on the evening of November 24. It was then shining as a star of the
third magnitude, in the constellation of the Swan, not very far from
the famous but faint star 61 Cygni—which first of all the stars in the
northern heavens had its distance determined by astronomers. The three
previous nights had unfortunately been dark; but Schmidt is certain
that on November 20 the star was not visible. At midnight, November
24, its light was very yellow, and it was somewhat brighter than the
well-known star Eta Pegasi, which marks the forearm of the Flying
Horse. Schmidt sent news of the discovery to Leverrier, at Paris;
but neither he nor Leverrier telegraphed the news, as they should
have done, to Greenwich, Berlin, or the United States. Many precious
opportunities for observing the spectrum of the new-comer at the time
of its greatest brilliancy were thus lost.
The observers at Paris did their best to observe the spectrum of the
star and the all-important changes in the spectrum. But they had
unfavourable weather. It was not till December 2 that the star was
observed at Paris, by which time the colour, which had been very yellow
on November 24, had become “greenish, almost blue.” The star had also
then sunk from the third to far below the fourth magnitude. It is
seldom that science has to regret a more important loss of opportunity
than this. What we want specially to know is the nature of the spectrum
given by this star when its light was yellow; and this we can now never
know. Nor are the outbursts of new stars so common that we may quickly
expect another similar opportunity, even if any number of other new
stars should present the same series of phenomena as the star in Cygnus.
On December 2, the spectrum, as observed by M. Cornu, consisted almost
entirely of bright lines. On December 5, he determined the position of
these lines, though clouds still greatly interfered with his labours.
He found three bright lines of hydrogen, the strong double sodium line
in the orange-yellow, the triple magnesium line in the yellow-green,
and two other lines—one of which seemed to agree exactly in position
with a bright line belonging to the solar corona. All these lines were
shining upon the rainbow-tinted background of the spectrum, which was
relatively faint. He drew the conclusion that in chemical constitution
the atmosphere of the new star was constituted exactly like the solar
sierra.
Herr Vögel’s observations commenced on December 5, and were continued
at intervals until March 10, when the star had sunk to below the eighth
magnitude.
Vögel’s earlier observations agreed well with Cornu’s. He remarks,
however, that Cornu’s opinion as to the exact resemblance of the
chemical constitution of the star’s atmosphere with that of the sierra
is not just, for both Cornu and himself noticed one line which did not
correspond with any line belonging to the solar sierra; and this line
eventually became the brightest line of the whole spectrum. Comparing
his own observations with those of Cornu, Vögel points out that they
agree perfectly with regard to the presence of the three hydrogen
lines, and that of the brightest line of the air spectrum (belonging to
nitrogen),—which is the principal line of the spectrum of nebulæ. This
is the line which has no analogue in the spectrum of the sierra.
We have also observations by F. Secchi, at Rome, Mr. Copeland, at
Dunecht, and Mr. Backhouse, of Sunderland, all agreeing in the main
with the observations made by Vögel and Cornu. In particular, Mr.
Backhouse observed, as Vögel had done, that whereas in December the
greenish-blue line of hydrogen, F, was brighter than the nitrogen line
(also in the green-blue, but nearer the red end than F), on January 6
the nitrogen line was the brightest of all the lines in the spectrum of
the new star.
