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