Chapter XXXV: Annular Nebulæ

The model annular nebula is the beautiful filmy ellipse situated between ß and γ Lyræ (N.G.C. 6720). Discovered by Darquier at Toulouse in 1779, it appeared to Sir William Herschel a simple hoop of light, quite dark within. The “gauze drawn over the hoop,” spoken of by Sir John Herschel, had been perceived by Schröter in 1797, and with long exposures the photographed ring fills up into a disc. Plate XXV. is taken from a singularly perfect representation obtained by Mr. W. E. Wilson in twenty minutes, while an hour’s exposure sufficed to blur beyond recognition the characteristic annular aspect of the structure. Yet the havoc thus wrought, in a pictorial sense, is compensated by the experimental significance of a result proving the interior of the ring to be far from vacuous of luminous or luminescent material.

PLATE XXV.

The Ring Nebula in Lyra. Photographed by W. E. Wilson, F.R.S.
Exposure, 20^m.
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The dimensions of the ring are about 87″ by 64″. It was shown in the Lick negatives as an exceedingly complex structure. “It seems,” Professor Keeler wrote,[1017] “to be made up of several narrower bright rings, interlacing somewhat irregularly, the spaces between them being filled with fainter nebulosity.” Many bright patches and condensations diversify the main annulus, which is, nevertheless, fundamentally continuous; it does not break up into detached knots of nebulosity. The light is strongest near the extremities of the transverse axis, and its failure at each vertex of the ellipse, conspicuously shown in our Plate, was, already in 1785, noticed by the elder Herschel.[1018] It is accompanied by symptoms of effusion, marking possibly equatorial outflows due to rotative acceleration. Lord Rosse in 1863, and Schultz in 1865, were alike struck with “nebulous radiations in the direction of the longer axis, which seemed momentarily almost to destroy the annular form.” They issued from the north-eastern side, while a similar appearance on the south-westerly side of the nebula was observed by Professor Holden at Washington in 1875.[1019] He perceived, too, with surprise that the south end of the minor axis, which Lord Rosse had represented as the best terminated, was unmistakably, after thirteen years, less clearly finished than the north end. Similarly, the Lick thirty-six-inch disclosed in 1888[1020] the whole southern margin as filamentous, by a kind of alternation with the corresponding state of the northern edge noticed at Parsonstown. This tufted or fringed effect was caught, for the first time photographically, by Professor Keeler. The oval imprinted on his plates was fringed on both sides, and his measurement of eleven nebulous projections from it will give the means of detecting any future variations in their luminosity or distribution.

The gauzy stuff in the interior is not a mere formless light-mist. A drawing published by Lord Rosse in 1844[1021] represented it as divided into longitudinal striæ, and their reality was confirmed by the Crossley photographs after fifty-five years. “I have tried,” Professor Keeler wrote, “to verify this band-structure by visual observation with the thirty-six-inch refractor, and have fancied at times that I could catch glimpses of it; but the observation is a most difficult one, the contrast of the bright and dark bands, exaggerated by the photograph, being almost too slight to affect the eye.” The surprising accuracy of the delineations made with the Rosse speculum, while its burnish continued unimpaired, was thus once more exemplified. It conveys a warning against lightly setting aside any of the earlier records concerning nebulæ obtained at Parsonstown, even when they imply seemingly improbable changes. It is certainly by no accident that the striæ within this nebula coincide in direction with its major axis; nor can the termination of the transverse axis by maxima, of the longitudinal axis by minima of brightness[1022] be regarded as casual features. All the details of the edifice, in fact, are arranged with obvious reference to its apparent shape, and this amounts to a demonstration that the elongation is real. The nebula, then, is not simply projected into an oval; it is not a circular formation viewed obliquely. There seems no escape from the conclusion that it is an ellipsoid of revolution—that the bands follow the line of the equator, and originate under conditions prescribed by the rotation of the body; while the partial interruption of luminosity at the ends of the oval mark outflows of matter where centrifugal velocity overrides the holding power of gravity. Everything, indeed, leads us to suppose that this nebula, like the rest of its kind, is actually a hollow spheroid of shining fluid, the marginal brightness resulting from the increased thickness of the luminous shell penetrated by the visual ray. The “hoop” and the “gauze” drawn round it are then two aspects of the same thing. Nebulous rings, as such, probably do not exist. They would be subject to perspective effects, no traces of which are to be found in the heavens. Annularity in nebulæ may accordingly be considered as a purely optical modification of a different structural plan.

