What indeed are the secrets of cool star spectra?  How can the M and
C-types look so different and yet have similar temperatures?  We find
more M-type dwarf stars than any other class, while the luminosity and
radius differences between the dwarfs and the supergiants are the most
extreme to be found in any temperature range.  So let's explore this
rich variety of stars, and discover some of their importance, even to
life.