Chapter XI

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.