Chapter XXVIII: Nebulous Clusters—the Pleiades

“Tangled in a silver braid” of shining world-stuff, the Pleiades stand out as the typical nebulous cluster. They give signs of not being indefinitely remote. The assembled stars have a common drift, which is most likely a perspective effect of the sun’s advance in the opposite direction. If this be so, their light spends just 200 years in reaching the earth, the rate of our progress towards the constellation Lyra being taken at twelve miles a second. Alcyone then radiates at the very least 190 times more powerfully than our sun; in its place, Sirius would appear fainter than the fifth magnitude; it would be outshone, not only by the _lucida_ of the group, but also by five of its companions—by Atlas, Merope, Electra, Maia, and Taygeta. Thus the glory of the Atlantids would be but slightly enhanced by the addition of the great Dog star to their number, and the scale of the system must be commensurate with the magnificent luminosity of its members.

A beginning has been made in the discrimination of the genuine Pleiades from their optical companions. The definite character of their proper motions has made actually feasible what in other similar collections is only remotely possible. The outcome of Elkin’s measures with the Yale heliometer in 1884–85[813] was to distinguish forty-five stars, including Alcyone, as inseparable travellers, while eight proved their independence by dropping out of the ranks. This group of forty-five members may be regarded as a nucleus round which additional stars will aggregate as their movements develop. How far it will extend, how many of the small stars swarming on long-exposed negatives it will eventually take in, remains conjectural. Its delimitation, however, should be practicable in the course of a decade or two; for the comparison of photographs taken about 1915 with those of 1885 and 1888 may be expected to bring numerously into view relative displacements consequent upon the abandonment, as it were in mid-ocean, of a multitude of pseudo-Pleiades exempt from the drift belonging to the true cluster. There is reason to think that this process of expulsion will have to be carried far. The self-selected assemblage will probably not be overcrowded. Exact numerical inquiry has led to many unexpected results, but to none more surprising than that of the thinning-out of faint stars within the area of the Pleiades. Professor Bailey counted nearly 4000 on a photograph including it taken with the Bruce twenty-four-inch lens in 1897; but their density, as a detailed examination made evident, fell off notably and systematically inside the precincts of the system. “It therefore appears,” Professor Pickering wrote,[814] “that the total number of stars in the region of the Pleiades is actually less than in adjacent portions of the sky of equal area, and it is much less than the corresponding number in many parts of the Milky Way.” Regarded, then, as a physical entity, the cluster includes only the brightest of the spangled points thrown together into the field. The spangles of the background would indeed presumably be still more numerous but for the absorbent effect of the nebulous masses attached to the brilliant stars in front of them. Their paucity, at least, must be somehow accounted for, and this explanation of it, suggested by Professor Pickering, seems admissible. M. Stratonoff,[815] too, was led, by a study of stellar distribution in their neighbourhood, to the conclusion that the physical associates of Alcyone are comparatively few; and the conclusion is the more interesting from the sure prospect of bringing its truth to the test.

The Pleiades might be described as not merely a nebulous cluster, but as a cluster of nebulæ, so numerous and so sharply characterised are the cloudy forms collected within its borders. All save one are photographic revelations. The exception is the “Merope nebula,” discovered by Tempel 19th October 1859. A mere “breath stain” on the sky, it is, to telescopic vision, a highly elusive object; yet it is always there, striated and definite, when looked for by chemical means, and the hypothesis of its variability has long ago been abandoned. The Maia nebula has something of the same striped aspect, but clings in a strongly curved whorl to the star which forms its nucleus. Mr. H. C. Wilson of the Goodsell Observatory described as follows a photograph of this object taken by him 30th January 1894.[816] “The region about Maia is especially interesting. A very bright horn-shaped patch of nebula runs out from the west edge of the star-image immediately northward, and extends to a distance of 3′ north of the star. The nebula here is full of irregularly parallel streaks similar to those about Merope, but making only a very small angle with the meridian. Some of them run to and beyond the bright stars north of Maia. A series of rather broad and diffuse patches extend from the middle of the group on a diagonal toward the north-west, reaching to a comparatively bright pair of stars in that direction.”

A second Merope nebula, totally unlike the first, was discovered by Professor Barnard with the Lick thirty-six-inch refractor 14th November 1890.[817] It is round, clearly terminated, and centrally condensed, 30″ in diameter, and presents the general effect of a distant comet. With the adjacent star it forms so close a combination as to indicate, almost of necessity, the slow progress of mutual revolution. Mr. Burnham, who measured the new nebula at Lick in the autumn of 1891, regarded it as “one of the most singular objects in the heavens,” and “unique with respect to its nearness to a bright naked-eye star.”[818] It is the brightest nebula in the Pleiades, and came out well on plates taken by Professor Keeler in 1898 with the Crossley reflector.[819]

Another cosmic species singularly exemplified in this cluster might be called “ribbon nebulæ.” They run in narrow, straight bands from star to star, in one case stringing together six or seven, “like beads on a rosary,” and they pursue with fair accuracy, along parallel lines, an east and west direction. What manner of communication they establish between the suns they connect it is impossible to divine. They may conceivably be mere survivals of a prior order of things, belonging rather to the past than to the present; but no structures more curious have been brought to our notice by the camera than these long, luminous highways built as if for the purpose of facilitating intercourse between the cities of space. An _unfinished road_ starts from Electra towards Alcyone; it has been completed over only about one-third of the way. Or is it the wreck of a celestial causeway which formerly reached its destination, but has been gradually, for some ages past, falling out of use and repair? The question is a daring one; ultimately, however, the comparative study of analogous objects may supply hints for answering it, at least by a plausible surmise. The cluster is besides crowded, especially in the neighbourhood of Alcyone, with irregular or nondescript nebulæ, which choke the background as if with rolling fog. But in general the tendency is unmistakable to assume filamentous shapes, such as were shown with peculiar distinctness in two photographs taken by M. Stratonoff at Tashkent early in 1896 with multiple exposures of respectively ten and seventeen and a half hours.[820]

