Chapter XXXVI: The Orion Nebula

The first place among irregular nebulæ is, by universal consent, accorded to the gleaming formation in the Sword handle of Orion. Although incidentally referred to by Cysatus of Lucerne in 1618, it received little attention until Huygens, in 1656, affixed his note of admiration, and executed a drawing of the bright central part, still known as the “Huygenian region.” Here is situated the trapezium, the hub, as it might be called, in which all the spokes of the great wheel are inserted. A photograph of the group is reproduced in Plate XXIV. Fig. 2. It was taken by Professor W. H. Pickering from Mount Wilson, California, 29th September 1889, with the thirteen-inch Boyden telescope. The exposure allowed was only ten minutes, and already the enveloping haze was beginning to cloud the images of the stellar sextett. The companions (which seem to vary in light[1071]) of the two brighter stars are distinct in the original negative, and can be made out in the figure, one as a tiny blotch, the other as a mere deformation of the lowest and largest disc. The physical association of these six stars may be assumed without much risk of error; but there is nothing to show that any real tie exists between them and four adjacent star-points, at the limit of vision with the Lick thirty-six-inch, detected by Barnard and Alvan Clark in 1889.

The nebula, as it developed on a plate exposed, under the same circumstances, during 2^h 36^m, is shown in Plate XXVI. The stars of the trapezium are here completely submerged; only their influence can be traced, or suspected, in the symmetrical arrangement of the expansive wings of light stretching away from their place. These are by no means vague or indefinite outflows. Some of the long streamers have sharp inner edges, peculiarly curved and notched. And the texture is generally filamentous, like that of solar prominences, the characteristic forms of which—as Mr. Ranyard effectively pointed out[1072] are faithfully imitated in some of the minor features of the nebula. An outlying mass to the north (N.G.C. 1977) not only claims affinity by some degree of structural resemblance, but is seen, on Professor Pickering’s plate, to be linked on to it by a faint intermediate extension. The gap between them is absolutely black to telescopic vision.

PLATE XXVII.

Nebulous Formation in Orion. Photographed, 14th January 1890, by W. H.
Pickering.
]

Improvements and modifications in photographic methods have led, by successive steps, to a remarkable increase in the compass assignable to this extraordinary formation. Portrait-lenses have played a leading part in the process. Professor Wadsworth’s reasonings[1073] have made it clear that small apertures, owing to the greater contrast afforded by them with the sub-sensible illumination of the sky, are positively advantageous for obtaining impressions of faint, extended objects. They have the further prerogative of a wide field of view, giving room for the grasp and delineation of large contours; so that, in certain branches of celestial portraiture, they render indispensable and invaluable services. By their aid the nebulous stars ι and c Orionis were proved to be dimly connected with the trapezium nebula; a sharply indented streamer became apparent, issuing towards it from the belt star, ζ Orionis, long recognised as a separate focus of nebulosity; while Max Wolf derived evidence of a luminous union between the nebulæ surrounding ζ and ε Orionis.[1074] Finally, the combined picture, shown in Plate XXVII., of all the nebulous patches in the constellation was obtained by W. H. Pickering, 14th January 1890.[1075] The instrument employed was a Voigtländer lens, 2·6 inches in aperture, and of 8·6 inches focal length; the time of exposure, 6^h 22^m. As the upshot, the fragmentary condensations previously known became united into a vast spiral formation, 15° across. Starting from near Bellatrix (which lies outside the field), it sweeps round by κ Orionis and Rigel to the south, then bends upward to η Orionis, and most probably effects a junction with the nuclear mass in the Sword handle, although the further course of the stream is rendered indistinct by the fogging of the plate. After an interval of nearly five years, Professor Barnard independently repeated and confirmed the Mount Wilson experiment.[1076] His small “lantern lens” showed, with exposures of 2^h and 1^h 15^m, “an enormous curved nebulosity encircling the belt and the great nebula, and covering a large portion of the body of the giant.” In comparison with it, he remarked, the old trapezium nebula “is but a pygmy.” The astonishing extension given to the luminous structure in Orion was, however, of less importance than the disclosure of its architectonic plan. Nor can it be supposed that we are, even now, fully acquainted with it. Further developments may be looked for. The nebulous connections of the various parts will doubtless be more clearly expounded in future photographs. Possibly an effective delineation might be obtained in sections with instruments, like the Crossley or the Meudon reflector, too restricted in angular grasp to embrace the whole at one view. The study of details would thus be made feasible, their subordination to the general design being at the same time kept well in sight. The relations of the members to the main body of this nebula offer a problem of extreme complexity. The stars of the trapezium, for instance, have an appreciable proper motion of about 5″ a century. The nebulous plumage attached to them must evidently partake in their displacement; but this cannot be unhesitatingly asserted of the appendages to the belt stars, still less of the ocean-river of nebulosity flowing outside them. Professor W. H. Pickering discussed the movements of nineteen stars involved in the spiral, though not of course necessarily connected with it.[1077] And so far as any common character could be ascribed to them, it was that of recession from the trapezium. It would be ridiculous to found any conclusion upon so flimsy a basis; yet the indication harmonises with hints, or nascent symptoms of a centrifugal tendency among clustered stars such as the Pleiades. The determination and comparison of their radial movements will be the best means of promoting knowledge on the subject; and since Pickering’s nineteen stars are all, except one, brighter than 6·2 magnitude, their spectroscopic measurement should present no difficulty.

