never been observed in true stars, we may say that all these
observations are in harmony with the view that hydrogen should
be and is the principal element in the outer stratum of the
very young star. A few of the stars whose spectra contain
bright hydrogen lines have also a number of bright lines whose
chemical origin is not known. They appear to exist exactly at
this state of stellar life: several of them have not been found
in the spectra of the gaseous nebulae, and they are not
represented in the later types of stellar spectra. The strata
which produce these bright lines are thought to be a little
deeper in the stars than the outer hydrogen stratum.

A slightly older stage of stellar existence is indicated by the
type of spectrum in which some of the lines of hydrogen, always
those at the violet end, are dark, and the remaining hydrogen
lines, always those toward the red end, are bright. The
brightest star in the Pleiades group, Alcyone, presents
apparently the last of this series, for all of the hydrogen
lines are dark except H alpha, in the red. In some of the
bright-line stars which we have described, technically known as
Oe5, Harvard College Observatory found that the dark helium and
hydrogen lines exist, and apparently increase in intensity, on
the average, as the bright lines become fainter. Wright has
observed the absorption lines of helium and hydrogen in the
spectra of the nuclei of some planetary nebulae, although the
helium and hydrogen lines are bright in the nebulosity
surrounding the nuclei. We may say that when all of the bright
lines have disappeared from the spectra of stars, the helium
lines, and likewise the hydrogen lines, have in general become
fairly conspicuous. These stars are known as the helium stars,