Chapter XXXIX: Variable Nebulæ

The occurrence of local changes of brightness is reasonably certain, as we have seen, in some of the great irregular nebulæ, and may be suspected in others. Variability affecting smaller objects in their entirety must then be admitted as possible. No doubt the phenomenon would introduce ideas difficult to adjust and unexpected; but the heavens are full of surprises. The immediate question to be put regarding it is, Does it really subsist? The answer must be given with extreme circumspection. The visibility of nebulæ depends upon contrast; the blackness of the sky has as much to do with it as the brightness of the filmy masses projected against it. They are besides apt to disappear with high magnification, and that for two reasons. First, because of the diffusion over a larger area of the same quantity of light; secondly, because of the restriction of the background in narrower fields of view. Hence there are drawbacks to the employment of large telescopes in nebular observation. The history of Tempel’s Merope nebula, marked by vicissitudes ascribed again and again to intrinsic causes, now fully recognised as non-existent, is a warning against hasty conclusions on so delicate a point. The lesson has indeed been so thoroughly learned that changes of the sort have of late been announced only with a certain timidity, and under reserve. Caution in the matter can, indeed, hardly be blamed for exaggeration, in view of Swift’s remark that, after the Krakatão eruption, many faint nebulæ absolutely disappeared.[1146] Nor need we go beyond Chacornac’s “temporary nebula” for an exemplification of optical caprices. On 19th October 1855 the French observer noticed a striated haze (N.G.C. 1988) attached to the star ζ Tauri, which seemed to have gained brightness in the ensuing January. No one else, however, saw it, and it had vanished by 20th November 1862. According to Tempel it never shone in the sky, but was a telescopic creation—a false image of an eleventh-magnitude star near ζ Tauri;[1147] and Burnham unhesitatingly adopts this opinion.[1148] Thirty years later its reality could have been tested by photographic means; but astronomers in those days had to rely upon the fallible human retina.

Chacornac’s phantom formation emerged near the site of two genuine nebular Novæ. On 11th October 1852, Hind detected, close to a star, then of the tenth magnitude, but since registered as an irregular variable under the designation T Tauri, a dim, round nebula (N.G.C. 1555) which brightened steadily until 1856, when it was obvious to general observation. A comparatively rapid decline ensued. Auwers[1149] could barely discern the object with the Königsberg heliometer in January 1858; to d’Arrest,[1150] using the eleven-inch Copenhagen refractor, it was wholly invisible 3rd October 1861; in 1862 it was vainly sought at Paris and at Malta with Foucault’s and Lassell’s great mirrors; Hind himself was unable to find it; Secchi, under the pure Roman sky, was equally unsuccessful; only at Pulkowa it continued to glimmer just perceptibly for a few months longer. From 1863 the sky in its place seemed a dead blank. At last, 15th October 1890, Mr. Burnham requested his colleague, Professor Barnard, to examine the region with the Lick thirty-six-inch, whereupon a nebulosity about 50″ in diameter, and so faint as to be at the limit of vision, was detected[1151] (see Fig. 49). Burnham too saw it, but believed that he could not have done so independently, his splendid powers of sight being better adapted to the discernment of concentrated than of diffused light-rays.

The nebula was again observed by Barnard—and with somewhat increased facility—in February 1895.[1152] Seven months later he was amazed to find it utterly gone! His search was repeated on three nights, under supremely good conditions, with the same negative result. And the object, so far from evading the grasp of large apertures, is peculiarly fitted for observation with them, owing to its small size and compact shape. Nevertheless, the forty-inch Yerkes refractor failed to show it at all in 1897, and barely enabled Barnard to catch a glimpse of it, 28th September 1898.[1153] Finally, Professor Keeler obtained faint images of it on two Crossley plates, exposed during four hours each, in December 1899.[1154] A copy of his drawing from them is given in Fig. 50. It exhibits the nebula as composed of three vaguely defined patches, united by a dim haze, the camera having, as usual, descried structural complexities inappreciable by the eye. On 20th January 1900, the great refractor just availed to bring it into view, and it has not since been heard of. To Professor Keeler it appeared inconceivable that in its present obscure state it could ever have been seen with small telescopes; and indeed the evidence of variability is conclusive. Will it ever recover any of its lost brightness? It may be doubted. The changes so far undergone by it have been, though comparatively slow, strictly analogous in character to those of “new” stars; a presumption hence arises that it will share their fate of permanent extinction. There is much reason to suppose that Hind noted in 1852 an early stage of its kindling; that its maximum in 1855–56 was solitary, its declension irretrievable.