A glittering point of light occupies the centre of the annulus in Lyra. Many anomalies are connected with its visibility. The first heard of it is in 1800, when Von Hahn of Remplin, in Mecklenburg, was surprised by its disappearance.[1023] He attributed the change, not to loss of light in the star, but to the nebulous clouding-over of the black background upon which he was accustomed to see it relieved. Unperceived by the Herschels, it was next seen by Lord Rosse in 1848,[1024] and attracted Father Secchi’s attention at Rome in 1855.[1025] A ten-inch Steinheil sufficed, in 1865 and 1867, even in unfavourable weather, to show Hahn’s star to Hermann Schultz;[1026] yet it unaccountably evaded the deliberate scrutiny, ten years later, of Professor Asaph Hall,[1027] armed though he was with the twenty-six-inch Washington equatorial. The same instrument, however, displayed its evasive sparkle to Mr. A. C. Ranyard, 23rd August 1878;[1028] while to Dr. Vogel, with the Newall telescope in 1875, and the Vienna twenty-seven-inch in 1883, it remained consistently imperceptible.[1029] Very remarkable, too, is its non-appearance to Dr. Spitaler at Vienna in 1885, when he carefully delineated the nebula, as well as in 1886, during frequently repeated verifying observations.[1030] The interior seemed then to contain only dimly luminous floccules; nevertheless on 25th July 1887 the star caught his eye at the first glance. It had, in the meantime, 1st September 1886, been photographed by Von Gothard, and has since abstained from capricious disappearances. It is indeed of such exceptional actinic power that the camera cannot easily lose sight of it. Fainter visually than the fifteenth magnitude, it needed only an exposure of one minute to come out distinctly on a Crossley negative, and it left a dim impression in half that time.[1031] With all exposures the image was clearly defined, although in photographs taken with other instruments it had usually presented a hazy disc. Its light appears to be of normal stellar quality. Its maximum of intensity, that is to say, falls in the yellow part of the spectrum. Both Keeler and Barnard agreed that, with the Lick and Yerkes refractors, the focus for the star was about one-fifth of an inch shorter than for the encircling nebula. And a similar disparity exists between the nuclei and discs of most planetaries. This, however, leaves their special chemical effectiveness unexplained; for Keeler’s suggestion of its being due to ultra-violet emissions lacks the support of known facts.

In the field with the Lyra nebula, Barnard perceived, 2nd October 1893, a second of about the fourteenth magnitude, 30″ in diameter, and somewhat irregular in shape.[1032] Professor Keeler’s longest-exposed negative showed the new object to be “a left-handed, two-branched spiral.”

In 1891 Mr. Burnham measured the nucleus of the ring in Lyra with reference to an external star of the twelfth magnitude which closely follows it. Eight years later, Professor Barnard employed the forty-inch Yerkes telescope to repeat his determinations,[1033] and was inclined to attribute a slight discrepancy to real motion in the nebula. But the Potsdam plates lent no confirmation[1034] to a suspicion which will probably remain long unverified. His experience was entirely negative as regards light-variability in any part of the object. Alleged changes in the stellar kernel he translated into genuine changes in visual facilities. “The fact,” he wrote, “that the nucleus is seen on a nebulous background makes steadiness of the atmosphere a most important factor in its distinctness—far more so than in the case of an ordinary star in the open sky. When the seeing is exceptionally good, the nucleus appears with a distinctness strikingly in contrast with its ordinary condition, so much so that one has to guard against deception in supposing a real change of light.”

