Chapter XXIII: Peculiar and Irregular Variables

The stars varying in periods comprised between thirty and a hundred days are not numerous, and they are often peculiar. Among them are to be found such remarkable objects as R Sagittæ, R Scuti, U Geminorum, and S^2 Cygni. Of R Sagittæ, with its double period and reversing minima, something has already been said. The possibility that it is in reality a “short-period variable” on a magnified time-scale is suggested by its resemblance to β Lyræ, and emphasises the question as to its spectroscopic duplicity. Its irregularities, though considerable, do not appear to transcend the limit of what might be explicable in a gravitational system.

FIG. 39.—Light-Curve of R Scuti (Flanery).
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This, however, cannot be said of R Scuti; and since the two stars are analogous in their mode of variation, a rationale clearly impossible for one must be regarded, for that reason alone, as highly improbable for the other. The fluctuations of R Scuti, first observed by Edward Pigott in 1795, extend from 4·4 to about 9·7 magnitude, and have a nominal period of seventy-one days. But they cannot be even empirically embraced in any formula. As Mr. Flanery remarked in 1896, “No set of elements yet devised will fit this star long.”[707] Each in turn has to be rejected as unserviceable. Thus, on 6th May 1897, the star showed a complete inversion of phase;[708] it was at a low minimum instead of at the computed maximum. It then rose to an unforeseen brightness of 5·4 on 11th June, diminished to half-light during eighteen days, and finally, remounting the slope it had just descended, shone duly at the predicted maximum of 17th July. Again, it varied less than a magnitude for two months after the maximum of 29th August 1896, but underwent a precipitate decline at the end of the stationary spell. The light-curve, from Mr. Flanery’s observations July to October 1895, is given in Fig. 39. It is that of a star which refuses to be bound by the shackles of any definite theory. Faint and brighter minima alternate, as Argelander long ago perceived; and they perhaps, now and again, exchange relative values, like those of R Sagittæ. If, then, we double the period, and call it 142 instead of 71 days, the star might rank, despite its vagaries, as an analogue of β Lyræ and R Sagittæ. For a subordinate minimum, placed midway between two maxima, is a feature common to all three, though the other circumstances of variation are in each star widely different. Such resemblances in the midst of diversity are extremely perplexing to students of stellar light-change. The similarity of some of the phenomena suggests a uniform principle of explanation; but the attempt to extend its application serves only to undermine the credit it originally possessed. The eclipse rationale, for example, suits β Lyræ passably well, and might be accommodated to the less equable phases of R Sagittæ, but is wholly incompatible with the disordered fluctuations of R Scuti. This, however, in view of their fundamental resemblance to those of the accurately variable star in the Lyre, raises the question whether eclipses can be regarded as occurring in the one case, when they assuredly do not occur in the other. Over and over again this difficulty presents itself. No theory seems elastic enough to bear the strain put upon it by the variety of the facts. Each member of a group of related stars adds its quota to the burthen of explanation to be borne; until finally the breaking point is reached, and a collapse ensues, leaving the ground encumbered with the débris of the original speculation.

FIG. 40.—Long and Short Maxima of U Geminorum.
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R Scuti might usefully be made the subject of detailed spectrographic investigation. Bright lines shine in the blue and violet sections of its light; but they have not been identified, and the flutings associated with them appear ill-pronounced, or even subject to effacement.

As a curiosity of the skies, R Scuti is much outdone by a small star in Gemini, the abnormal behaviour of which was noticed by Hind in 1855. Habitually tranquil at 13·1 magnitude, U Geminorum rises with amazing celerity to near the ninth once in two, three, or four months. A leap upward of nearly four magnitudes is often accomplished in a single day, and that without preliminary fluctuations. The descent is always much slower, but along a very changeable curve. Two types of maximum are shown in Fig. 40. In one, the episode of brightening occupies fifteen to twenty days, in the other it is terminated in nine or ten. And, as a rule, they alternate one with the other. Nothing, indeed, is certain about this star except its uncertainty. “Predictions in regard to it,” Mr. Parkhurst concludes from his experience, “can be better made after the fact.”[709] The greatest light varies from 8·9 to 9·7 magnitude;[710] the least to a rather larger extent. Thus on 28th February and 26th March 1897 the star must have been below fourteenth magnitude, since Father Hagen lost sight of it with the twelve-inch refractor of the Georgetown College Observatory.[711] No relation is perceptible between the amount and the duration of change; long and short maxima are indifferently high and low. They are fickle, too, in their time-connections. The period—if it can be called a period—may be as short as 71, or as long as 126 days. Their unpunctuality apart, the changes undergone by U Geminorum bear a strong resemblance to those of cluster-variables. There are the same relatively prolonged intervals of repose, followed by vehement spasms of activity, beginning abruptly, dying out gradually. It will be of much interest to inquire whether the rays of objects so singularly and so similarly affected approximate to uniformity in quality. Those of U Geminorum are in colour dull bluish white; they give, according to Pickering and Copeland, an ordinary continuous spectrum. Still it is possible that peculiarities might be revealed by special scrutiny with powerful instruments.

