Chapter XXXIII: Nebulous Stars

“There is a vast difference,” Professor Swift wrote in 1897, “between a nebulous star and a star in a nebula.”[937] Vast indeed, since a star and nebula in reality billions of miles apart may be thrown by perspective into the same visual line, while a true nebulous star claims the ownership of its luminous appendage on the evidence of obviously adapted form. The nimbus is not more unmistakably fitted to the head of a saint in a picture than nebulous halos are, very often, to the stars they encircle. The relationship is patent. That the glow emanates from the star, and is no casual adjunct to it, the instinctive logic of the eye suffices at once to decide.

Sir William Herschel was the first to give express attention to “stars with burrs.” They struck him as remarkable, not only in themselves, but for what they implied. They served as the basis of a memorable train of reasoning.[938] Their atmospheres, he argued, being plainly “not of a starry nature,” must be composed of a “shining fluid,” the same which is seen to be diffused through space in milky tracts, or curdled into fantastic shapes of chaotic irregularity. He was, in a word, led by them to the capital discovery of _nebulæ_ as a distinct sidereal order. On 13th November 1790, he came across the “singular phenomenon” which determined his abandonment of the view that the universe is constituted exclusively of stars variously aggregated. This was “a star of about the eighth magnitude, with a faint luminous atmosphere of a circular form, and of about 3′ in diameter. The star is perfectly in the centre,” he continued, “and the atmosphere is so diluted, faint, and equal throughout that there can be no surmise of its consisting of stars, nor can there be a doubt of the connection between the atmosphere and the star.” This exemplar object, situated in Taurus (N.G.C. 1514), he regarded as “decisive in every particular”; yet its nature is still to some extent dubious. It has even been classed of late as a planetary nebula, and certainly shares not a few characteristics of that family. The uncertainty of its status renders its study especially instructive.

A planetary nebula is definitely terminated; a nebulous star fades off into space. One shows a disc; the other is surrounded by an “atmosphere.” Moreover, a star, or stellar nucleus, is subordinate in the one formation, while it dominates the other. These distinctions, however, cannot always be unhesitatingly drawn, since the relative strength of the stellar and nebular elements varies widely in different objects. Hence the doubt as to the category in which Herschel’s typical specimen should be ranked. For the glow round it is uncommonly bright; d’Arrest found it to strike the eye with a four-and-a-half-inch refractor.[939] Nor is it equably diffused. The Parsonstown telescope showed it as spotted and patchy, and very curiously “ragged” at the edges;[940] and its aspect to Mr. Burnham in 1891 was essentially the same. A “broken and mottled” surface, about 126″ across, emerged in the field of the Lick thirty-six-inch.[941] He was inclined, though under reserve, to agree with Barnard in considering the object planetary. Its affinities might be settled by spectroscopic means; the attempt to do so could, at any rate, hardly fail to have an interesting result. So far, little has been ascertained about the spectra of true nebulous stars. Their halos, certainly in some cases, presumably in all, emit bright lines, and it might be expected that the same lines would show by absorption in the spectrum of the central star. The presumption has not, indeed, been fully verified; while the expectation founded on that presumption is disallowed by the facts scantily at our disposal. Stars with nebulous appendages are usually, if not invariably, distinguished by “early Orion” spectra—a combination already noted as significant in connection with the course of sidereal growth; but they show no special lines that could be attributed to light-stoppage by the immense bulk of rarefied incandescent matter interposed between our eyes and their shining photospheres. This is one of the many perplexities involving the luminous relations of nebular stuff, which, setting “Kirchhoff’s law” at defiance, exercises no absorption correlative to its emission.

