Popular Science Monthly/Volume 11/May 1877/The New Star in the Constellation of the Swan
By AMÉDÉE GUILLEMIN.
THE phenomenon of a new star appearing in the heavens is sufficiently rare to strike the imagination of the public, as well as to attract the attention of scientific men. On the one side, it possesses all the interest which attaches to the unexpected, to the mysterious unknown; and, on the other, it raises some very important questions as to the physical and chemical constitution of the stars, and as to the likeness between those distant suns and our own. But now more than ever before, more even than in the first moiety of the nineteenth century, is such curiosity justified, inasmuch as the new means of investigation in the hands of astronomers give promise of revealing, at least in a great measure, the nature of the strange transformations which give rise to these apparitions.
Before we consider the quite recent discovery made by Julius Schmidt, director of the observatory at Athens, let us make a brief review of the apparitions which preceded it.
Every one has seen in works on astronomy the account of the famous temporary star of 1572, which appeared during the month of November in the constellation of Cassiopeia, all of whose phases were observed by Tycho Brahe. Its extraordinary scintillation; its brightness, equaling and surpassing Vega, Jupiter, Sirius, and even Venus when in quadrature, so that it was visible at high noon; finally, its sudden diminution and disappearance after seventeen months of visibility, all conspired to give to this star an extraordinary celebrity.
In 1600 a new star appeared in the Swan, and was studied by Kepler; then it disappeared in 1621, was again visible in 1655, and at sundry times afterward; it is still visible.
Thirty years after the disappearance of the new star in Cassiopeia appeared the star in Serpentarius discovered by Brunowski in October, 1604, and which had for its observer and historian the great Kepler. It was visible for eighteen months, and, while it did not equal in brightness the star of 1572, it surpassed the stars of first magnitude, and even Jupiter itself.
In 1670 a third temporary star was discovered by the Carthusian Anthelme, in that part of the constellation of the Fox which is nearest to /3 of the Swan. At the time of its apparition, or rather of its discovery, June 20th, it was of the third magnitude. About August 10th it was only of the fifth magnitude, and three months later it disappeared, reappearing on March 17, 1671, with the lustre of a star of the fourth magnitude. The temporary stars of 1572 and 1604 had directed the attention of astronomers to the variableness of the light of stars; and already, in 1050, Bouillaud had approximately determined the period of Mira Ceti, or the star o in the constellation of the Whale. Cassini, who observed sensible variations in the star of the Fox, supposed that its period could be fixed at ten months; but it was sought for in vain in February, 1672; it did not reappear till the end of March, being at that time of the sixth magnitude; then it disappeared once more, and has since never been seen.
Between the star of the Fox and the star discovered on April 28, 1848, by Hind in Ophiuchus (or Serpentarius), 177 years elapsed. The star of 1848, which was of a dark yellow or reddish color, did not exceed the fifth magnitude, but the variations of its light were carefully studied during the whole period of its visibility. In 1850 it was hardly of the eleventh magnitude, and this magnitude it has since kept.
Here we may refer to the researches made since Cassini's time into ancient writings, whether European or Chinese, which show that many similar apparitions of new stars have been noted in chronicles and afterward forgotten: for instance, the star of the year 125 b. c., observed by Hipparchus, as we learn from Pliny; another, which appeared in the Emperor Hadrian's time; the new star seen in the constellation of the Eagle in the year 389, and which possessed a brilliancy resembling that of Venus; that seen in the Scorpion in the ninth century; the new stars of the years 945 and 1264, both of which made their appearance in very nearly the same part of the heavens between Cepheus and Cassiopeia.
Before we come to the two latest temporary stars, which have succeeded each other in an interval often and a half years, and which are worthy of a detailed description, let us briefly state the questions to which these apparitions have given rise among astronomers, and the hypotheses which have been offered for their solution.