FIG. 41.—Light-Curve of S^2 Cygni.
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U Geminorum ranked as a unique object until S^2 Cygni was discovered. Miss Louisa D. Wells in 1896 traced the fluctuations of the latter on the Harvard plates from 7·2 to below 11·2 magnitude; and the shortness of their apparent period of forty days combined so unusually with their wide range,[712] that they immediately became a cynosure for observers in that branch. The more closely they were watched, the more nearly they were found to conform to those of Hind’s variable. In both stars, stationary minima are interrupted, at intervals not wholly irregular, by sudden ascents of three or four magnitudes; and as in U Geminorum, so in S^2 Cygni, long and short maxima are coupled together in pairs, yet by no invariable law. Breaks in the alternate succession have been in each case recorded. In March and April 1897, for instance, S^2 Cygni rose to consecutive short maxima,[713] and a pair of long maxima again marred the rhythmical flow early in 1900. The curve for a double period is reproduced, from a drawing by Messrs. Parkhurst and Daniel, in Fig. 41. They comment on “the sharp turns in the curve at the beginning and end of maximum” as being “peculiar to this type of variable,”[714] which they judiciously assimilate to that prevalent in clusters. The mean period of about fifty-seven days attributed to S^2 Cygni has a wide margin of uncertainty. The interval from one maximum to the next may be curtailed to thirty-six, or expanded to sixty-three days, and that without traceable plan or method. Revolution in an orbit is hence absolutely excluded from among the possible causes of light-change. An hypothesis proposed by M. Hartwig of Bamberg[715] merely illustrates the baffling nature of the problem set by this star. He suggests that the maxima are occasioned by the brief kindling, at periastron passage, of a satellite pursuing a highly eccentric path. The irregularities of the period, he thinks, may be accounted for by a rapid gyration of the line of apsides. The blazing of “new” stars is, in his view, an analogous phenomenon;[716] but new stars exhaust their energy in a single display, while the variables in Gemini and Cygnus seem to have a limitless power of recuperation. The light of S^2 Cygni is tinged with blue; nothing has been published regarding its spectrum.

Although narrow in range (4·6 to 5·4 magnitude), the variations of υ Herculis are unsurpassed in singularity. Recognised by Schmidt in 1872,[717] they have proved to be “irregularly periodic”; their tendency to preserve definite time-relations appears to be continually resisted and sometimes overborne by countervailing influences. Occasionally they conform approximately to a forty-day cycle, then break loose, and become for a time utterly lawless. The minima are attended by extraordinary fluctuations; the maxima are normally tranquil. The star was marked “red” in the Copenhagen Catalogue, but is now pale yellow, and shows a helium spectrum. Scant attention has of late been bestowed upon it.

A southern star noticed by Dr. A. W. Roberts[718] in 1891 to vary from 6·8 to 8·0 magnitude in a period of 38½ days, is remarkable for its phenomenally quick rise. Only 5½ days are needed for the tripling of its light, while thirty-three elapse during the corresponding subsidence. Hence, if the average ratio for Cepheid variables of the times of increase and decrease held good for U Carinæ, its period would at once be abridged to eighteen days, and there could be no mistake about its membership of a class to which it is affiliated by the type of its variations.

The periodicity of R Lyræ, detected by Baxendell in 1856, often much perturbed, is never wholly effaced. Argelander found it to be comprised within forty-eight days, which Schönfeld reduced to forty-six; yet in 1872 Schmidt considered it uncertain between the limits of thirty and sixty days.[719] Pannekoek’s introduction of a periodic term for its regularisation[720] can be reckoned only a temporary expedient. The oscillation is of small amplitude, from 4·0 to 4·7 magnitude, but two striking outbursts of light, witnessed by Sawyer in November 1884,[721] imply essential instability. The simultaneous development of emission-rays might possibly have been observed had a prismatic eye-piece been at hand; but no spectroscopic examination was feasible at the critical moments. Ordinarily, the star, which is deeply tinted with orange, gives a superb colonnaded spectrum unmarked by bright lines.