One of the few nebulous stars bright enough for easy spectroscopic investigation is situated in Scutum Sobieski. Of 5·5 magnitude, it is enrolled in the Bonn Durchmusterung under the heading S.D. −10°4713. On a plate exposed by Professor Barnard with the Willard lens, 29th June 1892,[942] a large diffused nebulosity was seen to encircle it. The appendage must be visually very faint to have escaped notice so long. Its proper spectrum may then be nearly evanescent. The star it belongs to is No. 8198 of the Draper Catalogue, where it is set down, although doubtfully, as of the second type (Spectrum E). Now this is a point of crucial importance to theories of stellar development. The pronounced nebulous condition of a star near the solar stage would have a bearing on such inquiries that could not be ignored. Should it be established, current ideas will need revision. The spectral character of Barnard’s _nebulosa_ in the Shield promises, indeed, to afford a test by which to try the validity of reasonings on sidereal evolution. The test ought, with the least possible delay, to be applied.[943]

A seventh-magnitude star in Eridanus was perceived by Swift in 1859 to be almost centrally placed in a shining corona.[944] It is perhaps identical with N.G.C. 5315. Its spectral classification should present no difficulty. A similar object, equally adapted for spectroscopic inquiry, was detected by the elder Herschel in Cepheus (N.G.C. 7023). The nebulosity is particularly strong north and south of the star. Irregularities of a more marked kind are apparent in other instances. A tenth-magnitude star in Monoceros was found by Barnard visually nebulous in 1888, photographically in 1894.[945] A “small dark space,” however, interrupts the encircling halo. A subsequent exposure with the same instrument disclosed as “closely nebulous” the 9·5 magnitude star, D.M. + 23° 1313.[946] And here again the illumination is unevenly distributed, the “fuzzy” border to the star-disc being denser south and east than elsewhere. This object lies almost midway between η Geminorum and χ^2 Orionis. One in most respects analogous (N.G.C. 2247), detected by Swift, 24th November 1883,[947] came out noticeably “blurred” on the same plate with Barnard’s nebulous star in Monoceros. Two further specimens of the class were photographed by Barnard in Sagittarius. One—D.M. −19° 4948—is fainter than the ninth magnitude. It has a narrow fringe of light.[948] The second—D.M. −19° 4953—is of 7·6 magnitude, and is encompassed by a far-spreading halo, 15′ in diameter,[949] conspicuous with the camera, although nearly invisible to the eye.

Nebulous stars are frequently compound—perhaps more frequently than stars clear of cosmic fog. Sir John Herschel recorded at the Cape a close pair (N.G.C. 5367) as involved in a bright glow two minutes of arc in extent; and a faint star with an aureola, discovered by Tempel in Cetus (N.G.C. 707), proved, when scrutinised by Burnham in 1891, to have a minute attendant at an interval of 10″.[950] A still more interesting detection concerned a nebulous triplet in Auriga (N.G.C. 1931). Discovered by Sir William, and described by Sir John Herschel as “one of the most curious objects in the heavens,” it consists of three stars, the brightest of 9·5 magnitude, forming an equilateral triangle with a side of about 8″, placed precisely at the centre of a small circular nebula. Mr. Burnham had repeatedly inspected it with minor instruments,[951] but it needed all the power of the Lick refractor to bring into view a fifteenth-magnitude satellite at a distance of little more than 2″ from one of the stars of the triangle. As a test for “seeing” facilities, the pair is of unsurpassed delicacy. Again, a wide double star occupies the middle point of a pretty large faint nebula in Monoceros (N.G.C. 2182). The attendant may be only optically such; the circumstances are on this point indecisive. The chief star, however, was found by Mr. Burnham to be double in a perfectly unequivocal sense.[952] A companion of nearly its own magnitude (8·6) is separated from it by a spatial gap of less than half a second, and the two must assuredly revolve round their common centre of gravity. Here, indeed, we are confronted by a profoundly embarrassing question. The couple are evidently plunged in nebulous matter; their movements must then, according to received ideas, be impeded, with the result of an eventual collapse of the system. We can see no escape from the dilemma except by adopting the startling hypothesis that the nebulous fluid does not constitute a resisting medium. The difficulty greatly enhances the interest of spectroscopically determining the velocities of bodies nebulously connected.

