(en) F. Martins, New atmosphere models for massive stars: Line-blanketing effects and wind properties of O stars, Toulouse, France, Université Paul Sabatier, (Bibcode2004PhDT........21M)
(en) R. F. Garrison, « A Hierarchy of Standards for the MK Process », The MK Process at 50 Years. A Powerful Tool for Astrophysical Insight Astronomical Society of the Pacific Conference Series, vol. 60, , p. 3 (Bibcode1994ASPC...60....3G)
(en) William Wilson Morgan, Philip Childs Keenan et Edith Kellman, An atlas of stellar spectra, with an outline of spectral classification, Chicago, États-Unis, Univsersité de Chicago, (Bibcode1943assw.book.....M)
(en) W. W. Morgan, Helmut A. Abt et J. W. Tapscott, Revised MK Spectral Atlas for stars earlier than the sun, Williams Bay et Tucson, États-Unis, Yerkes Observatory et Kitt Peak National Observatory, (Bibcode1978rmsa.book.....M)