well known, discovered mostly at the Harvard Observatory
A few hundred stellar spectra resembling this description are
well known, discovered mostly at the Harvard Observatory. Their
details differ greatly, but they have certain features in
common. The bright lines of helium are extremely rare in stars,
but they have been observed in a few stellar spectra. The
bright lines of nebulium have never been observed in a true
star: they and the radiations in the ultra-violet known as at
3726A, seem to be confined to the nebular state; and the
absorption lines of nebulium have never been observed in any
spectrum. As soon as the stellar state is reached nebulium is
no longer in evidence. Stellar spectra containing bright lines
seem always to include hydrogen bright lines. This is as we
should expect; hydrogen is the lightest known gas, and it is
probably the substance which can best exist in the outer strata
of stars in general. The extensive outer strata of very young
stars seem to be composed largely of hydrogen, though other
elements are in some cases present, as indicated by the weaker
bright lines in a few cases. This preference of hydrogen for
the outermost strata is illustrated by several very interesting
observations of the nebulae. The nebulium lines are relatively
strong in the central denser parts of the Orion and Trifid
nebulae, but the hydrogen bright-lines are relatively very
strong in the faint outlying parts of these nebulae. The
planetary nebula B.D.--12 degrees.1172 is seen in the ordinary
telescope to consist of a circular disc (probably a sphere or
spheroid) of light and a faint star in its center. When this
nebula is observed with a slitless spectrograph the hydrogen
and nebulium components are seen as circular discs, but the
hydrogen discs are larger than the nebulium discs. In other
words, the hydrogen forms an atmosphere about the central star
which extends out into space in all directions a great deal
farther than the nebulium discs extend. The Wolf-Rayet
star-planetary nebula D. M. + 30 degrees.3639 looks hazy in a
powerful telescope, and when examined in a spectroscope the
haziness is seen to be due to a sharply defined globe of
hydrogen 5 seconds of arc in diameter surrounding the star in
its center. Wolf and Burns have shown that in the Ring Nebula
in Lyra the 3726A and the hydrogen images are larger as to
outer diameter than the nebulium images, but that the latter
are the more condensed on the inner edge of the ring. Wright
has in the present year examined these and other nebulae with
special reference to the distribution of the principal
ingredients. He finds in general that the radiations at 4363A
and 4686A, of unknown or possibly helium origin, are most
closely compressed around the central nuclei of nebulae; that
the matter definitely known to be helium is more extended in
size; that the nebulium structure is still larger; and that the
hydrogen uniformly extends out farther than the nebulium; and
that the ultra violet radiation at 3726A seems to proceed from
the largest volume of all. The 37726A line, like the nebulium
line, is unknown in stellar spectra; it seems also to be
confined to true nebulosity. Neglecting the elements which have
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.
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