Nine bright lines have been seen or photographed in the spectrum of the Lyra annulus, that of shortest wave-length at λ 3727 being, according to Von Gothard, the most intense.[1035] Hydrogen shines feebly; no C has been recorded, but the Rydberg ray at λ 4688 was detected by Campbell.[1036] Helium is unrepresented; since a violet line at λ 3869, which appears prominently and persistently in nebular spectra, cannot be ascribed to that substance. It chances, indeed, to coincide approximately with one belonging to a known series, but is itself of “rare and strange” origin.

Several analogues of the Lyra nebula have been discovered; none so large or so bright. The best imitation is situated in Cygnus (N.G.C. 6894). It measures 47″ by 41″, the inner vacuity 20″, and was marked “resolvable” at Parsonstown.[1037] Needless to say that the sparkling effect which conveyed the impression of a stellar constitution was altogether illusory. A fifteenth-magnitude star near the interior border of the ring to the north-west was measured by Burnham in 1891.[1038] The true nucleus is considerably fainter. First brought to view in a photograph taken by Dr. Roberts 31st August 1897,[1039] it duly reappeared in one of the Crossley pictures of 1899,[1040] and was visually discerned with the great Paris siderostat by M. Antoniadi, 17th July 1900.[1041] The elongation of the annulus from north-east to south-west was obvious to him no less than the faint haze with which it is filled. Nothing is known about the spectrum of the ring nebula in Cygnus. It doubtless resembles that of its prototype in Lyra.

Even more scanty is the information at hand regarding “a beautiful delicate ring” in Scorpio, about 40″ in diameter (N.G.C. 6337). It is in a field crowded with stars, two of which are projected upon, but may not belong to it. Neither occupies the proper position of a nucleus, and the one seen by Lassell at Malta had to him somewhat the aspect of a nebulous knot.[1042]

An annular nebula in Ophiuchus (N.G.C. 6369) was found by Mr. Burnham “very like the well-known example in Lyra, except in brightness.”[1043] A central star of 14·5 magnitude was probably then first noticed. The longest diameter of the ring measures 31″; the edges seemed to Sir John Herschel “a very little cottony,”[1044] and they are doubtless fringed with dim appendages, like those attached to its model in Lyra. This is again copied in an object discovered by Mr. Gale of New South Wales in 1897, or perhaps a little earlier. It is bright enough to make its late detection somewhat surprising.[1045]

Immersed in a fine cluster in Argo (Messier 46), a nebula of planetary aspect, about 60″ in diameter (N.G.C. 2438), drew the attention of Sir William Herschel. The Rosse reflector showed a central star dominating a vacuous interior, besides two stars sparkling on the condensed border.[1046] Lassell perceived in the object a resemblance to a large, dim, compound planetary in Eridanus (N.G.C. 1535), but with the qualifying circumstance that in N.G.C. 2438 only one “stratum of nebulosity” was discernible.[1047] This singleness of construction appears characteristic of perfected nebular rings, and such the inmate of the cluster in Argo has declared itself to be. In a photograph taken by Dr. Roberts, 24th February 1894,[1048] it is definitely and unmistakably annular. Three stars are projected upon, or contained within it, we cannot tell which; although one by its nuclear position gives some assurance of being there through organic relationship. Nor can we venture to assert that the nebula is really in the cluster. It may only be thrown accidentally into line with it. Still the fact that Sir John Herschel recorded in two cases similar collocations of planetary nebulæ with clustered stars[1049] inclines the balance of probability towards the side of genuine association. One of these groups (N.G.C. 5979) is situated in the constellation Circinus, the other (N.G.C. 2818) near the mast of Argo.