A 6·5 magnitude star in Cepheus (D.M. +57° 2309) appeared in a photograph taken by Barnard in 1893 “surrounded by a rather unsymmetrical dense nebulosity.”[953] A “hazy glow” could be seen with the Lick thirty-six-inch, which, in Burnham’s employment, had already revealed the star to be very unequally double at 4″.[954] A first-type spectrum is dubiously ascribed to it in the Draper Catalogue.

The nebulous triplet, ι Orionis, has been more completely observed than perhaps any of its congeners. It consists of a third and an eighth-magnitude star 11″ apart, with an eleventh-magnitude satellite at 49″, described by Admiral Smyth as “grape-red” in colour.[955] Sir John Herschel perceived the group to be “involved in a feeble nebula 3′ in diameter,”[956] and in the nebula (N.G.C. 1980) there was apparent with the Rosse reflector a central cavity containing the bright star-couple.[957] Possibly the effect was an illusion due to their effacing radiance; but this cannot be taken for granted, since “holes” in nebulæ are an attested phenomenon. And the early observations at Parsonstown approve themselves as singularly accurate through the confirmatory evidence of the best recent photographs. The spectrum of ι Orionis is of the helium type, and Dr. McClean identified in it three members of the Pickering series of hydrogen, besides many oxygen lines.[958] The surrounding glow emits the ordinary nebular rays, but they make no show, either directly or by reversal, in the dispersed stellar light. Yet before reaching outer space, that light has to traverse enormous volumes of incandescent or luminescent nebulium. The anomaly presented by the absence from the Fraunhofer spectrum of the solar coronal green line is here repeated in an emphasised form. A long nebulous streak, visible only on sensitive plates, links the hazy appendage of ι Orionis with the great formation in the Swordhandle.[959]

Far away in the northern part of the constellation, there is found in λ Orionis a combination very similar to that presented by the nebulous trio just considered. A yellowish and purple pair (Σ 738), of 3·7 and 5·6 magnitudes, at 4·2″, are immersed, with a comparatively remote twelfth-magnitude attendant at 29″, in a nebulous haze, photographed by Barnard in three hours, 17th September 1893.[960] The discovery was at once telescopically verified.

Five nebulous stars occur together in a narrow region of Sagittarius, and three of them are double. The two apparently single are Barnard’s stars, already mentioned. The pairs are N.G.C. 6589 and N.G.C. 6590, both first noticed by Swift,[961] and N.G.C. 6595, delineated seventy years ago by Sir John Herschel. The character of an object photographed by Dr. Roberts[962] near the spiral nebula M 81 in Ursa Major, needs to be more satisfactorily determined. Known to Herschel and d’Arrest as a condensed nebula (N.G.C. 3077), it appeared on the sensitive plate with a sharp, stellar nucleus in lieu of the woolly disc visually perceptible.[963] The spectroscope may perhaps help towards its rightful classification. Nebulous stars merge insensibly into stars with nebular appendages, such as ω Orionis and σ Scorpii. The former has a dimly luminous, curved spur running out from it, besides a larger mass hanging like a cloud above it to the north; the latter is winged with nebulosity, two pointed projections issuing from it in divergent directions.[964] The whole of these appurtenances were detected photographically by Professor Barnard. Both stars afford spectra marked by helium absorption. The relations of stars and nebulæ are manifold. Misty trains and tails of all sorts and sizes have stellar foci; they emanate from stars, or condense into stars; but nebulous stars are, properly speaking, what Herschel called “stars with burrs”; they give the usual sharply defined diffraction-discs, although a dim halo spreads more or less symmetrically round each. The nebular element in such a combination is entirely subordinate to the stellar; while in stars with appendages the disparity gradually becomes reversed.