Meanwhile the original “Fish-mouth” nebula raises, even when treated apart, questions of extreme intricacy. Seventy of the stars scattered through it were photographically determined by Scheiner, and compared in 1898[1078] with Gould’s similarly deduced places for them. The close agreement of the results showed them to have remained sensibly stationary during about a score of years, not sharing the drift of the central group. They must accordingly become, in course of time, detached from it, and from the encircling nebula. They constitute then no physical part of the structure, but are scattered casually over its surface. The trapezium-stars, on the contrary, are really embedded in the nebula. And the grounds for this assertion are twofold. Visual logic by itself certifies its truth. The wings of a bird do not start more definitely from the breastbone than the nebulous plumes from the stellar group at their base. The relation is emphasised by the circumstance that two of its members serve as the abutments of a luminous arch, photographed at Potsdam,[1079] which spans the interval between them. A similar nebulous bridge was remarked by Dr. Scheiner to connect two other stars in the outskirts of the formation. Then, as we have already seen, the spectrum of θ Orionis presents peculiarities indicative of a close affinity with the nebulous masses around, and indeed suggests, albeit doubtfully, an actual commingling of luminous substance. It follows that the nebula shares the proper motion of the stars, the small secular value of which (5″), at the probably vast distance of the involved objects, corresponds to a very considerable real velocity. Admitting for a moment the correctness of Professor Pickering’s estimate of one thousand years for the duration of the light-journey from the Orion nebula hither,[1080] we should have to ascribe to the trapezium and its belongings a lateral speed of forty-nine miles a second, the greater part of which would be inherent, since its direction precludes the supposition that it is a perspective effect of the solar journey. The receding radial motion of eleven miles per second, determined for the nebula by Keeler, seems, on the other hand, to belong almost entirely to the sun. Yet the line-displacements by which it is indicated vary, in some degree, from tract to tract of the formation. Recent spectrographic measures executed at Potsdam[1081] imply the progress of interstitial movements, the nature and laws of which it will be of the highest interest to ascertain.

Thirty bright lines, from λ 5007 up to the seventeenth member of the hydrogen series, have been photographed in the spectrum of the Orion nebula, besides the baffling groups faintly apparent on the Tulse Hill plate of 1888. The yellow helium ray (D_{3}), visually detected by Dr. Copeland in 1886, has at least three more refrangible associates; but several lines met with in planetary nebulæ are missing, notably those distinctive of the Wolf-Rayet class of stars. Thus the Pickering and the Rydberg series are alike unrepresented in this grand object. Nor does it glow with the light of any metallic vapour. Its hydrogen emissions are in some respects peculiarly conditioned. The red line is invisible; the green and blue lines are especially bright; beyond them there is a progressive decrease of intensity, though the complete series, up to Hρ, was photographed by Mr. S. A. Mitchell, with a concave grating of 11½ square inches, mounted on the twelve-inch refractor of the Yerkes Observatory.[1082] Thus the state of the nebula is such as to favour, apparently, the development of the quicker luminous vibrations. The inference has, indeed, been controverted. Dr. Scheiner maintained that the suppression of C in this spectrum is “due to purely physiological causes, and warrants no conclusions as to the physical conditions of luminosity,” in nebulæ.[1083] He regarded it, in fact, as an illustration of the “Purkinje effect,” by which the eye, owing to its differential colour-sensitiveness, loses sight of a red sooner than a green ray, when the light-source from which both proceed is gradually enfeebled. In the laboratory, accordingly, F survives alone in the spectrum of a hydrogen-tube at minimum illumination. The plausible deduction that there is no objective reality in the anomalous variations of comparative strength affecting the hydrogen lines derived from celestial objects is, nevertheless, untenable. It collapsed hopelessly, on being confronted with the simple fact, noted by Keeler,[1084] that the third hydrogen line (Hγ) always vanishes experimentally before the first, while in nebulæ it shines unfailingly, although F be imperceptible. The order of brightness, then, of the members of the series is not, in nebulæ, prescribed by physiological causes alone; the faintness of Hα is intrinsic. This is further proved by Professor Keeler’s observation of “Hβ, and several of the hydrogen series above it, glowing brilliantly in the spectrum of R Andromedæ, while not a trace of Hα could be found.” So that “in some of the variable stars we seem to have hydrogen in the same condition as in the nebulæ.”

