Chapter XXIV: Temporary Stars

A temporary star is a variable that rises sheer from profound obscurity to a single maximum. The maximum may be prolonged or multiple, but it must be essentially one. The occurrence of a second independent outburst would at once relegate the object to the category of irregular variables. The distinction is perhaps arbitrary, but we can only investigate by dividing. It will be best to plunge at once _in medias res_ with some account of an apparition which attained to epochal importance through the efficacy of photographic methods of research.

Nova Aurigæ sprang into conspicuousness with the stealthy speed of Jack’s Beanstalk. On 8th December 1891 Dr. Max Wolf took, at Heidelberg, a photograph of the sky round χ Aurigæ, showing stars to the ninth magnitude. The Nova was not among them; the spot destined for its occupation was vacant. Forty-eight hours later, as a Harvard negative attested, a fifth magnitude star filled the blank. By 20th January 1892, twelve records of this stranger’s presence were included in the same series; but inadvertently, for the documents were stored up unread, and it was only through their subsequent examination that a maximum of 4·4 magnitude was inferred to have taken place on 20th December. During nearly two months, then, a new star, readily visible to the naked eye, shone unnoticed in the heavens. It was finally perceived by an amateur, Dr. Thomas D. Anderson of Edinburgh, and an anonymous post-card, by which, on 1st February 1892, he conveyed the news to Dr. Copeland, formed the starting-point of widespread astronomical activity. The spectrum of the Nova was promptly photographed at Tulse Hill, South Kensington, Stonyhurst, Potsdam, and Mount Hamilton; eager study was devoted to its implications; and they were of a most unexpected kind. For no previous stellar apparition had been _analytically_ recorded; and the similar spectral phenomena, doubtless present in Nova Coronæ and Nova Cygni, eluded definite determination with the eye. But when the turn of Nova Aurigæ came, the spectrographic method was effectively at hand, and the peculiarities of its light could be rendered obvious, salient, and permanent. Some of them were indeed visually manifest. The spectrum was at once seen to blaze with bright lines, many of them greatly widened (see Plate XVII.); and nearly all came out photographically as attended by strong dark companions on their more refrangible sides. The entire hydrogen series, from crimson C to the last of its ultra-violet associates, was thus doubled, no less than the pre-eminent calcium pair, the sodium D, and a considerable number of lines since identified as originating from helium. An exceptional feature was the predominance of “green” helium; D_{3} and the rest of the lines belonging to the “yellow” set were comparatively faint; while λ 4922, λ 5016, and their fundamental, λ 6678, shone lustrously. Their superiority, never before observed in the spectrum of a heavenly body, results in the laboratory from heightening the exhaustion of the emitting gas; but this condition seemed to be excluded in Nova Aurigæ by the distended aspect of rays unmistakably proceeding from a more than ordinarily condensed atmospheric stratum. Unprecedented, likewise, was the simultaneous brightening of the _three_ D-lines. No stellar spectrum previously observed had shown the action of sodium otherwise than by absorption. Both characteristics, it is true, may have been present in earlier Novæ, but they first arrested attention in Anderson’s star.

PLATE XVII.

Visible Spectrum of Nova Aurigæ, 28th February 1892. Intensity-Curve
below (Campbell).
]