To what causes must we refer the nearly always sudden apparitions of these strange bodies, their variations of lustre, their intermittence, as also their changes of color? Why is it that, after alternations of great lustre and of paling, their light gradually faded away, and what is the cause of their ultimate disappearance?
A thousand conjectures have been made in the effort to answer this question. Among the weightiest of these, the one which compares temporary stars to variable periodic stars must be rejected at once—not on the ground that these two classes of stars are absolutely distinct, but because the periodicity being due, either to a movement of rotation or to occultation on the one hand, or to a phenomenon peculiar to the star itself on the other, the first hypothesis is clearly inadmissible for the explanation of new stars, and the second is precisely the question to be solved.
Tycho Brahe, struck with the suddenness of the appearance of the star of 1572, and its position on the edge of the Milky-Way, offered a bold hypothesis which is now abandoned. He believed in a creation or at least in the spontaneous incandescence of the nebulous matter of which he supposed the Milky-Way to be composed: when the new star vanished, the place it had occupied was void, or at least Brahe saw, in the absence of nebulosity at that spot, the result of the condensation of the matter the combustion of which had produced an appearance resembling a star. Humboldt justly compares this view, which at all events was an ingenious one, with the views held by W. Herschel as to the transformation of nebulæ into stellar masses. In Tycho Brahe's time it was not known that the light of the Milky-Way results from the aggregation of an indefinite number of stars, or stellar masses, and that it is within this immense agglomeration and in its vicinity that the nebulæ properly so called are rarest.
Besides, it has been proved that the stars known as "new" stars are anything but new. In the spot where the star of Ophiuchus made its appearance in 1848, there had previously been a star, noted by Lalande in Fortin's "Atlas Céleste" as a vanished star. So, too, the new star which appeared in May, 1866, in the Corona Borealis, and which at the start reached the second magnitude, had been already set down in catalogues as a star of the ninth magnitude; it still possesses the same lustre it had before it underwent, during the six months of its apparition, the extraordinary augmentation which attracts to it the attention of astronomers. Hence astronomers no longer believe in the creation or in the destruction of these stars. Before the sudden incandescence which makes them visible, they occupied the same places, and there they still remain after their more or less perfect extinction. It remains to inquire into the physical causes which produce these variations of lustre.Spectrum analysis has provided the first positive elements for the solution of this problem. As late as the year 1848 this method was as yet unknown; but when in 1866 the variable star of the Northern Crown (Corona Borealis) appeared, spectrum analysis was already so developed as to be profitably applied to the observation of it. The results obtained by Huggins and Miller in these researches were as follows:
"The spectrum of the variable star of Corona Borealis is found to consist of two superposed spectra, the one made up of four bright lines, and the other resembling the solar spectrum, each resulting from the decomposition of a group of luminous rays independent of the light which produces the other. The continuous spectrum, furrowed with groups of dark rays, indicates the presence of a photosphere of incandescent matter, almost certainly solid or liquid, and surrounded by an atmosphere of cooler vapors, which produce by absorption the groups of darker lines. So far the constitution of this star resembles that of the sun; but it offers an additional spectrum composed of bright lines. Here, then, is a second source of special light, and this source must be a luminous gas. Furthermore, the two principal bright lines of this spectrum show that this gas consisted mainly of hydrogen; and their great brightness proves that the temperature of the luminous gas was higher than that of the photosphere. These facts, taken in connection with the suddenness of the outburst of light in the star, its immediate
Fig. 1.—Spectrum of the Variable Star in Corona Borealis. (From Huggins and Miller.)
and rapid diminution of brightness, and its decline, in the course of twelve days, from the second to the eighth magnitude, lead us to the conclusion that the star was suddenly enveloped in hydrogen-flames. Possibly it was the scene of some mighty convulsion, with disengagement of an enormous amount of liberated gas. A great portion of this gas was hydrogen, which burned on the surface of the star by combining with some other element. The light given forth by this flaming gas was characterized by the spectrum with bright lines. The spectrum of the other portion of the star's light probably showed that this terrible conflagration of gas had surcharged and rendered more vividly incandescent the solid matter of the photosphere. When the free hydrogen had been exhausted, the flame died away by degrees, the photosphere became less luminous, and the star returned to its former state. . . . We must not forget," adds Mr. Huggins, "that light, though it travels with such great velocity, nevertheless requires a certain time to come to us from the star. Hence this grand physical convulsion, though new to us, was a thing of the past as regards the star itself. In 1866 the star had already for years been in the new conditions produced by this violent catastrophe."
