56 Recent Advances in Stellar Astronomy
remain, while the hydrogen lines get stronger and
stronger. Farther on, in the A type (Sirius) the metallic
lines all become weak and just beyond, in Class B9 the
lines of helium appear—to strengthen with rising tem-
perature, till at the top, in Class BO, practically all the
metallic lines are gone, the hydrogen lines are fainter
and we have the helium lines, the enhanced lines of oxy-
gen and nitrogen, and even traces of the enhanced lines
of helium. In the atmosphere of such a star—which
must be exceedingly hot—the atoms of the metals have
almost all lost two, or possibly three, electrons, and very
few or none are left in a condition to absorb any lines
in the visible spectrum or the nearer ultra-violet. Though
the hydrogen atoms are harder to ionize, most of them
have lost their one electron, leaving much fewer to pro-
duce the line absorption than in the case of the cooler
stars of Class A. The oxygen and nitrogen atoms have
lost an electron, and even helium—the hardest nut of
all to crack—is beginning to yield.
We obtain thus a clear and consistent picture of the
processes which underlie the spectral sequence. All that
needs further to be said concerns the bands which appear
in the spectra of the reddest stars. These are known (in
many cases at least) to be produced by compounds. When
an electron can be removed from a molecule, containing
several atoms held together by those forces which we
ordinarily call “chemical affinity,” the energy changes are
again quite different, and very complex, so that an enor-
mous number of lines are produced, grouped into bands
in the spectrum. At the temperature of the M-stars,
this happens for the titanium oxide vapor in the atmo-
sphere. But at the higher temperature of Class K the
violence of collisions with other atoms is too much for