These novelties, curious though they were, sank nevertheless into insignificance compared with one dominant trait. This was the large opposite displacements of the bright and dark sets of lines. Contrary motions of prodigious velocity appeared to be indicated, and for a time their prevalence was taken to be incontrovertibly attested. The outburst, by a consensus of opinion, resulted from the approach, and integrated the light of two components, one a bright-line star receding from the earth at the rate of about 230 miles a second, the other a dark-line star hurrying towards it with a speed of 320 miles. Soon, however, incongruities began to develop, and they grew and multiplied as time went on. To begin with, the spectral shifts underwent no alteration; the movements indicated by them—if they did truly indicate movements—persisted without abatement as the bodies animated by them withdrew from each other’s vicinity to a distance greatly exceeding that of Neptune from the sun. Now velocities, to continue uniform, must be inherent; that is to say, they cannot represent the merely temporary effects of gravitational pull, since orbital acceleration is strictly balanced by retardation. Evidently, then, the components of Nova Aurigæ did not simply fall together; they should have been fabulously massive to have produced, by their mutual attraction from infinity, a speed which continued at the rate of 550 miles a second three months after the periastral rush-past, the date of which presumably coincided with the first rise to brilliancy on 10th December. Professor Seeliger of Munich[746] calculated that 15,000 times the sun’s gravitative power, at the very least, must have been at work if the orbits traversed were parabolic; and the extravagance of the estimate sufficed, and was designed to compel its rejection. Hyperbolic motion was accordingly resorted to; the encountering bodies brought, it was supposed, their own velocities with them from the farthest bounds of space; and they were of so high an order that the increments due to mutual gravity left them sensibly unaltered. A pair of “runaway” stars, one moving towards, the other from the earth, must, it seemed, have accidentally passed each other almost within grazing distance. They were primitively unrelated; their quasi-collision could never be repeated; they were as unlikely as any two stars in the heavens to be similar in constitution. Yet their spectra affirmed their close affinity; both were of pure helium type; one might be called the _negative copy_ of the other. Nor was this all. Anomalies still more glaring presently disclosed themselves. Too obviously, on the adopted hypothesis, one pair of meeting stars could not suffice to explain the phenomena. Vogel stipulated for a triple encounter;[747] Campbell found evidence of the interaction of four luminous masses.[748] The more attentively, in fact, the spectrum was examined, the more complex it appeared. The bright lines were not simple emanations, but groups of differently refrangible rays; the dark lines were intersected by bright threads, variable in number and position. Several distinct sets of lines, each with its separate amount of shift, and each hence associated with a differently moving mass, thus stood out independently. “On the hypothesis of four bodies,” Professor Campbell wrote, “the principal system of bright lines was not displaced appreciably, and the star yielding it was practically at rest with reference to the solar system. Another system was displaced towards the red, a distance corresponding to a velocity of recession of about 315 miles a second. The system of fine bright lines, and likewise the system of dark lines, were displaced towards the violet, a distance corresponding to a velocity of approach of about 400 miles a second.” The analysed light of the Nova, on this showing, consisted of four superposed spectra disconnected in their origin. The case, however, was presented with diffidence; no conclusive force was claimed for it. The spectrum, indeed, if multiple at all, was more than quadruplicate. Victor Schumann pointed out[749] that not less than six, if as many as two stars were engaged in the outburst, and planets _ad libitum_ were thrown into the _mêlée_ by Vogel.[750] The collision-theory, in short, collapsed under the weight of the facts it had to carry; speculation was plainly off the track; a new principle of explanation had to be sought. It was difficult to find; yet some probable though partial truths had been laid hold of. Sir William Huggins, for instance, pointed out that the complexities of the spectrum might be in part due to “reversals” in the atmosphere of a single star.[751] Such effects of the stratification of glowing vapours are common in the sun, and may be artificially produced in a diversity of forms. Their presence in Nova Aurigæ was clearly recognisable. The opinion, too, expressed by Father Sidgreaves might safely be adopted that the spectrum of the new star “was, on the whole, what the solar chromospheric spectrum might be expected to show on a grander scale of disturbance.”[752] Finally, M. Seeliger’s general theory of stellar conflagrations,[753] as arising from the passage of compact bodies through “cosmical clouds,” or nebulæ, though strained to meet superfluous requirements, had fundamentally much to recommend it. Entire originality could not be claimed for it; Mr. Monck, and possibly others besides, had earlier proposed a similar view; but it was by Seeliger independently developed, and independently applied to the circumstances of the latest event. These were indeed particularly embarrassing to theorists. They seemed to imply the continuous progress of opposite radial movements of enormous velocity, and the problem of bringing them into play, whether by the rushing of inflamed gases or by the bodily transport of luminous globes, was, when considered in all its complicated bearings, formidable, if not desperate. Yet it had to be faced, for at that time spectral displacements were explicable only as effects of motion; they should perforce be interpreted on the radial velocity principle. This is no longer absolutely prescribed; the same phenomena have been found to bear other meanings—meanings not yet thoroughly intelligible, but promising, when they become so, to provide the keys to many enigmas.

FIG. 42.—Light-Curve of Nova Aurigæ.
]

We may now trace the further course of the apparition. During nearly three months it retained most of its brightness, despite wide fluctuations; then on 6th March a precipitate decline set in, bringing the object on 26th April to the limit of visibility with the great Lick refractor. In Fig. 42 the course of change so far is graphically portrayed. It was naturally believed to have reached its term; the Nova seemed definitively extinct; but in this, as in other respects, its behaviour defied anticipation. Observations resumed 17th August 1892, after its conjunction with the sun, showed, in the place of the vanished star, a stellar nebula of the tenth magnitude.[754] The recovered light was entirely altered in quality. No stranger disclosure has been made by the prismatic method than that of the spectral transformation of Nova Aurigæ between March and August. The dark lines of the former spectrum had become effaced; the bright lines were no longer chromospheric but nebular. Their character, moreover, denoted disturbance of a very peculiar kind. They were wide and hazy, and some at least could be resolved into groups projected upon a feeble luminous background.[755] Plainly the state of things producing the complex “reversals” noted in the spring had become intensified in the reappearance of the autumn. Yet the multiple bands composing the new spectrum corresponded most faithfully, in number and mean position, with the rays of a planetary nebula. The representation was essentially perfect. Out of nineteen Nova-lines measured by Professor Campbell, only one of subordinate importance appeared foreign to nebular light.[756] Both the Wolf-Rayet blue bands were present, as in certain planetaries; while the distinctive, though enigmatical, nebular line at λ 373, noticed as absent from the Tulse Hill photographs of the original spectrum, emerged as it altered, and was recorded by Von Gothard at Herény in November 1892.[757] All the metallic lines, all the helium lines (except a trace of λ 4472) had died out; only those of hydrogen survived; and hydrogen glows universally. The metamorphosis could hardly have been more complete. Professor Campbell might well say that the relation of the later to the earlier spectrum was “not apparent.”[758] It was certainly most obscure. At least, however, the conviction was acquired that the light “emanated from one source;” although subsequent experience obliges us to regard as fictitious the rapid approaching movement attributed to that source on what then seemed incontrovertible evidence. The presumed velocity reached a maximum of 190 miles a second in September, but in November 1892 had decreased to about half that value. And the slackening continued until, at the end of two years, the shifted lines occupied almost normal positions. Subsidence of physical agitation might safely be associated with the change. It was attended by spectral modifications of a different kind. In the transformed Nova bright lines at λ 436 and λ 575 were at first prominent. They also occur in nebulæ, but by exception and inconspicuously. Eventually they faded out, and the spectrum remained typically nebular save for the anomalous breadth of the lines.[759]