Certain complex formations, intermediate between planetary and annular nebulæ, have now to be considered. A striking specimen of the kind is met with in Andromeda (N.G.C. 7662). The disc, which includes perhaps more than one ring, measures 32″ by 28″. Alexander, about the middle of last century, and Lassell subsequently, thought the structure bi-annular. Vogel[1050] and Holden remarked its warped and twisted appearance, denoting possibly a multiple combination of rings thrown off in various planes as the outcome of long-past crises in a slow process of development. A central star surrounded by close spirals of nebulosity was seen at Parsonstown,[1051] but evaded the scrutiny of O. Struve in 1847, of Searle in 1866, and of Vogel in 1883. Lassell perceived it under the guise of a minute, bluish disc, Burnham as an ordinary fifteenth-magnitude star.[1052] Fig. 4 in Plate XXIII. reproduces Professor Keeler’s drawing of the Andromeda planetary with its visual spectrum.[1053] “This nebula,” he wrote, “is annular, with a bright inner ring and a very small nucleus. It is somewhat elongated north and south.” The fourth line in the spectrum is the “fundamental” of Rydberg’s hydrogen series; its unusual strength makes it the equal of the ordinary hydrogen lines on either side of it. The spectrum of the nucleus appears to be perfectly continuous, save for a possible bright knot about the place of D_{3}. Campbell, however, caught no glimpse of this radiation, although he determined, visually and photographically, eighteen bright lines in the spectrum of the nebula.[1054] We subjoin his list, with notes and comments between brackets.

BRIGHT LINES RECORDED IN N.G.C. 7662. ┌────────────┬────────────────────────────────────────────────────────┐ │Wave-length.│ Remarks. │ ├────────────┼────────────────────────────────────────────────────────┤ │ 540│Very faint, difficult (Wolf-Rayet line; Pickering │ │ │ series). │ │ 532│Very faint, difficult (possibly the chromospheric K │ │ │ 1474). │ │ 5007│First nebular line, very bright. │ │ 4959│Second nebular line, very bright. │ │ 4861│Hβ, very bright. │ │ 4744│Faint (origin unknown). │ │ 4715│Faint (unknown). │ │ 4688│Very bright (Rydberg hydrogen line). │ │ 4663│Very faint (unknown). │ │ 4643│Faint (nitrogen ?) │ │ 4472│Very faint (helium; prominent in Orion stars). │ │ 4364│Bright (unknown). │ │ 4341│Hγ, very bright. │ │ 4102│Ηδ, very bright. │ │ 4067│Very faint (unknown). │ │ 4026│Very faint (helium). │ │ 3969│Hε, very bright. │ │ 3869│Very bright (unknown). │ └────────────┴────────────────────────────────────────────────────────┘

The brilliancy of the last line, in view of the faintness and fewness of other helium emanations, confirms the inference that its association with that substance is inadmissible. Experiments in the laboratory would nevertheless be valuable on the behaviour, for instance, of the adjacent violet line of helium in a mixture of that gas with hydrogen.

Campbell perceived this nebula to consist “of two nearly concentric rings more or less broken up, with a fourteenth-magnitude stellar nucleus near its centre.” Ingall described it, 18th December 1885,[1055] as a “superb planetary,” of a bright greenish-blue tint, the centre not quite dark, and thus fitly to be called annular. On applying a power of 500, “an extraordinary structure appeared to unfold itself. The bright ring seemed very jagged and fringed at the edges, and the centre was mottled with unequal shades, often as if of _two_ dark centres.” The note made on it at Harvard College was: “Somewhat annular; edges hazy.”[1056] The blue colour of the object faithfully corresponds to the actinic energy of its rays. In two seconds of exposure to them the Crossley reflector gave a weak image, including a barely visible central star,[1057] and Professor Keeler obtained finished pictures in 20, 30, and 60 seconds. At Potsdam, in 1892–93,[1058] thirty-three impressions were taken for the purpose of parallax-determinations, but they yielded no positive result. They afforded only the information that the nebula could not have an annual parallax so large as one-fifth of a second; and, indeed, the true value of the quantity, judging from other indications, may very well fall short of one-tenth the assigned maximum.

