Chapter XXIX: NEBULOUS CLUSTERS—Continued

Many nebulous clusters besides the Pleiades are known, but none in which the relations of stars and nebulæ are so highly specialised. In general, the stellar collection seems as if independently organised, and plunged as a whole into an ocean of cosmic fog, without any strong tendency on the part of its members to form individual nebulous attachments or connections. When, however, the structural details of such formations come more fully to our acquaintance, they may be found to contain evidence of a closer association between particular objects of the two species than can at present be vouched for. Such investigations can be conducted effectively only by photographic means and with carefully adapted instruments. For the purpose of bringing out the full extent of the nebulosity, small portrait-lenses have prerogatives, illustrated practically by Professor Barnard,[831] theoretically by Professor Wadsworth.[832] But pictures on a larger scale than those obtained with them are needed for the disclosure of many topographical minutiæ huddled together by the strong concentration due to their short focal length. For securing these, reflectors of powerful light-grasp are unsurpassable. Thus two lines of photographic inquiry should be made to converge upon nebulous clusters; one directed towards determining the limits of the involving nebulosity, the other towards ascertaining its constructive peculiarities.

To a superficial view the object we are now about to describe seems more like a cluster and nebula than a nebulous cluster. The nebula is “Messier 8”; it is just visible to the naked eye, and from the oval vacancies which interrupt its light, it has received the descriptive designation of the “Lagoon Nebula.”[833] The cluster—separately catalogued as N.G.C. 6530—immediately follows it on the same parallel, but not in complete detachment. The two formations unmistakably overlap, and, to discerning vision, inextricably intermingle. Professor Barnard’s negatives, taken with the Willard lens in 1892, showed the compound object to be “a singular mixture of stars and nebulosity. East and west its diameter is about 45′, and north and south some 42′. The southern side is sharply defined and serrated, with three distinct pointed projections. From its north-following corner a wisp of nebulosity extends nearly to a group of nebulous stars, and possibly with a longer exposure would be found to connect with them.”[834] A picture on so small a scale could teach little regarding internal structure; the “lagoons,” in fact, appear in it nearly _silted up_ with diffused nebulosity. Many of the brighter stars are involved in the pervasive haze, and one in particular occupies too critical a position at the edge of “a very black hole” for the supposition of a mere chance arrangement to be permissible. It may be doubted, however, whether the 5·7 magnitude star, 9 Sagittarii, has more than an optical connection with the cluster, and the multitude of stellar points glittering in the background will almost certainly be separated from it by the slow discrimination of drifting movement. A preliminary step towards applying the test was taken by M. Comas Solà in 1898.[835] His photographic triangulation of the group fixes a starting-point for comparative inquiries which can yield tangible results only in the distant future. His plates showed the stars to be doubly implicated with nebulosity. One of the seventh magnitude acts as the focus to a cloudy mass, distinct, apparently, from a diffuse, elongated structure projected upon the central parts of the cluster, which it not impossibly encloses in annular folds.[836] The Lagoon Nebula gives a spectrum of bright lines. It will be interesting to learn whether they are displaced by motion, and whether, if so, corresponding spectral shifts in the clustered stars ratify the presumption of organic relationship between the two orders of formation.

The optical history of nebulous objects is often curious and instructive. As an open cluster, N.G.C. 2239, in Monoceros, was first observed by Sir John Herschel. An adjacent patch of nebulosity, seen by Swift in 1865,[837] again attracted Barnard’s attention in 1883. It proved to be a kind of knot on a great nebulous ring discerned by the latter in its entirety with the Lick twelve-inch refractor in 1889.[838] This encloses the star group like a ring-fence, and may be said to osculate with a second great filmy ellipse, of which only a section is perceptible. Then came the turn of the camera. Professor Barnard obtained a photograph of the complex arrangement 9th January 1894, which confirmed visual impressions while demonstrating their inadequacy. They were not misleading, but extremely partial. The photographed nebula is, in Professor Barnard’s words, “about one degree in diameter, and very irregular in brightness and outline.”[839] It involves the cluster with unequal condensations, which are especially heavy north of the bright stars. The nebulous knots, and the section of a large ellipse, fully depicted in the sketch of 1889, reappeared in the chemical picture. In this, the only effect of annularity left visible is that a vacant interior space seems reserved for the conspicuously grouped stars. Their chief being of the eighth magnitude, the determination of its spectral character should present no difficulty, and would be of particular value as a test of nebulous affinity.