As a star, Nova Aurigæ had been decidedly yellow; in its nebular stage it assumed a greenish tint. Its chromatic peculiarities accounted for the haziness of its aspect with many refractors. Outstanding rays, left unfocussed when the rest were united, created a spurious disc of which no trace could be perceived with a reflector.[760] The revived Nova maintained tenth-magnitude lustre with trifling fluctuations for upwards of four years; then it began once more slowly to decline, and in the latter part of 1897 looked “more like a most minute and faint planetary nebula than a star.”[761] In 1897 it had sunk below the thirteenth magnitude; in 1900 it touched the fourteenth.[762] A partial revival was noted at the Lick and Yerkes observatories in 1901, the peculiarity of its focus continuing to indicate the nebular quality of its light.[763]

Nova Aurigæ was neither the first nor the last temporary star to don, in fading, a garb of nebular light. Nova Cygni passed through an identical phase, and the example has since been frequently imitated. Chiefly, indeed, the methods of investigation brought to bear upon it, and the novel considerations to which, in consequence, it gave rise, rendered the apparition of 1892 memorable. Its dominant features were those of a model “blaze star”; but they were for the first time decisively ascertained. An evanescent phenomenon was rendered virtually permanent, and for dubious visual impressions, definite transcripts of fact were substituted. They are likely to be multiplied in the future; and it has meantime become abundantly clear that no special theory, fitting only the circumstances of Nova Aurigæ, can be true.[764] Whatever rationale of them may be ultimately adopted, it must be one capable of general application.

The photographic era of investigation began with Nova Aurigæ, that of photographic discovery with Nova Normæ. The former object was still visible when the latter came into notice. Virtually a replica of the earlier spectrum imprinted itself upon a Draper Memorial plate exposed at Arequipa by Professor Bailey 10th July, and examined by Mrs. Fleming at Harvard College 26th October 1893.[765] It included about a dozen bright lines, each shadowed by a dark one on its more refrangible side. In position, in relative shift, in character, the system of duplicate rays matched with singular fidelity that scrutinised and wondered at in the previous year. There was then nothing casual about its production. The spectral displacements and the augmented refrangibility of the dark lines belonged to the essence of the phenomenon. Portentous velocities, specially directed, and inherent in specially constituted masses, could have had nothing to do with them. The appearance of Nova Normæ, then, disposed of what was left of the encountering star-theory.

The conflagration was of a strictly temporary nature. Absence from a chart-plate proved the star to have been fainter than the fourteenth magnitude, 27th May 1893; and a spectrographic plate, taken 21st June, added the information that it had not at the later date reached tenth-magnitude brightness. Its rise to the seventh, on or before 10th July, was probably effected in a few hours; but though fortunately registered at an early stage in its career, it was not recognised for nearly four months, by which time it had sunk to one-sixth of its primitive lustre. The same marvellous change witnessed in Nova Aurigæ attended its decline. On 13th February 1894 Professor Campbell[766] succeeded in observing the star with the Lick thirty-six inch, in spite of its low meridian altitude of barely 2½ degrees. The spectrum was nebular. It consisted, to the eye, of the well-known trio of green rays, together with the golden line (λ 575), also emitted by the transformed Nova of August 1892. Measurements of them afforded no evidence of displacement; their wave-lengths were as usual. Towards the middle of 1895 Nova Normæ became telescopically invisible. It was already provided with a successor. Just as it glimmered out, a star in the constellation Argo lit up. Nova Carinæ was detected by the same means as, though with somewhat more delay than, Nova Normæ. Two impressions of its spectrum, which chanced to be secured, on 14th April and 15th June 1895 respectively, disclosed precisely the same arrangement of coupled lines, the dark set above the bright, so remarkable in earlier examples.[767] During those two months this had not appreciably varied, but a significant change of a different kind had come about. A vivid ray at λ 4700 (approximately), scarcely visible 14th April, was as bright as the hydrogen lines on 15th June. Its identity with the Wolf-Rayet azure band—the leader line of the Rydberg hydrogen series—may safely be assumed, and its kindling was most likely the prelude to a complete nebular transformation. But of this we have no certain knowledge, since the later history of Nova Carinæ remained unwritten. The conflagration was brief; it had burnt itself out before Mrs. Fleming’s examination of the plates transmitted from Arequipa gave the alarm of its occurrence. Hence no express observations were feasible.

