In the next main division, the Class F spectra, the metallic
lines increase rapidly in prominence, and the hydrogen lines
decrease slightly in strength. These stars are not so blue as
the helium and hydrogen stars. They are intermediate between
the blue stars and the yellow stars, which begin with the next
class, G, of which our Sun is a representative.

The metallic lines are in Class G spectra in great number and
intensity, and the hydrogen lines are greatly reduced in
prominence. The calcium bands are very wide and intense.

Another step brings us to the very yellow and the
slightly-reddish stars, known as Class K. These stars are weak
in violet light, the hydrogen lines are substantially of the
same intensity as the most prominent metallic lines, and the
metallic lines are more and more in evidence.

Stars in the last subdivisions of the Class K and all of the
Class M stars are decidedly red. In these the hydrogen lines
are still further weakened and the metallic lines are even more
prominent. Their spectra are further marked by absorption bands
of titanium oxide, which reach their maximum strength in the
later subdivisions of Class M.

The extremely red stars compose Class N on the Harvard scale.
Their spectra are almost totally lacking in violet light, the
metallic absorption is very strong, and there are conspicuous
absorption bands of carbon.

Deep absorbing strata of titanium and carbon oxides seem to