The number of alternative titles by which the star catalogued by Flamsteed as “15 Monocerotis” is known expresses the curious variety of its claims to distinction. It is variable, multiple, and nebulous. In the first capacity it is designated “S Monocerotis.” Its fluctuations from 4·9 to 5·4 magnitude, in a period of 3^d 10^h 38^m, were noted by Winnecke in 1867. These elements are indeed still somewhat uncertain. They need verification and revision. The variable is, moreover, the leading member of a triple combination, enrolled by Struve as “Σ 950.” With a green companion at 2·8″, and a bluish one at 16·6″, it makes an exquisite telescopic object, but gives no sign of orbital motion. It occupies a dominating position in a collection of fifty or sixty stars, ranging from the eighth to the thirteenth magnitude, one of which is subject to fits and starts of extensive variation.[840] By virtue then of its chiefship of a cluster, 15 Monocerotis is registered among nebulæ as N.G.C. 2264. Nor solely on this account. Both Sir John Herschel and Lord Rosse suspected it to be nebulous, and Bruno Peter, who measured forty-five components of the group at Leipzig in 1879–82,[841] found their leader visibly wrapt in a hazy envelope. But the wide extent of its nebulous connections was entirely a photographic revelation. A picture of the region about 15 Monocerotis, taken by Professor Barnard on 1st February 1894,[842] showed it to be involved in a great nebula some three degrees in diameter. “It clusters densely,” he says, “about the groups of stars, and then spreads out in a weak, diffuse light, with rifts in it, and irregularly terminated along the edges of a vast vacancy in the Milky Way. The condensation, which is very strong, is not at 15 Monocerotis, but twelve minutes south-preceding that star, where it becomes a compact mass with numerous wisps and holes in it.” The absence of nebulous concentration about the individual stars struck him forcibly, especially by contrast with the different state of things in the Pleiades.[843] The nebulosity is free and general; no single member of the stellar assemblage appropriates a special share, or carves from it an appendage of its own. Yet it is difficult to doubt that the apparent association is real and physical.

The Wolf-Rayet spectrum is, broadly speaking, reversed in that of 15 Monocerotis. It includes both the hydrogen series and the upper “blue” band; metallic lines scarcely appear, but those of oxygen, nitrogen, and silicon are unlikely to be absent. About the peculiarities of the less refrangible section nothing is yet known. Here, perhaps, symptoms of emission will be found; indeed, a bright C might be looked for with success not only in 15 Monocerotis, but in some one or two of its principal associates.

An eighth-magnitude star in Auriga was noticed by Auwers about 1860 to be projected on a hazy disc (N.G.C. 2175). The object is placed centrally in a group of smaller stars, enveloped in nebulosity strongly manifested in Professor Barnard’s photographs of 1894.[844] The combination is quite similar to that just described in Monoceros. The spectrum of N.G.C. 2175 was examined by Professor Keeler with an indecisive upshot.[845]

Sir John Herschel observed at the Cape “a very remarkable object” in the Milky Way, where it crosses the tail of Scorpio. It showed to him under the guise of “a decided, tolerably defined, semi-nebulous mass, with abundance of very small stars forming altogether a telescopic Magellanic cloud. It fills about a field, and has branches and sinuses.”[846] This miniature Nubecula (N.G.C. 6437) invites photographic delineation. Long exposures may disclose in it constructive particulars of extreme interest.

Nebulous clusters are connected, by insensible gradations, with certain tracts of nebulosity in the Milky Way, which, since they are situated in, or in the line of sight with a stellar stratum, necessarily appear more or less densely star-strewn. The stars strewing them do not, however, collect into groups capable of individualisation; hence they cannot be termed “nebulous clusters.” These suggest some kind of organisation; they are more or less isolated and coherent entities, and are distinguishable as such from layers and beds of stars, however closely packed. Not that a clear line separates the two kinds of formation; the multiplicity of the heavens is too great for this to be possible; but it is well to maintain differences ideally, even though they be blurred, here and there in the concrete, beyond our perplexed powers of recognition. Nebulous clusters, on the other hand, shade off, as their members become fewer, into nebulous groups such as the Orion Trapezium, and reduce, “in the limit,” to simple nebulous stars. These will be the subject of a future chapter.