Periodic cannot be sharply distinguished from irregular variables. Stars of an intermediate character are quite common. Some degree of precision in change may even be temporarily maintained by objects eventually found to be eminently unmethodical in their modes of procedure. Such are α Herculis, α Orionis, and β Pegasi. Each assumed on first acquaintance a false air of submissiveness to a time-law, which each very soon laid aside. Their fluctuations rather exceed half a magnitude, and are included in an indeterminate number of months; their progress can in no wise be anticipated. Similarly, stars credited on historical grounds with extremely long periods have of late paid not the smallest regard to them. An instructive example is met with in R Cephei. Catalogued by Hevelius in the seventeenth century, and by Groombridge in 1807, as of the fifth magnitude,[722] it thereafter lost light, and in 1840 had sunk to the tenth magnitude. Collating all the available data, Pogson in 1856 assigned to the diminished star a period of seventy-three years, and predicted its restoration to naked-eye visibility in 1880.[723] But the prediction remains unfulfilled; the obscurity of R Cephei seems likely to be permanent.

At Potsdam in 1898 MM. Müller and Kempf noticed a star in Perseus[724] as variable in an unprecedented fashion.[725] After an indefinite term of constancy at 6·3 magnitude, it began in 1892 to decline at the very slow rate of one-eighth of a magnitude yearly, and continued to do so for six years. The counter-process was comparatively rapid. In twenty months the object had regained its former status, so that the complete oscillation occupied 7⅔ years. This time, however, there was no long stationary maximum. Already by the end of 1899 fading had made some progress; but it remains to be seen whether any true periodicity can be established.

A period of five years, ascribed by Mr. Espin to 63 Cygni, has been rejected on further experience of fluctuations distinctively capricious. Many, perhaps most red stars, are unstable to the extent of half a magnitude; and 63 Cygni is a very red star. The fact that it is one and a half magnitudes fainter chemically than visually, supplies a kind of measure for the intensity of its colour.

The empirical rule that irregularity gains more and more the upper hand with increasing length of period[726] is illustrated by S Persei. Indeed the order of succession in the changes of this object is by no means satisfactorily ascertained. Safarik[727] and Hagen[728] hold them to be rudely periodical in about two and a third years; but most other observers prefer to consider them as entirely irregular.[729] Their range though wide, from near the seventh to the thirteenth magnitude, is seldom completely traversed. Experience alone can decide whether the rudimentary method traceable in these variations during fourteen years previous to 1894 will continue to regulate them in the future. Phases so unpunctual are liable to effacement. Quite possibly, the actual instability of S Persei represents a more or less transitory state, which may be succeeded by one of approximate constancy in shining.

The most illustrious of casually variable stars is η Carinæ, formerly designated η Argûs. Futile attempts have been made to accommodate it with periods. It has none. It is, in the full sense of the term, irregular. Its changes are perhaps modified by influences of an unimaginable nature connected with the vast surrounding nebula. But those influences undoubtedly act upon a body of inherently peculiar constitution. The spectrum of η Carinæ is of a kind associated in every other known instance with absolute whiteness. It resembles that of P Cygni;[730] many hydrogen and helium lines are brightened in it, yet the star shows the colour of a Mira-variable. Whether this was always so or not, we have no means of deciding. The first note of a distinctive hue in the southern wonder-star was made by Piazzi Smyth, 1st January 1845, when he announced from the Cape a fresh increase in its light. For a month back, he wrote, it had been brighter than Canopus, and very red.[731] Then in 1850 Gilliss found it to outmatch Mars in depth of tint; and Thome described it in 1887 as of a “dull scarlet,” passing into “bright orange” during a slight temporary rise. The history of this star is familiar to most of our readers.[732] It need not here be repeated. One fact in addition to those currently stated may, however, be mentioned. In his star-maps of 1603, Bayer marked η Argûs as of the second magnitude, probably on the authority of Petrus Theodorus of Embden, who navigated the Indian seas 1594 to 1596. The variable was then equally bright in the sixteenth and in the eighteenth centuries, and its comparative insignificance when Halley placed it in the fourth rank was due to a merely transient decline. Whether the splendour of its beams has ever before been so completely shorn away as it is now, might be questioned. Excesses entail exhaustion, and the flaring maximum which culminated in 1843 was followed by a reactive sinking towards the ashes of extinction. Since 1886, as the observations of Finlay, Innes, and Roberts testify, the star has wavered between 7·0 and 7·7 magnitude, and no sign of its speedy restitution to brilliancy is perceptible. Its future is beyond divination. The present minimum may be indefinitely prolonged, but further change is more likely in the case of so ruddy an object. Another great outburst cannot indeed be reckoned upon even for a remote age. A “temporary” character may so far belong to η Carinæ that its biography will include but one absolute maximum. The star is sensibly devoid of proper motion.[733] Its distance from the earth must accordingly be prodigious.

