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.
[Illustration:
FIG. 39.—Light-Curve of R Scuti (Flanery).
]
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.
[Illustration:
FIG. 40.—Long and Short Maxima of U Geminorum.
]
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.
[Illustration:
FIG. 41.—Light-Curve of S^2 Cygni.
]
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.