FIG. 46.—Drawn from Photographs by E. Calvert.
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The history of the Pleiades nebulosities does not end here. Professor Barnard had long been aware of a dulling of the sky-ground over a vast adjacent area; and at last, in December 1893, he put these vague perceptions to the test by means of a ten hours’ exposure with the Willard lens.[821] “The resulting picture,” he wrote, “showed a number of singular curved and streaky nebulosities, apparently connected with the Pleiades and extending all about the group.” Some of them he was able to trace for several degrees on either side, especially towards the east. Yet doubts were expressed as to whether spurious photographic effects were not in question. The phenomenon disclosed was, indeed, so amazing that some degree of scepticism was excusable. Its reality, nevertheless, had to be admitted. Confirmatory photographs were produced by Dr. Max Wolf,[822] by Mr. H. C. Wilson,[823] and by Professor Bailey. A skilful drawing by Mr. E. Calvert, embodying the combined results, is reproduced in Fig. 46. But its limits are too narrow to include the whole of these far-spreading formations. There seems no end to them. The interior nebulosities are left undepicted for the sake of clearness. We are thus brought face to face with the “startling fact” (as Professor Barnard calls it) “that the Pleiades and their involved nebulosities are but the central condensation of an enormous nebula, intricate in details, and covering at least a hundred square degrees of the sky.”[824] The magnitude of this mixed system staggers belief and confounds the imagination. We contemplate it with imperfect apprehension of its scope and significance. For the present, the relations between its various parts appear scarcely open to investigation. The possibility even of speculating upon them will offer itself only when acquaintance begins to be made with the spectral characteristics of the Pleiades nebulæ. That they are all alike gaseous may safely be assumed; nevertheless, their dispersed light, when it becomes practicable to examine it, will perhaps offer diversities full of interest.

Among the stars of the cluster a single spectral type markedly predominates. It may be distinguished as “late Orion.” Hydrogen absorption is prominent; helium absorption also asserts itself, but with less emphasis than in the earlier stars of the same class.[825] This quality of light is common to all the principal stars; those that deviate from it are of inferior grades of brightness, and may eventually, through the effects of proper motion, be sifted out from an assemblage to which they do not properly belong. Hence, when the group comes to be organised on a definitive basis, the _gross_ percentage of sixty-five helium stars may have to be raised very much higher. The first example of a mixed hydrogen series was met with in Alcyone. Professor Campbell was astonished to perceive in 1893 that its spectrum, otherwise marked only by absorptive action, included a glowing crimson C. Very remarkably, too, this solitary bright ray is coupled with a dark streak, situated, as usual in cases of duplication, on its more refrangible side. Which is the displaced line has still to be ascertained. Pleione, the only other member of the family showing signs of emission, is spectroscopically akin to γ Cassiopeiæ. Its bright rays bisect obscure bands.[826] The recession at the rate of eight or ten miles a second of the solar from the Atlantid system must occasion a perceptible shift towards the red of all the spectral lines of its members, to which constant element are superadded the varied, and perhaps varying effects of their individual motions. Spectrographic researches hold out, then, the best prospect of gaining, within a reasonable lapse of time, some insight into the working of this amazing piece of celestial mechanics. From a triangulation, executed with the Göttingen heliometer in 1889–91, Dr. Ambronn derived, as he thought, indications of a division of the cluster into several distinct parcels of mutually dependent masses;[827] but his measures, which included only sixteen stars, had too restricted a scope to be decisive of much.

No stars of assured variability, whether periodical or irregular, are found among the Pleiades. Yet light-changes, eluding definite recognition, are suspected to progress. Maia and Merope have both been held to fluctuate slowly; and Atlas, divided into a pair by Struve in 1827, is now single, with all powers, in the serenest skies. Only a twofold occultation, noted by Hartwig in 1876, seemed to intimate the obscure survival of the vanished companion. Possibly it may make itself _felt_ spectroscopically, even should it never again be seen. The motion-displacements, accordingly, yielded by Atlas deserve attentive scrutiny, since from them may be obtained the key to one of the long outstanding enigmas of double-star astronomy. Another member of the group appears to have an authentically variable attendant. In Wolf’s map of 1874 an anonymous 7·2 magnitude star due south of Alcyone is marked as a wide double; it was single in the Paris photograph of 1886, but again double in that of 1888, when the satellite had risen to eighth-magnitude brightness. Yet M. Chevremont, reviewing the collection with a small refractor in November 1895, could find no trace of it.[828] This was the most precisely defined among several cases of presumable light-fluctuation met with in the course of his survey. Again, M.M. Müller and Kempf constructed at Potsdam in 1899 a photometric catalogue of ninety-six Pleiades,[829] forty-two of which, given in the Bonn Durchmusterung as of 9·5 magnitude, seemed to have diminished so considerably in brightness that their mean magnitude could not be placed higher than 10·7.[830] Very little real change, however, may here have been concerned, since the ordering of stars in the lower ranks of the Durchmusterung is known to have been a highly casual process. It can scarcely be doubted, indeed, that some components of the cluster are, in a measure, variable, but their variability is of a kind not easily certified; it follows no method; it obeys no time-prescription; its effects, perceived when least looked for, cannot be counted upon to recur. Hence the problems of light-change set by the Pleiades are of a peculiarly baffling nature.