Vögel, commenting on the results of his observations up to March 10,
makes the following interesting remarks (I quote, with slight verbal
alterations, from a paraphrase in a weekly scientific journal):—“A
stellar spectrum with _bright_ lines is always a highly interesting
phenomenon for any one acquainted with stellar spectrum analysis,
and well worthy of deep consideration. Although in the chromosphere
(sierra) of our sun, near the limb, we see numerous bright lines, yet
only dark lines appear in the spectrum whenever we produce a small
star-like image of the sun, and examine it through the spectroscope. It
is generally believed that the bright lines in some few star-spectra
result from gases which break forth from the interior of the luminous
body, the temperature of which is higher than that of the surface of
the body—that is, the phenomenon is the same sometimes observed in
the spectra of solar spots, where incandescent hydrogen rushing out
of the hot interior becomes visible above the cooler spots through
the hydrogen lines turning bright. But this is not the only possible
explanation. We may also suppose that the atmosphere of a star,
consisting of incandescent gases, as is the case with our own sun, is
on the whole cooler than the nucleus, but with regard to the latter is
extremely large. I cannot well imagine how the phenomenon can last for
any long period of time if the former hypothesis be correct. The gas
breaking forth from the hot interior of the body will impart a portion
of its heat to the surface of the body, and thus raise the temperature
of the latter; consequently, the difference of temperature between the
incandescent gas and the surface of the body will soon be insufficient
to produce bright lines; and these will disappear from the spectrum.
This view applies perfectly to stars which suddenly appear and soon
disappear again, or at least increase considerably in intensity—that
is, it applies perfectly to so-called new stars in the spectra of
which bright lines are apparent, _if_ the hypothesis presently to be
mentioned is admitted for their explanation. For a more stable state
of things the second hypothesis seems to be far better adapted. Stars
like Beta Lyræ, Gamma Cassiopeiæ, and others, which show the hydrogen
lines and the sierra D line bright on a continuous spectrum, with
only slight changes of intensity, possess, according to this theory,
atmospheres very large relatively to their own volume—the atmospheres
consisting of hydrogen and that unknown element which produces the
D line.[16] With regard to the new star, Zöllner, long before the
progress lately made in stellar physics by means of spectrum analysis,
deduced from Tycho’s observations of the star called after him, that
on the surface of a star, through the constant emission of heat, the
products of cooling, which in the case of our sun we call sun-spots,
accumulate: so that finally the whole surface of the body is covered
with a colder stratum, which gives much less light or none at all.
Through a sudden and violent tearing up of this stratum, the interior
incandescent materials which it encloses must naturally break forth,
and must in consequence, according to the extent of their eruption,
cause larger or smaller patches of the dark envelope of the body to
become luminous again. To a distant observer such an eruption from the
hot and still incandescent interior of a heavenly body must appear as
the sudden flashing-up of a new star. That this evolution of light
may under certain conditions be an extremely powerful one, could be
explained by the circumstance that all the chemical compounds which,
under the influence of a lower temperature, had already formed upon the
surface, are again decomposed through the sudden eruption of these hot
materials; and that this decomposition, as in the case of terrestrial
substances, takes place under evolution of light and heat. Thus the
bright flashing-up is not only ascribed to the parts of the surface
which through the eruption of the incandescent matter have again become
luminous, but also to a simultaneous process of combustion, which is
initiated through the colder compounds coming into contact with the
incandescent matter.”
Vögel considers that Zöllner’s hypothesis has been confirmed in its
essential points by the application of spectrum analysis to the stars.
We can recognize from the spectrum different stages in the process
of cooling, and in some of the fainter stars we perceive indeed that
chemical compounds have already formed, and still exist. As to new
stars, again, says Vögel, Zöllner’s theory seems in nowise contradicted
“by the spectral observations made on the two new stars of 1866 and
1876. The bright continuous spectrum, and the bright lines only
slightly exceeding it at first” (a description, however, applying
correctly only to the star of 1876), “could not be well explained if we
only suppose a violent eruption from the interior, which again rendered
the surface wholly or partially luminous; but are easily explained
if we suppose that the quantity of light is considerably augmented
through a simultaneous process of combustion. If this process is of
short duration, then the continuous spectrum, as was the case with the
new star of 1876, will very quickly decrease in intensity down to a
certain limit, while the bright lines in the spectrum, which result
from the incandescent gases that have emanated in enormous quantities
from the interior, will continue for some time.”
