The MK Classification and Its Calibration

Abstract

I have been asked to describe the system of spectral classification as it has developed from the original Yerkes Atlas (Morgan, Keenan, Kellman, 1943) until to-day. I use the word “developed” because any system that is to remain useful must be flexible enough to adapt not only to improved techniques of measurement but also to new theoretical insights into the variables that actually determine the energy spectrum of a star in all its fascinating but sometimes frustrating detail.