Finally, Professor Campbell made the decisive experiment of directly comparing the nebulæ with an artificial spectrum.[1085] The two kinds of light being admitted through the upper and lower halves respectively of the same slit, their spectra were seen side by side in the eye-piece, and the corresponding lines in them could be at once equalised by merely altering the distance of the hydrogen-tube from the slit. Combined experiments with this apparatus by Keeler, Wright, and Campbell proved that (1) when the F lines from the nebula and tube were matched in brightness, the blue line above it (Hγ) was markedly stronger in the nebular than in the tube-spectrum; (2) the equalisation of the blue lines left the green line from the nebula conspicuously fainter than the same line from the tube. “The relative intensities,” it was concluded, “of the hydrogen lines from the nebula and from the tube are, therefore, not the same; the nebular lines are relatively the stronger toward the violet, the lines from the tube are relatively the stronger toward the red end of the spectrum.” Absolute measures showed the three principal lines in the spectrum of the Orion nebula to be of very low intensity. Their faintness completely neutralised, to Professor Campbell’s eye, their differences of tint. Hence the “Purkinje phenomenon,” which depends upon the perception of colour, cannot in any degree affect their comparative visibility. There is, however, another aspect to the question. The spectrum of the nebula varies, not alone from the standard of comparison supplied by the vacuum-tube, but also locally, within the formation itself. The relative strength of the constituent rays is different for its different sections. Dr. Runge of Hanover,[1086] who devoted special attention to the point during a visit to the Lick Observatory in September 1897, satisfied himself that the F line of hydrogen, which, near the trapezium, had only one-third (or possibly two-fifths) the intensity of the nebulium line, was ten times brighter in the faint outlying sections of the nebula. The relative gain was then twenty-five to thirtyfold; while the alleged physiological cause, admitting that it had full scope and play, could at the utmost have produced a gain of 1·8 times. The reality of the change was further certified by observing the second nebular line (λ 4959), which in the “Huygenian region” just equals F, to disappear as the slit was moved outward, while F continued to shine with a very sensible lustre. And since, in this case, the Purkinje effect was null (the lines being almost indistinguishable in colour), a demonstration was afforded of a genuine modification in the curve of emissive energy in passing from one part of the great nebula to another.

Professor Keeler devised a completely novel method of demonstrating its non-homogeneous character.[1087] Pictures of the nebula taken on orthochromatic plates protected by colour-screens from the blue radiations, were compared with impressions on ordinary unscreened plates, all being exposed with the Crossley reflector, though during very unequal times. The result was to show that, for equal intensity of the Huygenian region, that of the remote parts and outlying streamers fell greatly below its normal value in the screened photographs. “Conversely,” to quote the words of the ingenious operator,[1088] “where photographs made by the two methods, on the same night, show an equal extent of nebulosity, the Huygenian region is very much more intense on the orthochromatic plate. We infer, therefore, that in the remote parts of the nebula the two lowest nebular lines are weak, or the hydrogen lines strong, as compared with the Huygenian region. Thus the results of spectroscopic researches are confirmed, and are extended to parts of the nebula too faint for visual observation.” That is to say, the hydrogen image of the Orion nebula is larger than its image in nebulium. And this corresponds precisely with Campbell’s spectroscopic discovery about the planetary nebula S.D. −12° 1172,[1089] already adverted to. When nebulium comes to be examined in the laboratory—if that shadowy possibility be ever realised—the cause of the discrepancy may be laid bare. The most obvious is a difference of density between the two substances; yet we cannot unreservedly assume its validity, considering the noted effects of what we may call electrical preferences in modifying the spectra of attenuated gaseous structures.

Two facts, then, have been definitely ascertained regarding the hydrogen spectrum in the “Fish-mouth” nebula. The first is that its more refrangible constituents are preferentially developed as compared with the standard set by the vacuum-tube. The second is that of its persistence in regions of the nebula too dim to glow with any other species of light. Both peculiarities urgently demand explanations, which can probably be afforded only by arduous experimental work. Dr. Scheiner has not neglected this side of the inquiry; and although the outcome of his efforts is negative, it serves none the less to answer a fundamental question. He postulates in nebulæ extreme rarefaction and excessively low temperature; and to test the effect of the latter condition he plunged hydrogen-tubes into liquid air and examined the spectrum. He found it entirely unchanged by cooling to −200° centigrade.[1090] This, he pointed out, harmonises with the view that the luminosity of gases originates solely through internal movements of the individual molecules, and is hence independent of external temperature. Moreover, one of the few means available for terrestrially altering the relative strength of the red and green hydrogen lines is that of electrical differentiation. Professor J. J. Thomson’s observation[1091] of C bright, F invisible near the positive, F bright, and C invisible near the negative electrode, offers a clue which has not yet been followed up, for the threading of the labyrinth.