It thus appears that Herr Vögel regarded the observations which had
been made on this remarkable star up to March 10 as indicating that
first there had been an outburst of glowing gaseous matter from the
interior, producing the part of the light which gave the bright
lines indicative of gaseity, and that then there had followed, as a
consequence, the combustion of a portion of the solid and relatively
cool crust, causing the continuous part of the spectrum. We may
compare what had taken place, on this hypothesis, with the outburst of
intensely hot gases from the interior of a volcanic crater, and the
incandescence of the lips of the crater in consequence of the intense
heat of the out-rushing gases. Any one viewing such a crater from a
distance, with a spectroscope, would see the bright lines belonging
to the out-rushing gases superposed upon the continuous spectrum due
to the crater’s burning lips. Vögel further supposes that the burning
parts of the star soon cooled, the majority of the remaining light (or
at any rate the part of the remaining light spectroscopically most
effective) being that which came from the glowing gases which had
emanated in vast quantities from the star’s interior.
“The observations of the spectrum show, beyond doubt,” he says, “that
the decrease in the light of the star corresponds with the cooling
of its surface. The violet and blue parts decreased more rapidly in
intensity than the other parts; and the absorption-bands which crossed
the spectrum have gradually become darker and darker.”
The reasoning, however, if not altogether unsatisfactory, is by no
means so conclusive as Herr Vögel appears to think. It is not clear how
the incandescent portion of the surface could possibly cool in any
great degree while enormous quantities of gas more intensely heated (by
the hypothesis) remained around the star. The more rapid decrease in
the violet and blue parts of the spectrum than in the red and orange
is explicable as an effect of absorption, at least as readily as by
the hypothesis that burning solid or liquid matter had cooled. Vögel
himself could only regard the other bands which crossed the spectrum
as absorption-bands. And the absorption of light from the continuous
spectrum in these parts (that is, not where the bright lines belonging
to the gaseous matter lay) could not possibly result from absorption
produced by those gases. If other gases were in question, gases which,
by cooling with the cooling surface, had become capable of thus
absorbing light from special parts of the spectrum, how is it that
before, when these gases were presumably intensely heated, they did not
indicate their presence by bright bands? Bright bands, indeed, were
seen, which eventually faded out of view, but these bright bands did
not occupy the position where, later on, absorption-bands appeared.
The natural explanation of what had thus far been observed is different
from that advanced by Vögel, though we must not assume that because
it is the natural, it is necessarily the true explanation. It is
this—that the source of that part of the star’s light which gave the
bright-line spectrum, or the spectrum indicative of gaseity, belongs
to the normal condition of the star, and not to gases poured forth,
in consequence of some abnormal state of things, from the sun’s
interior. We should infer naturally, though again I say not _therefore_
correctly, that if a star spectroscope had been directed upon the
place occupied by the new star before it began to shine with unusual
splendour, the bright-line spectrum would have been observed. Some
exceptional cause would then seem to have aroused the entire surface
of the star to shine with a more intense brightness, the matter thus
(presumably) more intensely heated being such as would give out the
combined continuous and bright-line spectrum, including the bright
lines which, instead of fading out, shone with at least relatively
superior brightness as the star faded from view. The theory that, on
the contrary, the matter giving these more persistent lines was that
whose emission caused the star’s increase of lustre, seems at least not
proven, and I would go so far as to say that it accords ill with the
evidence.
The question, be it noted, is simply whether we should regard the kind
of light which lasts longest in this star as it fades out of view as
more probably belonging to the star’s abnormal brightness or to its
normal luminosity. It seems to me there can be little doubt that the
persistence of this part of the star’s light points to the latter
rather than to the former view.