The year 1895 was prolific of temporary stars. One was retrospectively announced to have appeared in the constellation Perseus in 1887.[768] A bright-line spectrum, dimly self-imprinted upon a Draper Memorial plate exposed at that period, was at first thought to signify the rise to maximum of an ordinary Mira-variable. But the originating object soon vanished, to all seeming, definitively, and its brief incandescence asserted itself, by its non-recurrence, as that of a new star. There was a further note of distinction. The fifth line of hydrogen (Hε), never apparent in an ordinary variable, glimmered in the peculiar spectrum of the star of 1887, which included besides an unknown line at λ 4060.

The last record obtained of Nova Carinæ preceded the first of Nova Centauri by just a fortnight. They differed, however, widely in character. The spectrum singled out by Mrs. Fleming, 12th December 1895, from a crowded spectrographic picture taken at Arequipa 18th July in the same year, resembled that of an exceptional nebula, 30 Doradûs. It seemed abortively or imperfectly stellar.[769] The source, too, from which it emanated was situated in a nebulous environment. With the fading of its brightness—which probably never greatly exceeded the seventh magnitude—an outlying portion of a known nebula (N.G.C. 5253), momentarily effaced by the blaze, as a fire is “put out” by the sun, shimmered into view, like a halo round the dying star.[770] It still survives, while not a trace can be seen of its quondam inmate. By the time that Nova Centauri came to be recognised, it was unfortunately already far advanced on its return to obscurity. It had sunk below the eleventh magnitude on 22nd December, when Professor Campbell secured the first of three observations of its spectrum. He described it as continuous, though peculiar, for the blue section was of disproportionate strength, and the yellow showed inequalities, as if through the superposition of bright lines.[771] A parting glimpse of the strange star was caught by Professor Hussey at Lick on 16th July 1896. It was then excessively faint, and immersed in nebulosity. The analogy between this apparition and that which illuminated the great Andromeda nebula in 1885 was unmistakable, and both diverged significantly from the type of Nova Aurigæ.

It was reverted to by Nova Sagittarii. This star was photographed at Arequipa 8th March 1898, and identified by Mrs. Fleming as “new” a year later.[772] Yet it was at maximum fully the equal of Nova Aurigæ, and might have been seen at a glance by any one familiar with sky scenery. It waned, however, very speedily. It was of only 8·2 magnitude on 19th April, when a spectrographic record chanced to be secured, and had declined to the eleventh before its deliberate investigation became possible. Plate XVI. Fig. 4, reproduces two spectral photographs of Nova Sagittarii, obtained within forty-eight hours of each other. Six or seven hydrogen lines are shown in them, broad and bright, but without the dark companions usually visible in such spectra. The second band from the right is the more refrangible of the Wolf-Rayet blue radiations (λ 4643); the K of calcium is absent, but six sharp rays, identified with chemically unclaimed lines in the spectrum of Nova Aurigæ, came out distinctly on the negatives. Although taken at so brief an interval, the records disagreed in some particulars. Thus the unknown line at λ 4060, emitted by the first Nova Persei (1887), was strongly absorbed in Nova Sagittarii on 19th April, but had vanished on 21st April; while the chief nebular line, invisible at the earlier, shone at the later date. The initiated change did not stop here. In March and April 1899 a purely nebular spectrum was derived from the semi-extinct object by Professors Campbell and Wright.[773] The regular cycle had been run through; a planetary nebula replaced the faded star. In one important respect, nevertheless, Nova Sagittarii had departed, so far as the extant data could inform us, from the pattern set by Nova Aurigæ. The spectrum, as already mentioned, was single, not duplicated by absorption. But there is good reason to believe that this difference did not originally exist; it supervened with the declension of light. In “temporary” spectra the bright lines always seem to survive the dark; the chiaroscuro effect is produced only near the period of maximum, and this period had terminated before the star of 1898 was spectrographically registered. Earlier impressions, we may be sure, would have displayed the coupled lines symptomatic of the enormous disturbances attending these extraordinary outbursts.

