MASSIVE STAR POPULATIONS IN NEARBY
GALAXIES - André Maeder and Peter S. Conti (1994)
A DIGITAL SPECTRAL CLASSIFICATION ATLAS -
R. O. Gray
AN ATLAS OF STELLAR SPECTRA - Morgan,
Keenan, Kellman (1943)
ON THE RELEVANCE OF THE MK SYSTEM AND PROCESS
TO THE THEORY OF STELLAR ATMOSPHERES - Dimitri Mihalas
THE MK SYSTEM AND THE MK PROCESS -
W. W. Morgan
WHAT IS WRONG WITH THE MK SYSTEM? - Philip
C. Keenan (1984)