Let it also be noticed that the changes which had been observed thus
far were altogether unlike those which had been observed in the case of
the star in the Northern Crown, and therefore cannot justly be regarded
as pointing to the same explanation. As the star in the Crown faded
from view, the bright lines indicative of glowing hydrogen died out,
and only the ordinary stellar spectrum remained. In the case of the
star in the Swan, the part of the spectrum corresponding to stellar
light faded gradually from view, and bright lines only were left, at
least as conspicuous parts of the star’s spectrum. So that whereas one
orb seemed to have faded into a faint star, the other seemed fading
out into a nebula—not merely passing into such a condition as to
shine with light indicative of gaseity, but actually so changing as
to shine with light of the very tints (or, more strictly, of the very
wave-lengths) observed in all the gaseous nebulæ.
The strange eventful history of the new star in Cygnus did not end
here, however. We may even say, indeed, that it has not ended yet. But
another chapter can already be written.
Vögel ceased from observing the star in March, precisely when
observation seemed to promise the most interesting results. At most
other observatories, also, no observations were made for about half
a year. At the Dunecht Observatory[17] pressure of work relating to
Mars interfered with the prosecution of those observations which
had been commenced early in the year. But on September 3, Lord
Lindsay’s 15-inch reflector was directed upon the star. A star was
still shining where the new star’s yellow lustre had been displayed
in November, 1876; but now the star shone with a faint blue colour.
Under spectroscopic examination, however, the light from this seeming
blue star was found not to be starlight, properly speaking, at all.
It formed no rainbow-tinted spectrum, but gave light of only a single
colour. The single line now seen was that which at the time of Vögel’s
latest observation had become the strongest of the bright lines of
the originally complex spectrum of the so-called new star. It is the
brightest of the lines given by the gaseous nebulæ. In fact, if nothing
had been known about this body before the spectroscopic observation
of September 3 was made, the inference from the spectrum given by the
blue star would undoubtedly have been that the object is in reality a
small nebula of the planetary sort, very similar to that one close by
the pole of the ecliptic, which gave Huggins the first evidence of the
gaseity of nebulæ, but very much smaller. I would specially direct the
reader’s attention, in fact, to Huggins’s account of his observation
of that planetary nebula in the Dragon. “On August 19, 1864,” he says,
“I directed the telescope armed with the spectrum apparatus to this
nebula. At first I suspected some derangement of the instrument had
taken place, for no spectrum was seen, but only” a single line of
light. “I then found that the light of this nebula, unlike any other
extra-terrestrial light which had yet been subjected by me to prismatic
analysis, was not composed of light of different refrangibilities, and
therefore could not form a spectrum. A great part of the light from
this nebula is monochromatic, and after passing through the prisms
remains concentrated in a bright line.” A more careful examination
showed that not far from the bright line was a much fainter line;
and beyond this, again, a third exceedingly faint line was seen. The
brightest of the three lines was a line of nitrogen corresponding in
position with the brightest of the lines in the spectrum of our own
air. The faintest corresponded in position with a line of hydrogen. The
other has not yet been associated with a known line of any element.
Besides the faint lines, Dr. Huggins perceived an exceedingly faint
continuous spectrum on both sides of the group of bright lines; he
suspected, however, that this faint spectrum was not continuous, but
crossed by dark spaces. Later observations on other nebulæ induced him
to regard this faint continuous spectrum as due to the solid or liquid
matter of the nucleus, and as quite distinct from the bright lines into
which nearly the whole of the light from the nebula is concentrated.
The fainter parts of the spectrum of the gaseous nebulæ, in fact,
correspond to those parts of the spectrum of the “new star” in Cygnus
which last remained visible, before the light assumed its present
monochromatic colour.
Now let us consider the significance of the evidence afforded by this
discovery—not perhaps hoping at once to perceive the full meaning of
the discovery, but endeavouring to advance as far as we safely can in
the direction in which it seems to point.
We have, then, these broad facts: where no star had been known, an
object has for a while shone with stellar lustre, in this sense, that
its light gave a rainbow-tinted spectrum not unlike that which is given
by a certain order of stars; this object has gradually parted with its
new lustre, and in so doing the character of its spectrum has slowly
altered, the continuous portion becoming fainter, and the chief lustre
of the bright-line portion shifting from the hydrogen lines to a line
which, there is every reason to believe, is absolutely identical with
the nebula nitrogen line: and lastly, the object has ceased to give
any perceptible light, other than that belonging to this nitrogen line.