“Rejected” nebulous stars are still worth attention in view of the possibility that they may be subject to genuine change. The case of 55 Andromedæ is particularly instructive, if only as illustrating the propagation of error. This is a 5·6 magnitude star, qualified as _nebulosa_ by Flamsteed and Piazzi, and regarded by Sir John Herschel as a typical specimen of a hazy star. It figures as No. 428 in his _General Catalogue of Nebulæ_ (1863), but was omitted by Dreyer from the revised edition of that work. Sir William Huggins, nevertheless, observed it in 1864 to be “a fine nebulous star with a strong atmosphere;”[965] and since he used a very perfect refractor, his confirmation of what Herschel had seen with a reflector had an independent value which might seem to exclude the hypothesis of association in optical illusion. Yet neither Lord Rosse in 1848 nor d’Arrest in 1856 had perceived any trace of nebulosity, and Schjellerup, during some years previous to 1866, always found the star sharp.[966] So again it appeared to Mr. Burnham in 1879–80,[967] and so it seems likely to remain. We can, however, scarcely persuade ourselves that several eminent observers conspired to blunder; and Schjellerup’s theory that Piazzi merely repeated Flamsteed’s note, which crept, he supposes, into the British Catalogue by a transference from the great Andromeda nebula, is rendered unacceptable by the circumstance that 55 Andromedæ follows the nebulous structure held to have been confused with it at an interval of considerably more than one hour of right ascension. Nor even if so extraordinary a mistake had been made, was Piazzi capable, one would think, of copying it unawares. His high astronomical reputation suffices in itself to clear him from the charge of such astounding carelessness. The spectrum of the star resembles that of the sun, a type never yet unequivocally associated with nebulous attachments. On the other hand, the normal quality of the light renders their optical creation more difficult of explanation. Thus the nebulous aspect of 55 Andromedæ must stand over as one of the unsolved problems of astronomical history.

A similar, but less convincing case is that of 8 Canum Venaticorum. On four separate occasions Sir John Herschel noticed this fourth-magnitude star to be surrounded by a “considerable atmosphere.” Yet since no one before or after him has vouched for its presence, he was presumably deceived. The spectrum of 8 Canum is of the solar class.

Finally, a 7·5 magnitude star in Cetus, discovered as nebulous by Stephan at Marseilles in 1880[968] (N.G.C. 988), appeared to Burnham and Barnard in 1891 devoid of any such peculiarity.[969] A photograph taken with suitable exposure would serve decisively to test its present condition. The criterion might indeed fail with stars so bright as 55 Andromedæ and 8 Canum Venaticorum; for their imprinted discs would become, through chemical irradiation in the time needed to bring out faint glows, sufficiently distended for their obliteration.

In connection with nebulous stars two lines of inquiry open out. First, the spectroscopic. The scanty evidence at our disposal is to the effect that the stellar rays of such objects are of the “Orion” kind; that they show the quality believed to characterise suns in a primitive stage, while their aureolas shine like gaseous nebulæ. But these generalisations rest on a very narrow basis of fact, and probably admit of interesting and significant exceptions. Indeed, each nebulous star should be treated as a separate spectroscopic problem, destined to afford in the course of its solution insight into many obscure secrets. A second branch of research relates to the structural peculiarities of stellar halos. Their luminosity is seldom, perhaps never, equably distributed. Its irregularity sometimes goes so far as to produce the effect of dark vacuities, photographically attested to be no mere visual deceptions. What their true nature and origin may be, is a subject for inquiries likely to be long and arduous. It is scarcely credible that they are what they appear to be, obscure tunnels, striking, in the direction of the earth, right through the heart of immense spheres of shining tenuous matter. The alternative view is preferable that the so-called “atmospheres” of stars are really effluences—that they consist essentially of spiral coils wound closely enough to merge ordinarily into an approximately uniform surface, while leaving in certain circumstances conspicuous gaps between their luminous folds. If this be so, nebulous stars fall into line with cometary nebulæ, the trains of which take a more or less completely annular shape; but their nearest allies are unquestionably to be found in the planetary family; and this brings us to the subject of our next chapter.