Their frequency, established by the camera, is a fact of subversive import, but has already ceased to excite surprise. The recognition of the autographs of new stars has become a regular part of the business conducted at such a “Solomon’s House” as the Harvard College Observatory. In the _Annual Reports_ thence issued the mention of new stars has grown to be habitual and familiar. Of Nova Sagittarii the last had only just been heard, when Nova Aquilæ was announced to have appeared.[774] This was about fifteen months after date. The star made its _début_ on a Draper Memorial chart-plate 21st April 1899 as an object of the seventh magnitude, and displayed the quality of its light in a spectrograph taken 3rd July. It was characteristically that of a Nova, although nebular lines had begun to come in, and the transformation was completed in the course of September. During a year, Nova Aquilæ wore the aspect of a planetary nebula, sinking gradually from the tenth to the twelfth magnitude. It was last observed by Professors Campbell and Wright with the Lick thirty-six-inch refractor and a 60° prism spectroscope. “The visible spectrum,” the former wrote,[775] “consisted of extremely faint continuous light in the green, and of three bright bands in the positions of the three principal nebular lines. The relative intensities of the three bands agreed approximately with the corresponding intensities in the well-known nebular spectra. The bands were not monochromatic, but on the contrary were very broad, perhaps fully twice as broad as the bands in the nebular spectrum of Nova Aurigæ in August 1892.”

It may be useful to tabulate the results, in the discovery of new stars, of the photographic surveys executed at Harvard College and at Arequipa, its southern dependency.

(i.) Nova Persei. Appeared 1887; ninth magnitude; hydrogen lines and λ 4060 bright. Temporary character recognised 1895.

(ii.) Nova Normæ. Spectrum photographed 10th July 1893; character detected 26th October 1893. Seventh magnitude; showed twelve bright lines coupled with more refrangible dark ones; spectrum nebular 13th February 1894.

(iii.) Nova Carinæ. Spectrum photographed 21st April 1895; examination followed in October. Hydrogen lines bright with more refrangible dark companions. Faded from eighth to eleventh magnitude between April and July.

(iv.) Nova Centauri. Spectrum photographed 18th July 1895; irregularly continuous; resembled that of 30 Doradûs. Character detected 12th December 1895. Maximum magnitude 7·2. Situated within a nebula.

(v.) Nova Sagittarii. Photographed 8th March 1898, when of 4·7 magnitude; detected March 1899. Spectrum photographed 19th and 21st April 1898. Hydrogen lines bright; no dark companions; λ 4060 dark. Spectrum nebular, 13th March 1899.

(vi.) Nova Aquilæ. Photographed 21st April 1899, when of seventh magnitude. Bright-line spectrum photographed 3rd July 1899; recognition ensued in July 1900. Spectrum nebular, October 1898.

In the course of seven years, then, five temporary stars were photographically discovered, and it is safe to assert that all would have escaped direct visual notice. Apparitions of the kind are, we hence learn, hardly to be counted as exceptional; their occurrence cannot depend upon rare contingencies, but must enter, to some extent, into the regular economy of nature. The conditions precedent are doubtless widely prevalent, and they are the same for outbursts of all magnitudes indifferently. The famous Novæ of history—the stars of Hipparchus, of Tycho Brahe, of Kepler—were due to just such prepared accidents as result, more frequently and less obtrusively, in the appearance of an extra dark dot on a chart-plate, or in the emergence, on a prismatic negative, of a “peculiar” spectrum among a crowd of normal ones.

FIG. 43.—Photographs of the same field taken at Harvard College before
and after the outburst of Nova Persei. No. 1, 19th February 1901.
No. 2, 26th February 1901.
]

