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
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,
or stars of Class B. Proceeding through the subdivisions of
Class B, the helium lines increase to a maximum of intensity
and then decrease. The dark hydrogen lines are more and more in
evidence, with intensities increasing slowly. In the middle and
later subdivisions of the helium stars silicon, oxygen and
nitrogen are usually represented by a few absorption lines.
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