The capricious disappearances of R Coronæ surprised Pigott in 1795. Usually of about the sixth magnitude, the star is liable at any moment, without note of warning, to drop to the thirteenth. The intervals of maximum lustre sometimes last for years. One extended from 1817 to 1824, another from 1843 to 1845.[734] But in the last-named year, and again in 1852, R Coronæ vanished from view with Argelander’s comet-seeker, regaining brightness on each occasion slowly, and, as it were, with difficulty. Of late its descents have been less profound. Schmidt observed a minimum at twelfth magnitude in August 1883;[735] Sawyer recorded on 13th October 1885 one arrested at 7·4 magnitude.[736] Having been visible to the naked eye nearly throughout 1893, the star sank to the ninth magnitude about 7th March 1894, and after an intermediate partial recovery, to 10·25 on 1st August.[737] By the end of the year the phase of instability seemed to have terminated. These lawless fluctuations, taken in connection with the extraordinary spectral changes ascribed to it, render this object one exceptionally inviting to careful study.

An analogue to it, but with a much narrower range of mutability, is the lucid white star ε Aurigæ. Ordinarily of the third magnitude, it fades at long and uncertain intervals to one-quarter of this brightness. One such diminution was observed by Heis in 1847; another by Schmidt in 1875.[738] There is nothing in the quality of the light to account for them. The spectrum is modelled on that of Procyon, only with an increase of definiteness, and marked differences of relative intensity in the lines.[739]

The vicissitudes of T Tauri derive special interest from their presumable connection with those of a group of nebulæ. Discovered by Hind 11th October 1852, it dwindled during fifteen years from tenth to twelfth magnitude _pari passu_ with the fading of the adjacent “temporary” nebula, but attained in March 1868 a second and higher maximum, coincidently with the brightening of “Struve’s nebula,” another member of the collection. Again it declined, and was left unnoticed from 1877 until Burnham and Barnard, directing the Lick thirty-six inch to its place in October 1890, perceived it as the faint nucleus of a small condensed nebula,[740] which four and a half years later survived only as a feeble glow round the almost extinct variable. The glow was resolved by the Yerkes refractor into a little wisp of nebulosity, attached brush-wise to the star;[741] and partial impressions of it came out on plates exposed by Professor Keeler for four hours with the Crossley reflector, 6th and 29th December 1899.[742] “Can it be,” Professor Barnard asks in surprise, “that the star becomes essentially a nebula as it sinks in light?” The question goes to the root of cosmic relations, and it is raised under more than one aspect by investigations of stellar variability. The associations and transformations of T Tauri are hence of profound significance, and should be diligently supervised until they can be linked together by some rational principle of causation.

Irregular variability has a wide and indefinite reach. It includes changes almost instantaneous, and changes well-nigh millennial in their development. The light of certain stars has undergone a slow secular decline. A noteworthy instance is that of Θ Eridani, identical, as Dr. Anderson has conclusively shown,[743] with Ptolemy’s “Last in the River” (the Arabic _Achernar_), the title and honour of which have been usurped by the more southerly, and now far brighter α Eridani. Al-Sûfi in the tenth century reckoned Θ among the thirteen brightest stars visible in Irak; and it was still of the first magnitude in 1437, the epoch of Ulugh Beigh’s Catalogue. Yet it had sunk to the third when Halley visited St. Helena in 1677, and of the third it still remains. Two other stars which have undeniably faded with the lapse of centuries are β Leonis and δ Ursæ Majoris,[744] and their fading may even now be imperceptibly progressing. Nor is their eventual restoration by any means assured. Accessions of lustre are rarer, and often transitory. On 6th August 1868, 83 Ursæ Majoris, a sixth-magnitude star near Mizar, was seen by Birmingham to be the equal of δ Ursæ; though for that night only. The next, it had visibly gone off, and before long the whole of its added splendour had departed. Its amount was very considerable. The star attained, during its unexplained rise, to threefold its customary brilliancy. One of Burnham’s close pairs, z Virginis, underwent in 1866 a similar phase. This was before it was known to be double, so that our curiosity as to whether both the nearly equal components shared in the brightening remains ungratified. Red stars not infrequently drop abruptly to a lower rank. Thus the Danish astronomer Torwald Köhl had for years observed B.D. + 20° 1083, in the constellation Taurus, as of 7·7 magnitude, when on 22nd January 1898 he was taken aback to find it not much above the ninth.[745] Sixty per cent of its rays had been, as a consequence of some inexplicable collapse of energy, subtracted or suppressed. A step towards the bourne from which there is no returning may be taken in such cases. Stars vanish, but they seldom or never reappear. Yet renovation plays its part no less than decay. Waning stars have their correlatives in waxing stars. Alcor, the Rider, and Benetnasch, the third Horse of the Wain, are among these. Pollux, too, seems to have bettered its position, and Alcyone is far more predominant than of yore in the Atlantid family. Night’s robe will still be profusely spangled, even though a few of its gems grow dim.