Chapter X: Wolf-Rayet Stars

A very remarkable star was described by Professor Pickering in 1896.[446] Through the measurement of its absorption lines a companion hydrogen series to that already known was, for the first time, recognised; while both the Wolf-Rayet bands in the azure showed in it by direct emission. Only the presence of a dark K obliges us to separate ζ Puppis from Wolf-Rayet stars proper, and to consider it as a linking instance between them and helium stars of the earliest variety. Just this trace of calcium-absorption differentiates the hybrid spectrum of the star in the Poop from spectra of Class viii., which include no legible metallic impressions. Their absence is of especial importance as extending to nebulæ. Nebular chemistry is entirely non-metallic.

The Pickering series in Wolf-Rayet stars is not infrequently bright in its lower members, submerged by absorption higher up. The bands in the blue, on the other hand, are always bright. They form a multiple group, the mutual relations of which await more complete disentanglement. The lowest member is, or may be, the “fundamental” of Rydberg’s hydrogen series at λ 469. Yet the circumstance that it is not really solitary tends to discountenance this identification.[447] The “lazulite” ray at λ 465 is never entirely effaced, and Campbell has provided it with several associates of still shorter wave-lengths.[448] Some of these fall suspiciously near nitrogen lines. The possibility is not then excluded that all these enigmatical blue effluences may be the joint products of glowing hydrogen and nitrogen, although this view, like every other that can be proposed, is beset by serious difficulties. Among the less refrangible of the Wolf-Rayet bright lines there are two (at about λ 569 and 559) which appear to coincide with rays photographed by Professor Hale in carbon stars. Neither has been chemically interpreted. One of them, the greenish or “citron” line (λ 569), seems in some way correlated with the blue band at λ 465. A more definite connection can be traced between the latter and the yellow line at λ 581. They are emphasised in the same stars, while the brilliancy of the Rydberg beam at λ 469 is accompanied by a special vivifying of the green Pickering line at λ 541. This rule is quasi-universal; it may stand, at least temporarily, as a useful principle of order amid half-intelligible phenomena.

Hydrogen is most diversely imprinted on the Wolf-Rayet spectra. Its lines, according to Professor Campbell, “have nearly every known character. In many of the stars they are dark. Again, they are dark with bright borders, and suggest strongly that they are doubly reversed. The bright hydrogen lines vary from faint to very bright, from monochromatic lines to very broad bands, and from those clearly single to those apparently multiple.” This splitting-up of widened lines is evidently analogous to the tripling of the brilliant hydrogen rays observed in Mira, and once more recalls the possibility that certain peculiar stellar spectra may be produced in powerful magnetic fields. Helium is not very prominent in spectra of this type. It frequently shows by emission in D_{3}, and occasionally, even in the same star, by absorption at λ 4472; but its display is subordinate to that of other known and unknown elements.

The grand exemplar of the Wolf-Rayet class is γ Argûs (_alias_ γ Velorum), a star of 2·4 magnitude, giving a resplendent spectrum ablaze with yellow and blue lines. It was first effectively studied by Professor Campbell in 1893–94,[449] although at the Lick Observatory the star barely attains an altitude of six degrees, and can be observed for only a few minutes on any one night. The main facts that struck him were the brilliancy of C, the transitional character of F, and the unmitigated darkness of all the upper hydrogen lines. Similarly, the initial term of the Pickering series at λ 541 shows by emission, the rest by absorption, while a bright D_{3} contrasts with a number of more refrangible dark associates. As usual, both blue bands are visible, but the weight of radiation falls upon that of shorter wave-length, the Rydberg line being comparatively inconspicuous. Miss Cannon found several of the hydrogen lines in both series to be dark with illuminated borders, an arrangement, as she remarked, the inverse of that prevailing in γ Cassiopeiæ and its allies.[450] Mr. McClean recognised oxygen absorption in this star,[451] and the feature is not unlikely to prove, on fuller inquiry, common to all the members of its class. The absence of H and K is more than probable, and suggests comparisons and reflections.

A 7·5 magnitude star in Cygnus (D.M. + 43° 3571) shows, like γ Argûs, a mixed succession of hydrogen lines, but modified, perhaps, by double reversals.[452] With the Lick thirty-six inch Professor Keeler perceived its spectrum as “an extremely complicated range of absorption bands and faint bright lines,”[453] the unusual width of which struck both him and Professor Campbell. Thus the azure bands actually overlap, forming a single indistinct glow one hundred tenth-metres broad. A spectrograph of this object, taken by Mr. Ellerman with the forty-inch Yerkes refractor, is described[454] as totally unlike any spectrum of the fourth type. Whether the dissimilarity is of a nature to be generalised so as to exclude all idea of kinship between these stellar families is more than we can tell at present.

