THE GENERAL QUESTION OF BRIGHT LINES IN STELLAR SPECTRA.
The more closely we study the phenomena of bright lines in stellar
spectra, the more fully convinced we become that no single or simple
principle avails to explain them. They are evidently produced under
varied circumstances, at different elevations above the stellar
photospheres, and in manifold forms of connection with the adjacent
absorbent layers. Reviewing rapidly the chief types of emissive spectra,
we can, to some extent, gather their implications.
The _sine quâ non_ for the display of bright lines is the presence of a
stratum in the star’s atmosphere outshining the photosphere. The
difficulty is indeed very great of attributing this superiority in
brightness to a superiority in temperature; but it can be evaded by the
use of Wiedemann’s convenient term “luminescence,” signifying a state of
glow unconditioned in the strict sense by heat. The rationale of
“luminescence” is still uncertain, but the introduction of the new idea
it represents marks an important departure from the old groove of
thought. Now we can trace two modes of emission in the sun, faintly
indicated, it is true, but instructive as being within reach of
comparatively immediate study. In the first place, some of the
Fraunhofer lines seem to be relieved against vague effusions of
light,[464] originating, almost certainly, beneath the reversing layer,
among the interstices of the photospheric clouds. In the second place,
the violet calcium lines, and occasionally one or two lines of hydrogen,
are doubly reversed in the chromosphere. Both kinds of effect are
reproduced in stellar spectra.
Carbon stars show bright lines, which may be described as chromospheric;
the gases emitting them surmount the vapours generating the noted dark
bands. Moreover, these rays appear to be simple and uncompounded of
bright and dark elements; they are not perceptibly affected by reversals
or absorptions. The same may be said of the bright lines in
Mira-variables. Yet the locality of their origin is widely different.
They are essentially photospheric and deep-seated, shining from beneath
the dusky flutings they diversify. The leading characteristic of
bright-line helium stars is the duplication of their spectra. The vivid
rays have dark companions. And this, not through the optical conjunction
of two distinct bodies, but as the result of physical conditions
prevailing in a single globe. In such stars, then, there are complex
stratifications of emitting and reversing vapours diversely affected, we
cannot tell how, by heat, pressure, magnetism, or electricity. The
gradual penetration into the secrets of nature that must accompany their
study offers an enticing, and a far from hopeless prospect to the rising
generation of astrophysicists. But it will involve an indefinite
expenditure of time and labour. The conditions of bright-line production
in Wolf-Rayet stars are extremely hard to define. They probably vary
greatly in individual specimens. The state of some probably resembles
that of helium stars showing analogous symptoms of gaseous
incandescence. The corresponding reversals, however, are less emphatic,
and have indeed been rather suspected than perceived. Other members of
the Wolf-Rayet class (for Campbell’s star presumably has fellows)
possess vast gaseous envelopes, uniformly glowing, and scarcely
arresting light.
As to the theory of bright lines in stellar spectra, it is only certain
that they testify to a real excess of incandescence in certain layers of
the stellar atmospheres. They are not optically created by the
concentration, through distance, of far-reaching, cool, gaseous
appurtenances. This is proved by the example of the sun, by the study of
Campbell’s star in Cygnus, in which not a cool, but a strongly glowing
appurtenance is actually visible, and by the phenomena of spectral
variability, totally inexplicable on the view that mere extent of
gaseous surroundings is competent to produce bright lines. It is much
easier, however, to deny than to affirm—to perceive incongruities with
fact than to trace the lines of a true hypothesis. This cannot be done
off-hand; much preliminary toil must be undertaken. Of prime necessity
is the continuous study of the sun’s facular rays, of their originating
conditions, their displacements, their periodical changes. Laboratory
inquiries will proceed simultaneously—inquiries into the nature of
“luminescence,” into the temperature of radiating gases, into the
spectral effects of varied modes of electrical illumination, all which
topics may demand subterfuges of treatment not yet easily imaginable.
But need will stimulate invention, and knowledge will advance along the
arduous ways by which alone future progress is possible.