In another of its elements besides hydrogen, the spectrum of the Orion nebula is suspected to vary regionally. The ultra-violet ray at λ 3727 is second to none in importance when photographed with suitable apparatus. Nevertheless, it was missing from a plate exposed by Sir William and Lady Huggins 28th February 1889,[1092] although the impressions upon it of two much weaker lines near its place were clearly to be seen. The anomaly of its total absence has not recurred. Professor Keeler vainly went over the ground in 1892–93,[1093] groping with his slit for the blank district. Professor W. H. Pickering, it is true, derived indications of local diversities in the intensity of the line from a photograph taken without a slit, 10th July 1888; but they were rather suggestive than conclusive.

Suspected light-variations in the Orion nebula have not been confirmed by modern research. There is absolutely no photographic evidence of change, and visual discrepancies have not been attested with sufficient precision for the support of any positive inferences. Professor Holden was indeed disposed, after an exhaustive comparison of his own with numerous recorded observations, to believe in luminous instability of a partial kind;[1094] but the effects considered may have been only apparent. Their production is at once rendered intelligible by Professor Ormond Stone’s pertinent remarks:—“The general appearance of the Huygenian region,” he wrote in 1896,[1095] “is very much like that of a so-called ‘mackerel sky.’ Many of the condensations have pretty well-defined nuclei, whose light diffuses, blending with the surrounding nebulosity when the seeing grows poor. I have frequently been surprised to find how greatly the definition changes the relative brightness of the different condensations. This explains the many apparently contradictory estimates.” So far as it is possible to judge, then, the brightness of this nebula is not subject to change, either general or local. Moreover, its component parts seem absolutely fixed in outline and position. Internal movements, if in progress, will need the lapse of ages to become sensible to the eye. No relative shiftings of knots or nuclei have been detected; no alteration in shape of outgrowths and effusions. Yet they are of an eminently unstable aspect, and might be supposed no less essentially transitory than the appurtenances of comets. And so most probably they are, although the unit of time by which their duration is measured be long, and the scale of their construction unimaginably vast.

Some of the stars, however, scattered near the trapezium preserve anything but a constant lustre. One, catalogued as T Orionis, fluctuates irregularly from 9·7 to 13·0 magnitude; and others vary as unmistakably, though to a less extent. None are periodical, so that they belong to a different category from the flash-lights of globular clusters. An attentive study, photometric, photographic, and spectroscopic, of the Orion variables could not fail to be fruitful and instructive. Their instability is the more noteworthy from the whiteness of their light. T Orionis (also known as “Bond 822”) is perhaps a unique example of a star untinged with red losing and regaining nineteen-twentieths of its visible radiance. Among its obviously variable neighbours are the objects numbered by Bond in his survey of the nebula, 641, 647, 654, and 679. Scarcely any sustained attention has yet been paid to the group, notwithstanding the many questions of interest connected with it. Enough only is known to make it certain that its members exhibit no community of character in their vicissitudes save that of exemption from any traceable law of order. Thus Bond 654 was noticed by Holden, as it had been noticed by Otto Struve twenty years earlier, to rise occasionally, from habitual quasi-extinction, to brief maxima of about twelfth magnitude. But Ormond Stone recorded none of these sudden brightenings, although he observed the star, 30th September 1886, to rank higher than the twelfth magnitude. Possibly the manner of its variability is itself variable. The star Bond 647, on the other hand, has gained largely in average lustre since 1837, when Sir John Herschel’s measures, reduced to Struve’s scale, made it of 13·1 magnitude. Otto Struve chronicled its disappearance in 1863, after a prolonged maximum at 12·5 magnitude;[1096] Bond found it to be of 11·6 magnitude in 1867; Ormond Stone, of 10·9 magnitude, 1886 to 1894;[1097] and there is nothing to show that this steady rise has reached its culminating point. A compensatory decline may be anticipated, though not with entire confidence. The situation of these stars lends a special meaning to their variations. That they are physically connected with the nebula they are seen projected upon, cannot indeed be proved, but it may legitimately be assumed. Hence every advance in knowledge of their vicissitudes cannot but help to elucidate the still obscure relations of nebulous environment to stellar light-change.