolution spectra, for the most part, are presented in a flux versus wavelength format; this format provides additional information on the energy distribution of the star, and is to be preferred for the cooler stars, as these stars have essentially no continuum points in their spectra. For ease of illustration, the fluxes have been normalized to unity at one consistent point in the spectrum.
Since the publication of the two most recent photographic spectral atlases mentioned in the paragraphs above, the MK system has undergone extensive revision and refinement. Important work in refining and extending the MK system to dimensions beyond the traditional two-dimensional temperature/luminosity grid has been carried out by Keenan and co-workers in the addition of abundance indices for the late-type stars, by Gray (1989), in the extension of the MK system to metal-weak F and G-type stars, by Henry, Kirkpatrick & coworkers in their careful redefinition and extension of the MK system to the M-type dwarfs and most lately the "L-type" stars, and by Walborn in the classification of the O-type stars. Later editions of this atlas will attempt to illustrate more fully these developments.