A star of about the same brightness, distinguished as “Argelander-Oeltzen, 17,681,” was swept up in Sagittarius by Pickering in 1881. In its spectrum the golden ray at λ 581 and the lazulite beam at λ 465 predominate almost exclusively. Vogel could see no others with the great Vienna refractor in 1883;[455] nevertheless, Campbell succeeded, ten years later, in measuring twenty bright lines in this wonderful spectrum.[456] Only one among them, and that of secondary importance, can be attributed to helium; but many due to that substance may be included in the unexplored absorption spectrum of “A.O. 17,681.”

A star of 6·4 magnitude in Canis Major, catalogued as “Lalande 13,412,” shows, instead of the unknown blue and yellow rays at λ 465 and λ 581, the “new” hydrogen lines at λ 469 and λ 541.[457] The spectrum includes, besides, a more refrangible blue band, centred about λ 461, but diffuse and divided. These multiple azure effulgences in the Wolf-Rayet stars offer a problem of singular interest. They possess none of the structure of genuine flutings; they seem apt to spread unsymmetrically. Is this an effect of pressure on the emitting vapour? Or does it arise from some property inherent in it, or some mode of action exerted upon it? The answers may be long postponed, but cannot fail to prove interesting.

A unique specimen of this class was photographically detected in Cygnus in the course of the “Draper Memorial” surveys.[458] It is extremely faint—below the ninth magnitude—and was enrolled in the Bonn Durchmusterung as D.M. + 30° 3639. Nevertheless, its spectrum offers more than common facilities for exact observation, owing to the sharpness of its component rays. Thirty were measured by Campbell in 1893,[459] and they include, with many common to the type, several that appear to be individual to the star. The two brightest lines, however, are F and λ 569, the “citron” line strong in γ Argûs. The Pickering and Rydberg series are faint, while the alternative blue band at λ 465 glows intensely. But the distinctive feature of the star is that it is _spectroscopically nebulous_. Observed on the F-line like a solar prominence, Professor Campbell found it to present a very appreciable disc,[460] which, on narrowing the slit, became reduced to a line, as shown in Figure 20. The length of this line is about 5″ of arc, and it measures the apparent diameter of the incandescent envelope of hydrogen which surrounds the body of the star. Only the hydrogen lines behave thus exceptionally; all the other spectral rays show as mere bright points upon the continuous background, which they do not transcend by a hair’s-breadth. That is to say, hydrogen is the sole glowing constituent of the enormous appendage revealed by the powerful appliances available at the Lick Observatory. It has been seen nowhere else, but Runge[461] and Keeler[462] separately verified its existence.

FIG. 20.—Hydrogen-Envelope of the Wolf-Rayet Star, D.M. + 30° 3639
(Campbell).
]

There are a few circumstances worth noting in connection with this extraordinary appearance. First, the vast spread of incandescent hydrogen round the star has no effect in thickening the representative lines of that substance in the star’s spectrum. They are, on the contrary, particularly fine and narrow. The envelope, we can hence infer, is not an atmosphere; there is no appreciable downward pressure of its strata. Again, it must be—in the ordinary sense—hotter than the photosphere it surrounds; for the bright lines emanating from it are not reversed where they cross the prismatic thread due to the nucleus, as they should be if the nucleus were at a higher temperature than its envelope. Finally, it may be possible, by researches into the parallax and proper motion of this star, to form some estimate of its actual distance, and consequently of the real extent of its gaseous surroundings. Thus might be opened a novel line of inquiry destined to lead future students of the skies far afield.

The chemistry of the Wolf-Rayet stars—judging from partial interpretations of the disclosing script—is of the simplest. They have as unfailing constituents hydrogen and helium; oxygen is at least occasionally present, and the detection of nitrogen may be expected with some confidence. Sir William and Lady Huggins showed in 1890[463] the inadmissibility of a carbon origin for any of the blue bands, and defined their positions by exact measurements. Professor Campbell’s observations, visual and photographic, of thirty-two members of the class contributed materially to promote acquaintance with their peculiarities; yet they stimulated, rather than satisfied curiosity.

The distribution of these objects is strongly selective. They are virtually confined to the Milky Way. The rule is emphasised by its apparent exceptions, for the single specimen of the type deviating considerably in position from the galactic plane has proved, on closer inquiry, to be situated in a galactic offset; while the twenty-two grouped in the Magellanic Clouds belong to aggregations of the galactic order, and subject presumably to galactic conditions. The Milky Way, then, and the Nubeculæ afford analogous and exclusive facilities for the development of such bodies. They seem, moreover, to be provided more freely in some regions than in others, since the objects in question tend to collect into knots or groups, the finding of one Wolf-Rayet star being generally the prelude to additional detections in the same neighbourhood. Yet they rarely or never form binary combinations. They are loosely associated without any suggestion of mutual circulation. Nor has any of them, so far, given signs of spectroscopic duplicity. They are singularly steady light-givers. No Wolf-Rayet star is under the slightest suspicion of variability. Atmospheric incandescence on the largest scale is compatible in them with the perfectly uniform working of arrangements for the transference of heated matter from the interior to the surface of the radiating bodies. Considering the frequent and extreme instability of many bright-line stars of other varieties, this fact can hardly be too strenuously insisted upon.