But this obscure kind of manifestation was far transcended by the “New Star of the New Century” (to quote Father Sidgreaves’s designation of Nova Persei, No. 2). This object worthily commemorated the turning of a leaf in the book of ages. The sidereal heavens had harboured no such brilliant “guest” since Kepler’s star shone in Ophiuchus. Dr. Anderson, the discoverer of Nova Aurigæ, was still more lucky with Nova Persei, for he caught it on the rise. It was, however, already nearly the equal of Algol when he sighted it in the early morning of 22nd February 1901; while twenty-eight hours previously, as a photograph taken by Mr. Stanley Williams attested, it must have been fainter than the twelfth magnitude. Confirmatory evidence was derived from the Harvard College series, two specimens of which are reproduced in Fig. 43 by the kind permission of Professor Pickering. In the earlier photograph, taken with an exposure of sixty-six minutes 19th February, no trace of anything unusual is perceptible; in the second, to which, seven days later, a shorter exposure was given, the Nova is of dominating importance. It continued to gain light rapidly for about a day and a half after its detection at Edinburgh. On the evening of 23rd February, it was observed through drifting clouds at Harvard College to be brighter and bluer than Capella.[776] In thirty hours it had increased by two and a half magnitudes; during that one night it took rank as the premier star of the northern hemisphere. But its supremacy was quite transient. Increase was at once followed by decrease; there can have been no appreciable pause at maximum. Already on 24th February, the star had lost fully one-third of its light; the inevitable downward course was entered upon, and was pursued, although with singular intermittences. But the most remarkable characteristic of Nova Persei was its spectral variability. It showed, to begin with, a spectrum of the ordinary Orion type—a continuous prismatic strip, scarcely encroached upon by narrow lines of hydrogen and helium absorption. Some of these, it is true, proved on close examination to have their lower edges slightly brightened, but there was no other sign of disturbance. On the following night it was noticed that the K of calcium, previously imperceptible, stood out markedly obscure, after which an extraordinary change took place. During the twenty-four hours that followed the maximum of 23rd February, the character of the spectrum was completely transformed. Hydrogen now blazed in it; and the range of broad bright lines representing its emissions was duplicated, just as in Nova Aurigæ, by a range of more refrangible shadow-bands. These were strongly displaced towards the violet, and their displacement appears to have increased progressively for some days. It finally corresponded, if interpreted on the Doppler principle, to an approaching velocity of about 1000 miles a second, while the bright lines, so far as their distended condition allowed a judgment on the point to be formed, retained pretty nearly their usual places. The Doppler principle, however, had plainly a very restricted application to the case of Nova Persei. Dr. Vogel[777] used it, with excellent judgment, to determine the star’s true radial motion from measurements of fine dark reversals of H and K, which seemed entirely exempt from alterations of a physical kind. He found it to be about twelve miles a second in a direction away from the sun. He was less successful in his endeavour to explain the conspicuous shifting of the dark hydrogen lines on the pressure-theory as adapted by Wilsing. For pressure does indeed alter the refrangibility of the rays emitted by vaporous strata submitted to it, but the alteration is always in the same sense. The refrangibility of light is diminished by condensation; the resulting spectral displacements are towards the red. Nevertheless, those of the absorption lines in Nova Persei were towards the blue. Nor was the masking arrangement devised for the purpose of smoothing away this fundamental contradiction, one that would work. The star itself, through the gradual unfolding of its peculiarities, emphatically disavowed it. A Stonyhurst spectrograph of 7th March showed the dark hydrogen lines well separated from the bright;[778] they were unmasked, yet none the less shifted. Their altered positions were thus seen to be those of the rays in their entirety, and not merely of outlying wings left visible, while their central parts were concealed. Subsequently to 7th March these lines thinned off, and in about a fortnight disappeared finally. The spectrum then remained in much the same condition as that of Nova Sagittarii at the date of the earliest record of it; the distinctive chiaroscuro effect was gone; the bright lines no longer _cast shadows_.

FIG. 44.—Light-Curve of Nova Persei, 22nd February to 6th March 1901.
]

FIG. 45.—Light-Curve of Nova Persei, 17th March to 4th May 1901.
]

Sections of the light-curve of Nova Persei are delineated in Figs. 44 and 45. They seem to tell of a long succession of efforts towards recovery, constantly defeated by the invincible advance of exhaustion. These variations in magnitude were associated with remarkable spectral modifications. The light of this wonderful object was, from the first, of extremely unstable quality. The emitting and absorbing vaporous layers were evidently in a state of continual flux. Their turmoil was betrayed by changes of intensity, width, and wave-length in both dark and bright lines, most of them taking place unconnectedly, so to speak, and capriciously. By degrees, however, their confusion reduced itself to some kind of partial order. During the month of March, Nova Persei assumed the character of a variable fluctuating extensively in a period of three days, and the spectrum was found to vary quite decidedly in correspondence with the light. The phenomena of its alternations are illustrated in the subjoined reproductions of photographs taken by Father Sidgreaves (see Plate XVI. Fig. 5). The second and fourth are distinctive of minimum epochs; the first and third characterise much higher grades of brightness. Among the features of the minimum spectrum are the fading of the continuous radiance, the displacement upward of the first ultra-violet member of the hydrogen series (Hζ), and the development of a blue band at λ 463. The two types of spectrum continued to succeed each other with approximate regularity from 19th March to 3rd May. Numerous anomalies, indeed, presented themselves. They were described in detail by Fathers Sidgreaves[779] and Cortie,[780] by Lockyer, Pickering[781] and Hale,[782] but no clue to their physical interpretation has been found. Only those affecting one narrow region of the spectrum can here be further adverted to. The appearance of the D-lines in Nova Persei was most remarkable. Special attention was paid to them at the Yerkes Observatory, and they are delineated in Plate XVI. Fig. 3, from a photograph taken on an “Erythro” plate with the forty-inch refractor of that establishment. The relations of the various lines shown in it need some brief explanation. The broad bright band is due to the radiation of sodium; the fine dark lines projected upon it are obvious reversals by a cooler and rarer overlying stratum of the same metallic vapour. These threads of absorption are slightly displaced towards the red; they indicate about the same rate of recession from the earth that was derived by Vogel from the similar reversals of H and K. The fact, then, admits of no doubt that the kindled mass was travelling away from us with a small velocity.[783] This remained constant—so far as our information goes—from first to last; but the position of bright D underwent a marked alteration. The broad bright band, traversed almost centrally in the early days of March by hair-like reversals, shifted in the course of a month so notably towards the blue that the fiducial lines (as they might be called) lay in April close to its less refrangible edge; while the absorption line concealing D_{3} had meantime become diffused towards the violet. We have no inkling of a possible cause for these changes. They were steadily progressive, and so disclaimed any immediate connection with the periodical variations of the Hζ line. According to Father Sidgreaves’s view, indeed, these last originated from no change of refrangibility, but from the brightening of a cyanogen band situated just above the hydrogen line it illusorily modified. The sodium band, however, actually moved upward; the emitting molecules quickened their vibrations through some unknown kind of influence. Only the less refrangible members of the helium-series glowed in this marvellous spectrum. None higher up than the blue “Orion” line at λ 4472 could be seen; while the blood-red λ 6678, and D_{3} (when sodium-absorption thinned off) shone intensely.

