Sir John Herschel applied the term “falcated” to two curious nebulæ belonging, undoubtedly, to the later recognised “spiral” class. He perceived besides in oval nebulæ the annular lines of structure emphasised in Dr. Roberts’s photographs. He remarked, further, that “as the condensation increases toward the middle, the ellipticity of the strata diminishes.”
His study of the Magellanic Clouds gave the first idea of their composition. He showed them to be aggregations on a vast scale of every variety of cosmical product. “When examined through powerful telescopes, the constitution of the Nubeculæ, and especially of the Nubecula Major, is found to be of astonishing complexity.” He drew up a preliminary catalogue of 1,163 stars, nebulæ and clusters included in them, the conjunction of which was really decisive as to nebular status. For he showed, from the elementary principles of trigonometry, that, taking the Greater Cloud to be roughly spherical in shape, its nearest and remotest parts could differ in distance from ourselves by little more than one-tenth the distance of its centre. The fact was thus demonstrated that seventh and eighth-magnitude stars and irresolvable nebulæ co-exist within those limits. He stopped short, however, of the conclusion drawn by Whewell and Spencer, that the stellar and nebular sub-kingdoms are not only locally intermixed, but inseparably united.
The Magellanic Clouds are the most conspicuous features of the barren south polar heavens. Round the Lesser Cloud especially, the sky, Herschel said, “is most oppressively desolate.” And again: “The access to the Nubecula Minor on all sides is through a desert.” One of the separate inmates of the Larger Cloud is the “great looped nebula,” compared by Herschel to “an assemblage of loops,” the complicated windings of which make it “one of the most extraordinary objects which the heavens present.” To the eye of the present writer it resembled a shining strip of cellular tissue hung up against the sky. The “lace-work nebula” in Cygnus is of the same type; but here the tracery of nebula is closely followed by a tracery of stars. Truly, “A most wonderful phenomenon!” as Herschel exclaimed in contemplating it.
The first photographs of the Magellanic Clouds were taken in 1890–91 by Mr. Russell of Sydney. They contained an extraordinary revelation. Both objects came out in them as gigantic spirals. Their miscellaneous contents are then arranged according to the dictates of a prevalent, though unexplained cosmical law. The Nubecula Major is a double vortex, and the extent of its outlying portions, invisible except to the camera, is at least eight times that of the central mass; but they conform to the same helical lines.
Herschel catalogued 1,203 stars strewn over the surface of the famous Argo nebula, and devoted several months to its delineation. This he found “a work of great labour and difficulty.” While at the telescope he often half surrendered to despair “of ever being able to transfer to paper, with even tolerable correctness, its endless details.” “Language cannot easily convey,” he said, “a full impression of the beauty and sublimity of the spectacle this nebula offers when viewed in a sweep, ushered in by so glorious and innumerable a procession of stars, to which it forms a sort of climax.” Only the Orion nebula may be thought to surpass it in “magnitude, complexity, and brightness.” Its most characteristic feature is an abrupt vacuity, of a “lemniscate oval” shape, from which it derives the name of the “Keyhole Nebula.” The value of Herschel’s drawing of this grand object has been accentuated by its photographic portrayal. Their comparison betrays, in fact, the occurrence in the interval of what appears to be a vast change. Already, in 1871, Mr. Russell missed with surprise a prominent feature in the Feldhausen picture; and its failure to appear on photographic plates exposed for eight hours, yet impressed with innumerable stars previously unseen, raised something more than a presumption that, as Mr. Russell said, “a well-defined and brilliant portion of this nebula vanished between 1837 and 1871.” Its disappearance was independently verified by Dr. Gill, Royal Astronomer at the Cape. With a total exposure of more than twelve hours, in March, 1892, he secured a magnificent representation of this wonderful object, fundamentally agreeing with Herschel’s, save only as regards the mass of bright nebulosity vainly looked for by Mr. Russell. The “swan-shaped” or “trident-like” structure was clean gone! That is to say, the matter composing it had ceased to be luminous. It should be added that Mr. Ranyard, whose special experience lent weight to his opinion, thought it unsafe to trust much to comparisons of drawings of such baffling objects, either among themselves or with photographs.
Before leaving the Cape, Herschel witnessed an event testifying surprisingly to the vitality of this nebula. In a condensed tract close to the dark “keyhole,” he was accustomed to see the bright star Eta Argûs. It gave no sign of being variable until, on December 16, 1837, he perceived with amazement that it had, all at once, nearly tripled in brightness. After this sudden leap, it mounted gradually to the level of Alpha Centauri, then slowly declined. It just matched Aldebaran when Herschel lost sight of it in March, 1838. A second, and even more vigorous outburst was watched by Sir Thomas Maclear in 1843. It then overtopped every star except Sirius, and for seven subsequent years rivalled the splendour of Canopus. No notice was at first taken of its colour; but it was redder than Mars in 1850, and reddish it still remains, in its low estate of invisibility to the naked eye. But since bright lines of hydrogen show in its photographed spectrum, we may suspect that—
“Even in its ashes live its former fires,”
and that, consequently, its vicissitudes are not yet terminated. The instability of its character was virtually discovered at Feldhausen. Except by Burchell, the African traveller, no previous suspicion of it had been entertained; the numerous facts denoting that the star’s past behaviour had been abnormal were collected by Sir John Herschel after it had been caught in flagrante delicto. In his belief, it had no physical connection with, but was merely projected upon, the nebula. But since then the nebular relations of blazing stars have been strongly underlined. The mass of circumstantial evidence now accumulated on the point fully warrants the assertion that Eta Argûs makes an integral part of the formation it once illuminated.
A cluster in the constellation of the Cross, unique in the varied and brilliant tints of its principal components, was compared by Herschel to “a gorgeous piece of fancy jewellery.” Within the space of 1/48th part of a square degree, he determined the places of no less than 110 of them, referred to Kappa Crucis, a rosy orb round which they are irregularly scattered. The colour-effects in this beautiful ornament of the sky need large apertures for their full display.
An object showing to the eye as a hazy star of the fourth magnitude was entitled by Bayer in 1603 Omega Centauri. Herschel’s twenty-foot disclosed it as “a noble globular cluster, beyond all comparison the richest and largest in the heavens.” Dr. Gill obtained an admirable photograph of it May 25, 1892. The stars composing it are literally countless. On a plate exposed for two hours at Arequipa, Mr. Solon I. Bailey reckoned nearly 6,400; yet he made no allowance for those “too faint and closely packed” to be perceptible except as a “mottled grey background between the distinct images.”