A “sky-blue likeness of Saturn” in Aquarius (N.G.C. 7009) is built very much on the lines of the Andromeda planetary, but with the addition of “ansæ.” These, in August 1888, were resolved by Holden and Schaeberle, with the aid of the Lick refractor, into a pair of attendant nebulosities, situated in line with the major axis of a strongly elliptical body, and subsensibly united to it by evasive gleams of illumination.[1059] Yet the likelihood is small of their being really satellite-globes revolving in the same track at an invariable interval of two right angles. The probabilities of the case oblige us to believe rather that the original interpretation of them as the extremities of an annular appendage came nearer the truth. An analogy indeed suggests itself between them and the nebulous effusions from the vertices of the ring formation in Lyra.[1060] And here again we are assured that they mark an equator—that the disc they seem attached to must be the projection of a rotating spheroid. Intricacies of interior arrangement are, however, visible, showing the progress of manifold activities. Two dark cavities, extended parallel to the major axis, and helical wisps of nebulosity, were observed by Vogel at Vienna in 1883.[1061] Keeler saw and photographed a somewhat distorted condensed ring measuring 26″ by 16″,[1062] and Scheiner’s plates recorded curious spoke-like projections from an intensely actinic central star.[1063] The spectrum differs from that of the Andromeda planetary only by the inclusion of the enigmatic “last line” at λ 3727. From the displacement of the green ray of nebulium Keeler determined for the Saturn nebula a movement of approach towards the sun at the rate of thirty-one miles a second, only a small proportion of which can be due to our own journey through space.

Sir William Herschel observed in the constellation Gemini in 1787 “a star of the ninth magnitude, with a pretty bright nebulosity equally dispersed all around” (N.G.C. 2392). Lord Rosse found in it a dark hole close to a slightly eccentric nucleus.[1064] D’Arrest thought the object might be called annular;[1065] Lassell perceived a ring surrounding a bluish disc;[1066] Secchi described it as a star with an annular aureola.[1067] H. C. Key, using an eighteen-inch silver-on-glass reflector, noticed about 1868[1068] a concatenation of bright and dark rings besides the patch of interior obscurity detected at Parsonstown. To Burnham the nebula in Gemini appeared “one of the most beautiful objects of the kind in the heavens.”[1069] He assigned to it a diameter of 45.″ Barnard, finally, was impressed by its “magnificent and beautiful” effect in the Yerkes telescope.[1070] It disclosed to him a ninth-magnitude star encircled not quite symmetrically by a brightish oval ring, partially incomplete towards the south. “This ring,” he continued, “which is well defined inside and out, is surrounded by a vacuity, and this in turn by an almost circular broad ring of light less intense than the inner ring, and with a distinct break in it north-preceding. It breaks up into a clouded or unequal surface, and is very irregular on its inner edge, but fairly uniformly circular on its outside edge. The inner ring is filled with a nebulous light which has a black spot in it, south-preceding the nucleus.”

That the perforated effect is no illusion may be taken as proved. Nor is it subject to change. It has obtruded itself now for half a century upon one observer after another, under divers conditions, both instrumental and climatic. This nebula then, and a few others like it, betray the action of some force strange to our experience, by which the matter contained in an extensive region is either expelled thence, or its light-giving faculty suppressed.

The annular nebula in Lyra is the only member of the class that has been satisfactorily investigated. Until spectrographic possibilities are further developed there seems little chance of dissipating the perplexities that still envelop its nature. That is, by direct means; for indirectly much may be done. Comparisons, for instance, of detailed results for sister-objects ought to prove highly instructive as to the laws governing the construction of all alike. Most of them have been singularly neglected, considering the interest attaching to their peculiarities. The hooped nebula in Cygnus, the “ghost” in Scorpio, Gale’s annulus, the ring in Ophiuchus, should be photographed with long exposures, on a scale sufficiently large to bring into view specialties of texture and build. Their agreement in certain fundamental relations would thus be tested, and its importance as a guide to theories of their mode of origin cannot be overrated. Their self-delineation would, however, doubtless accentuate besides that variety in similarity which, throughout the whole created world, illustrates the wealth of the resources disposed of by Nature, and the inexhaustible inventiveness of the Mind revealed in Nature.

PLATE XXVI.

Photograph of the Orion Nebula (W. H. Pickering)
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