FIG. 49.—Sketch of T Tauri and Hind’s Nebula, 15th October 1890
(Barnard).
]

Strange to say, the phenomenon was duplicated. While looking fruitlessly for Hind’s nebula, Otto Struve came upon another unfamiliar object (N.G.C. 1554) 4′ east of its predecessor’s empty place. This was early in 1868; and the Nova—for d’Arrest was sure of its previous non-existence[1155]—was kept in view until 1877, when absolute obliteration covered it. Even Barnard’s quest for it in 1890, 1895, and 1899 was ineffectual. Its former position is marked in Keeler’s drawing (Fig. 50) by the thirteenth-magnitude star _b_, but no nebulous impression was there made upon the plates. Dr. Roberts had indeed already, in 1890,[1156] vouched for its photographic disappearance. When last seen by Tempel,[1157] 8th November 1877, the nebula was 90″ across, and d’Arrest had expressly recorded the presence in it of an eccentrically situated, though definite nucleus.

FIG. 50.—Region of T Tauri and Hind’s Variable Nebula. Drawn from
Photographs by Professor Keeler.
]

Hind’s and Struve’s nebulæ were bright enough, as Professor Barnard recalled wonderingly,[1158] to be ranked, soon after the middle of the last century, in Herschel’s first and second classes respectively. Both were watched “and measured by the best observers then living,” and “were easily visible in ordinary telescopes.” Yet one survives only as a fitful shimmer; the other is utterly, and probably for ever, quenched. They are perhaps obscurely connected, and the system—if such it can be termed—may include the nebulous variable T Tauri; although nebulæ and star alike seem to fluctuate in complete independence one of the others.

These are the only authentic instances of temporary nebular developments; but allegations of nebular light-change are common. Some have proved groundless; not a few, however, rest on a solid substratum of fact. The following may serve as specimens.

On 17th October 1785 William Herschel discovered, not far from Algol, “a pretty bright star with two faint branches” (N.G.C. 1186). Sir John verified the observation, while estimating the star at only fourteenth magnitude (about twelfth on the modern scale). Yet neither it nor its appendages could be seen with the Parsonstown reflector, and d’Arrest, after diligent and repeated search, affirmed decisively, _Nostra ætate in hac regione tale quid non exstat in cœlo._[1159] The lost object, nevertheless, came again into view in 1891. On 31st January and 26th February of that year, Bigourdan, using the thirteen-inch Paris equatorial, perceived it as a twelfth-magnitude star, with an unmistakable “fan” of nebulosity spreading from it over 1′ of arc. Only a fortnight later, Spitaler made a drawing of the object with the Vienna twenty-seven-inch, and described it as an elongated nebula, not regular enough to be called elliptical, 2′ in extent, and including a focal star of the eleventh magnitude.[1160] And Burnham, in the following August, “readily found in the proper place a _tenth_-magnitude star involved in a faint elongated nebula” measuring at least 2′ or 3′.[1161] During the course of 1891, accordingly, the star seems to have been progressively gaining light and the nebula compass. But if the star only were variable, the attached nebula would have appeared to shrink and become effaced as the bright point within it acquired intensity. Its simultaneous increase could not have been counterfeited. Proof was afforded by it that the growth, like the previous failures of luminosity, were due to influences diffused throughout every part of the formation.

The next variable nebula was a “find” of Barnard’s. It was conspicuous to him 30th November 1888. He judged it equal to a ninth or tenth-magnitude star, and remained convinced that its lucidity was of recent origin. Three years later it had parted with quite four-fifths of its lustre, and had faded down nearly to evanescence.[1162] It is situated in Cetus. No information is at hand as to whether its decline has continued since 1891. Unless arrested, it must, before the century closed, have carried it out of sight even of the chemical retina, and the object should then probably be relegated to the class of “temporaries.” The question is of great interest, and might be answered by taking one long-exposed photograph with a portrait-lens or a large reflector.