The colour of Nova Persei changed from white to red a few days after its maximum on 23rd February, and red it remained for some months. The tinge, indeed, lightened to clear orange in its spasms of recovery, but deepened to a purplish glow at each subsequent decline. It faded completely in July 1901, when the spectrum of the Nova resembled that of a planetary nebula.[784] Plate XVIII. Fig. 2 illustrates, from the Lick observations, the predominance in it, on 11th August, of the leading nebulium-radiation. This was in accordance with precedent; but the wildest flights of imagination were outrun by what ensued.

On 22nd and 23rd August 1901, Professor Max Wolf took long-exposure photographs of the Nova with the sixteen-inch Bruce objectives lately mounted as a twin-telescope at Königstuhl. Both plates showed the presence of detached nebulous masses to the south-east of the star;[785] and these proved to be only the brightest parts of a vast spiral formation photographed by Mr. Ritchey at the Yerkes Observatory on 20th September, under the form represented in Plate XVIII. Fig. 1.[786] Its intimate structural relationship to the star is patent; but there was more to come. Renewed impressions obtained 7th and 13th November with the same instrument—a twenty-four-inch reflector of his own construction—showed Mr. Ritchey that the nebula was expanding with portentous speed;[787] and an identical discovery was made by Mr. Perrine from a comparison of the Yerkes photograph of 20th September with a Crossley picture secured by a duplicated exposure 7th and 8th November.[788] The movements indicated were at the rate of about one minute of arc in five weeks; and they were maintained until the dimly shining spires affected by them faded into chemical invisibility. Moreover, the spinning of the nebulous web was found to have been an initial accompaniment of the stellar outburst. Two close coils of it were discerned by Mr. Perrine on scrutiny of a negative exposed for ten minutes 29th March 1901.[789] And since they were closer in the ratio of their expansion between September and November, it became evident that the process had gone on unchecked during seven or eight months; while the tracing of it backward showed it to have actually commenced almost simultaneously with the kindling of the conflagration. That is to say, the rate of flow of the nebulous streams indicated their issue from the Nova about 17th February, or five days previously to its visible manifestation.

PLATE XVIII.

1. Nebulosity round Nova Persei (Ritchey).
2. Spectrum of Nova Persei, 11th August 1901, with corresponding
Intensity-Curve (Campbell and Wright).
]

Speculation regarding this unique phenomenon has naturally been active; and an explanatory hypothesis of considerable plausibility was hit off independently by Professor Kapteyn[790] and Mr. W. E. Wilson.[791] It affirms the nebula to have been pre-existent, and to remain unchanged. But since we see it by the reflected light of the Nova, its various spires and condensations have come successively into view as the flare of the explosion travelled outward in widening circles. Hence an illusory effect of radial expansion was produced, while, in point of fact, the temporarily illuminated cosmic folds were as immovable as aligned snow-peaks, in turn set aglow by the rising sun. The parallax corresponding to this rationale is 0″·011; it implies that the Nova actually blazed in the third year of James I. (1605), and rose to a culminating splendour eight thousand times that of our sun. In this, at least, there is no improbability. The star is devoid of sensible parallax or proper motion; and its distance from the earth cannot, accordingly, be expressed in “numbers that have name.”

A strictly temporary character may safely be ascribed to its nebulous appurtenances; they were either evolved from its mass, or rendered luminous through its influence. Professor Max Wolf suggested the propagation of electric waves of the Hertzian type as the cause of the far-spreading excitement attested by the gleaming annuli. Professor Very[792] inclined to regard them as constituted by material corpuscles, such as give rise to cathode-rays, ejected from the star under the stress of light-pressure or electrical repulsion. There was, however, as Mr. Louis Bell[793] remarked, no evidence of acceleration, consequently none of the continued action of a repulsive force; while the supposition that the nebula round Nova Persei had been photographed by reflected light was, in his opinion, discredited by the absence of polarisation-effects, as well as by the long persistence of strongly nebulous patches close to the star, which, unless they had been self-luminous, should have vanished with its fading. Their light, accordingly, is surmised by Mr. Bell to have been of an auroral nature. It developed as a secondary consequence of electro-magnetic strains, propagated through space with the velocity of light, from a sun-like centre of disturbance.