Now it cannot, I think, be doubted that, accompanying the loss of
lustre in this orb, there has been a corresponding loss of heat. The
theory that all the solid and liquid materials of the orb have been
vaporized by intense heat, and that this vaporization has caused the
loss of the star’s light (as a lime-light might die out with the
consumption of the lime, though the flame remained as hot as ever),
is opposed by many considerations. It seems sufficient to mention
this, that if a mass of solid matter, like a dead sun or planet, were
exposed to an intense heat, first raising it to incandescence, and
eventually altogether vaporizing its materials, although quite possibly
the time of its intensest lustre might precede the completion of the
vaporization, yet certainly so soon as the vaporization was complete,
the spectrum of the newly vaporized mass would show multitudinous
bright lines corresponding to the variety of material existing in the
body. No known fact of spectroscopic analysis lends countenance to the
belief that a solid or liquid mass, vaporized by intense heat, would
shine thenceforth with monochromatic light.
Again, I think we are definitely compelled to abandon Vögel’s
explanation of the phenomena by Zöllner’s theory. The reasons which
I have urged above are not only strengthened severally by the change
which has taken place in the spectrum of the new star since Vögel
observed it, but an additional argument of overwhelming force has
been introduced. If any one of the suns died out, a crust forming
over its surface and this crust being either absolutely dark or only
shining with very feeble lustre, the sun would still in one respect
resemble all the suns which are spread over the heavens—it would show
no visible disc, however great the telescopic power used in observing
it. If the nearest of all the stars were as large, or even a hundred
times as large, as Sirius, and were observed with a telescope of ten
times greater magnifying power than any yet directed to the heavens,
it would appear only as a point of light. If it lost the best part of
its lustre, it would appear only as a dull point of light. Now the
planetary nebulæ show discs, sometimes of considerable breadth. Sir
J. Herschel, to whom and to Sir W. Herschel we owe the discovery and
observation of nearly all these objects, remarks that “the planetary
nebulæ have, as their name imports, a near, in some instances a
perfect, resemblance to planets, presenting discs round, or slightly
oval, in some quite sharply terminated, in others a little hazy or
softened at the borders....” Among the most remarkable may be specified
one near the Cross, whose light is about equal to that of a star just
visible to the naked eye, “its diameter about twelve seconds, its
disc circular or very slightly elliptic, and with a clear, sharp,
well-defined outline, having exactly the appearance of a planet, with
the exception of its colour, which is a fine and full blue, verging
somewhat upon green.” But the largest of these planetary nebulæ, not
far from the southernmost of the two stars called the Pointers, has
a diameter of 2⅔ minutes of arc, “which, supposing it placed at a
distance from us not greater than that of the nearest known star of our
northern heavens, would imply a linear diameter seven times greater
than that of the orbit of Neptune.” The actual volume of this object,
on this assumption, would exceed our sun’s ten million million times.
No one supposes that this planetary nebula, shining with a light
indicative of gaseity, has a mass exceeding our sun’s in this enormous
degree. It probably has so small a mean density as not greatly to
exceed, or perhaps barely to equal, our sun in mass. Now though the
“new star” in Cygnus presented no measurable disc, and still shines
as a mere blue point in the largest telescope, yet inasmuch as its
spectrum associated it with the planetary and gaseous nebulæ, which
we know to be much larger bodies than the stars, it must be regarded,
in its present condition, as a planetary nebula, though a small one;
and since we cannot for a moment imagine that the monstrous planetary
nebulæ just described are bodies which once were suns, but whose crust
has now become non-luminous, while around the crust masses of gas shine
with a faint luminosity, so are we precluded from believing that this
smaller member of the same family is in that condition.