This is not the only case in which an accession of brightness has been thought to be demonstrated by the lateness of discovery. A nebula was encountered by Tuttle in Draco, 1st September 1859 (N.G.C. 6643), which, in d’Arrest’s opinion, should certainly have been caught in the meshes of the Herschelian nets unless in their time comparative obscurity had enveloped it. Similarly, a nebula in Camelopardalis detected by Barnard in 1889,[1163] and again independently by Denning in 1890,[1164] could not long, Barnard considered, have been thus readily apparent. He recommended its being kept under surveillance as a probable variable, but as yet it has shown no sign of being so;[1165] unless, indeed, Swift’s earlier observation of the same object, recorded without date by a simple entry on a star-map,[1166] indicated a previous maximum. Another instance of a possible rise in the scale of luminosity is afforded by a small, fairly bright nebula in the Camelopard, first observed by Denning 30th September 1891, and casually again four times in the ensuing month. Yet his many previous reviews of the sky-contents in that neighbourhood had failed to elicit any trace of its existence. Like Barnard’s and Tuttle’s “new” nebulæ, however, it has apparently come to stay; and since variability is in sidereal bodies usually an ineradicable property, the hypothesis of an ascent from invisibility cannot safely be accepted until a corresponding descent has been entered upon.

The irregular variability, on the other hand, of two nebulæ adverted to by Winnecke in 1877–78[1167] is almost incontestable. The first (N.G.C. 3666) is in Leo. Elliptic in shape, in size 90″ by 40″, it was marked “very bright” by the elder Herschel 15th March 1785, but “extremely faint,” 23rd March 1830, by the younger, who added the comment, “This nebula must have changed greatly if ever it belonged really to the first class.” But its waning was not definitive. Boguslawski inscribed it as a _bright object_ in 1840 on the Berlin Academy star-map of that region; and Winnecke found it, 10th April 1878, of unquestionable primary rank. Yet meantime, in 1863, d’Arrest had described it as _subobscura_, and manifestly of third-class lustre; while again, on 24th May 1887, Dr. Dreyer perceived its diminished radiance only with the utmost difficulty. Its further history remains untold.

Winnecke’s second variable (N.G.C. 955) is an inmate of the crowded nebular district in Cetus. It consists, Burnham says,[1168] of long, narrow “nebulous wings on either side of a bright central condensation.” “On the whole,” he continues, “it is rather a curious object, and should be easily found and seen.” This was in 1891, and agrees quite well with Dreyer’s notice of the object in November 1887 as “fully of the second class.”[1169] The case for change rests upon its invisibility to Schönfeld in December 1861, and to Vogel in November 1865; although in 1856, 1863, and 1868 it had been seen at a glance by Schönfeld himself, no less than by d’Arrest and Winnecke.

Winnecke’s nebulæ were at first held by him to be periodical; but this they certainly are not. No fixed relation to time has so far been shown to govern nebular fluctuations. They either consist—according to the best evidence at command—of a solitary maximum, analogous to the outburst of a new star, or of irregular accessions and losses of light. No case of cyclical recurrence is on record. Photography is clearly destined to play an important part in the investigation of this difficult subject; its aid will be peculiarly welcome where visual faculties are often baffled, embarrassed, and deceived.

Nebular variability is indeed a phenomenon not only evasive to the senses, but startling to thought. It cannot be even remotely assimilated to the light-changes that progress in certain globular clusters; it is independent of geometrical conditions, of orbital movements, of planes and periods. Its cause defies conjecture; we can only be sure that it acts upon a prodigious scale. Thus Hind’s nebula in Taurus measured at least 2′ across. Its parallax was almost certainly less than one-tenth, and may not have exceeded one-hundredth of a second. The larger value would give, for the smallest admissible linear diameter of the object, 1200 astronomical units (radii of the earth’s orbit) or 111,000 million miles. Centred on the sun, it would extend on every side to twenty times the distance of Neptune, the equation of light within the vast formation being six and a half hours. Yet it kindled as a whole, through the pervading influence of some far-reaching event. Did another dark nebula sweep through it? We dare not pronounce. Its mysterious brightening, however, hints at the existence of an indefinite multitude of similar bodies lurking in the obscurity from which, by some rare chance, it emerged. It introduces us, in fact, to a realm of invisible nebulæ, impenetrable by observation, and hence pre-eminently adapted for the sports of scientific fancy.

Hind’s and Struve’s vanished nebulæ were presumably of gaseous composition, like the adjacent glow round T Tauri;[1170] Winnecke’s variable pair doubtless shine with the white light characteristic of the elliptic family to which they structurally belong. Their remoteness from the Milky Way points to the same conclusion. Luminous instability does not then appear to be associated in nebulæ with any special radiative quality. Those giving continuous, and those giving discontinuous spectra may equally be affected by it.