What cannot be gainsaid is that the apparition, with its strange attendant circumstances, ranks as the most interesting on record. Even the classic Nova Aurigæ was far outdone by the surprising diversity of the phases, both spectral and luminous, exhibited by its brilliant successor. Their study afforded much new knowledge, and opened wide tracts for future experiment and research. A critical point of difference between the observations made on the two occasions was the timeliness of those on the latter star. They were in full swing while it was still mounting towards the summit of its splendour. As a consequence, the order of its spectral phenomena was clearly ascertained and proved wholly unexpected. At the outset, the light was of ordinary quality; waning had already set in before the displacements and distension of its linear ingredients announced the prevalence of disturbance. This fact is full of meaning; let us weigh it well. We learn from it that the sudden development of luminous energy was essentially of photospheric origin. Gaseous eruptions and explosions followed, and produced the duplicated spectrum characteristic of temporary stars. But they were merely incidental to the primary event, which occurred independently of them. This disclosure narrows the field of speculation as to the nature of that primary event, and is so far enlightening. Further, we now know that the symptoms associated with prodigious radial velocities were not inherent, but consequential. The obscure body abruptly kindled to vivid incandescence gave no evidence of rapid motion; it seemed a leisurely traveller through space.

We may now attempt to arrange and generalise the items of information lately gained about Novæ. Three have appeared in nebulæ or clusters—Nova Scorpii in 1860, Nova Andromedæ in 1885, and Nova Centauri in 1895. Set apart by their nature no less than by their situation, they were evidently transient adjuncts to the formations in which they were immersed, and _probably_ imitated their luminous peculiarities. Our knowledge of them, however, is partial and unsatisfactory. Passing on to “blaze stars” proper, we note the following circumstances as common to the class.

1. They have their habitat in the Milky Way. Nova Coronæ alone had any considerable galactic latitude.

2. None have any sensible parallax or proper motion. They must then be vastly remote.

3. They rise from and relapse into approximate obscurity. The one _known_ star distinguished by a temporary flare was Nova Coronæ. Several, nevertheless, continue perceptible in their effete state.

4. The bright lines of Novæ are coupled with dark lines of shorter wave-lengths. This mode of juxtaposition is invariable.

5. The spectra of Novæ resemble, in their early stages, that of the solar chromosphere, later that of nebulæ, the emissions of hydrogen and helium ultimately yielding their predominance to the green rays of nebulium. The Wolf-Rayet blue bands generally make an intermediate appearance.

Now what may we legitimately conclude from these varied phenomena? Very little, unfortunately, of a positive nature; we must be content, in the main, with negative inferences. Yet it is no small advantage to clear the ways of thought by abolishing untenable hypotheses. It may then safely be stated that the remarkable spectral shiftings in temporary stars are not effects of translatory motion; they supply no argument for the duplicity of the light-source. Neither do they originate through pressure, which tends to damp down vibrations, not to accelerate them; and it is chiefly a shortening of wave-lengths that has to be explained. Staggered by this difficulty, Dr. Wilsing suggested[794] the alternative view that Novæ are not incandescent, but “luminescent” bodies. The distinction, first made by E. Wiedemann,[795] is valid and valuable, although next to nothing is known about the essential conditions upon which it rests. All that can be said is that they involve the production of light to a great extent without heat. But the supposition that they are found in temporary stars is an extremely hazardous one. It is countenanced only by the one fact that emissions due to luminescence are, or may be, accompanied by corresponding absorptions of greater refrangibility, and so present a colourable imitation of the perplexing chiaroscuro spectrum displayed during stellar outbursts.

On the whole, the most promising theory of their occurrence is that stars in the Milky Way occasionally get entangled in the diffused nebulosities with which that region abounds, and blaze through the resistance offered to their motion, just as meteors kindle to brief splendour in shooting athwart our cloud of “circumfluous air.” We must, it is true, be content for the present to accept it in principle; attempts to elaborate it in detail can only, until much fresh knowledge has been acquired, result in failure. Even M. Seeliger’s[796] ingenuity did not avail to conduct him to a successful issue. He demanded too much from the star-and-nebula hypothesis—demanded, indeed, more than the conditions (as we now know) actually required; and it hence incurred unmerited discredit. Admitted provisionally, it will perhaps serve as a guide to ultimate truth. An important discussion of the possibilities connected with it, and of the manner in which they might serve the purposes of spectroscopic interpretation, was published by Mr. J. Halm of the Edinburgh Observatory in July 1901.[797] Proceeding from the hypothesis that a Nova becomes visible when “a dark body impinges upon, and penetrates into a mass of nebular material,” he constructed a vorticose system of radiating and absorbing gases travelling with the great intruded globe, and skilfully adapted in all its parts to give rise to the observed phenomena. His arguments are not, scarcely indeed could be, in all respects convincing; yet they form a contribution of stimulating quality to the general doctrine of temporary stars. This has wide bearings. It should include, not only a rationale of the actual conflagrations, but also some definite and consistent view as to the previous state and history of the bodies subjected to them. They are glibly designated “dark stars,” but embarrassments supervene when we attempt to give precision to our conceptions of what constitutes a “dark star.” We will, nevertheless, essay the task in the next chapter.