Popular Science Monthly/Volume 38/January 1891/Star-Streams and Nebulae

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IT is wonderful what a mass of evidence confirmatory of the nebular hypothesis in its broadest sense has been accumulated within the past few years. Most of this new testimony in favor of an old theory has been furnished by Astronomical Photography, that giant that sees the invisible, which has recently risen to the aid of astronomers with the startling suddenness and unexpectedness of the Arab fisherman's afrite escaping from the despised bottle. Perhaps the most notable of these celestial photographs, in the direct light that it throws upon the nebular hypothesis, is Mr. Roberts's already famous picture of the Andromeda nebula. Nobody can look upon the vast nebulous spirals that this photograph reveals, surrounding a great central condensation, and showing here and there a brighter knot where a satellite of the huge focal mass is in process of formation, without feeling that Laplace and Kant were not very far astray in their guess as to the mode of formation of the solar system.

But, although stars in abundance are scattered over and around the Andromeda nebula, there is little in their appearance to suggest a connection between them and the nebula. It is different with the nebulæ in the Pleiades and in Orion. In the wonderful photographs of the Pleiades by the Henry brothers of Paris one not only sees masses of nebulous matter clinging, so to speak, to some of the more conspicuous stars, but in one place a long, straight, narrow strip of nebula has stars dotted along its whole length, like diamonds strung upon a ribbon. It becomes more difficult to resist the conclusion that in this strange nebulous streak, with its starry file, we possess an indication of the mode of origin of the many curious streams and chains of stars with which the heavens abound, when we look at another amazing revelation of celestial photography. I refer to Prof. Pickering's photograph of Orion, taken with a portrait-lens from a mountain in southern California.

In this photograph a tremendous spiral nebula is revealed, covering a space on the sky fifteen degrees in diameter, and embracing the whole of the constellation with the exception of the head and shoulders and the upraised arms of the imaginary giant. The well-known nebula in the Sword, the three bright stars in the Belt, the brilliant first-magnitude star Rigel, together with its less splendid neighbor Beta of Eridanus, and Kappa Orionis, forming the lower left-hand corner of the great quadrilateral of Orion—are all included within the boundaries of this vast nebula. The nebula in the Sword is seen to be only an exceptionally bright condensation in the nebulous system surrounding it.

But for our purposes the thing to be particularly noticed is the arrangement of the stars within the nebula. Any one who has viewed Orion with a powerful opera or field glass must have been struck with the curious marshaling of many of the smaller Star Garland in the Belt of Orion. stars. This is particularly noticeable around the Belt, where the star Epsilon, itself long known to be enmeshed in a faint nebula, is environed with a garland of little stars., which, defiling in a beautiful double curve, finally stop near Delta, the next star above in the Belt. But, indeed, one does not need a glass in order to perceive similar rows of stars in Orion. The most conspicuous of these, after the three stars in the Belt themselves, are those that outline the giant's left arm and the lion's skin that he is supposed to bear upon it. Another row, not so striking, is, however, more interesting just at this point, because it follows the curve of the great outer spiral of the newly discovered nebula. This file of stars really begins below the Belt at Eta, and, curving round between the Belt and Gamma or Bellatrix in the left shoulder, includes the stars 27, 22, Ψ12, 33, 38, and ω, besides others too faint to be visible to the unassisted eye. The connection between these stars and the nebula seems too evident to be doubted. The spiral form of the latter furnishes an explanation of the geometrical arrangement of the former. So with the chain of telescopic stars described above as winding around the bright stars in the Belt—the nebular forms account for the configuration of the stars.

In the cut of Orion's Belt, above, an attempt has been made to represent the appearance of the assemblage of small stars around Epsilon, the center star of the Belt. All the stars there shown can not be seen with an ordinary opera-glass, but a strong field-glass will reveal them and many more besides. In fact, with a powerful glass the complication of curving star-lines becomes rather confusing to one attempting to draw them, and the cut must be regarded rather as an "impressionist" picture than as one showing every star accurately in its place and of precisely the right magnitude. Still, it will be found an approximately correct representation. The reader should bear in mind the fact that the star Epsilon, the center of this remarkable sidereal array, has long been known to be surrounded by a strong nebulosity, and that in the photograph referred to this spot appears as one of the principal foci of the great spiral nebula. These considerations naturally lead to the conclusion (which has also been reached upon other grounds so far as the larger stars are concerned) that Epsilon and the other leading stars of Orion, with the exception of Betelgeuse, which lies beyond the boundaries of the nebula, are at practically the same distance from us as the small stars surrounding them, all being members of one system.

There are many such star-streams to be found in the sky where as yet no related nebulæ have been discovered. But one can hardly doubt, in view of the evidence which the photographs we have referred to furnish, that the forms of the streams are derived from the pre-existing forms of the parent nebulæ. In many cases, of course, the process of nebular condensation has been finished, and we can never expect to discover any evidence of the nebula having once existed beyond the peculiar configuration of the stars to which it gave birth. In other cases, as in this of Orion, photography may yet reveal to us the existence of faint nebulous spirals still connected with the star-groups. Prof. Holden's discovery of a starry ring connected with the celebrated ring nebula in Lyra is in direct accord with the revelations of photography in this respect. Another interesting example is furnished by Mr. W. F. Denning's discovery last September of a small nebula which is completely encircled by a ring of stars. It is impossible, when looking at Mr. Denning's sketch of this curious object in The Observatory, to think that the stars and the nebula there shown do not belong to a single system.

Among the most striking examples of curved or spiral stellar arrangement are the circlet of small stars surrounding Delta Canis Majoris and the exceedingly beautiful star-curves in the neighborhood of Alpha Persei, both of which are figured in my Astronomy with an Opera-glass. No one can survey the heavens with any kind of an optical instrument for half an hour without discovering many similar instances. If it should ever be demonstrated that the individuals composing these star-rows have all an identical parallax, or, in other words, are all at the same distance from us, so much additional strength would be given to the argument that they owe their origin to a nebula which resembled in shape the figure that they mark out. But the inherent probability that the stars concerned in such cases really do have practically the same parallax is so great that actual measurement could hardly make it stronger.

Looking at the matter still more broadly, it is clear that the Milky Way itself may be regarded as the starry residuum of a far grander nebula even than that of Orion, which once completely encircled our heavens; while the origin of such stellar streams as we behold in Eridanus, Pisces, and other constellations having their stars comparatively widely separated and few in number, may be referred to smaller nebulous masses once scattered over the region of space included within and extending on each side of the plane of the galactic circle.