The solar protuberances had not as yet been directly analyzed in 1866; it was not yet known that a continuous stratum of incandescent hydrogen envelops the bright photosphere of the sun, and that the emission of this gas, in the form of irregular jets, undergoes in the sun variations, phases, which are at least so far related to the sun-spots as to be coincident with them. What mighty physical revolution was it that suddenly produced the incandescence of the sun of the Corona, transforming it from a star of the ninth to one of the second magnitude?
What would become of our planet were such a revolution as this to take place in the sun, and were the calorific and luminous radiation to be suddenly increased a hundred-fold?
Fig. 2.—Spectrum of the New Stab in the Constellation of the Swan. (From Cornu.)
But let us come to the new star of the Swan, which is the main object of this article. Here is a very brief narrative of its discovery.
Julius Schmidt, director of the observatory at Athens, recently wrote to M. Le Verrier the following letter:
"On November 11th, at 5h 40m in the evening, I saw a star of the third magnitude in the zenith, near ρ Cygni. Observations with the refractor of the observatory, at 9h 30m, gave the position of the star as follows:
"It does not occur in the Bonn "Durchmusterung des Himmels." The star is strongly yellow in color. At midnight it was more intense than μ Pegasi (which is set down as of the third magnitude in Heis's catalogue). On November 20th the star was not visible. On the 21st, 22d, and 23d, the sky at Athens was overcast. From November 24th out, it has steadily declined in brightness, and on December 8th the star was a little below the sixth magnitude."
At Paris, too, the sky was almost constantly overcast for some days after the reception of Schmidt's letter. By taking advantage of infrequent and imperfect seasons of clear sky. Prosper Henry succeeded in observing the new star. Compared with the star 915 of Weisse's catalogue (hour 21), it had this approximate position:
It was of the fifth magnitude, and appeared to be of a greenish color, almost blue, as compared with a neighboring star (42,304 of Lalande).
The new star was also observed at Vienna by Littrow. To him on December 1st, as to Schmidt on November 24th, it appeared to be of from the third to the fourth magnitude; on December 3d it had fallen to the fourth magnitude, and on December 4th to the fifth magnitude. The right ascension was 24h 36sup>m 50.4sup>s; declination, +42° 16' 37.7" for 1886. O.
Fig. 3.—Map showing Portion of New Star in the Swan. Lézard—the Lizard; Cygne—the Swan; Étoile Nouvelle—the New Star.
We may observe here that there is nothing in common between this new star and those wliich made their appearance in 1600 in the neck of the Swan, and in 1670 near β of the same constellation. The only point of agreement between them is their nearness to the Milky-Way.
In comparing Schmidt's observations with those of Prosper Henry, we note two important facts: the first is, the diminution of lustre, varying in eight days from the third to the fifth magnitude; and, second, the notable change of color, from a pronounced yellow to greenish blue—a change so patent that it cannot be referred to individual differences of judgment, or to atmospheric influences.
The new star of the Swan has been made the subject of a very interesting spectroscopical study. As early as December 2d, Cornu applied a spectroscope to the great equatorial in the eastern tower of the Paris Observatory. Cazin, too, employed for the same purpose the nine-inch equatorial telescope (the object-glass of which is by Leon Foucault). Both of these observers arrived at the same conclusion. The spectrum of the star, observed during a brief interval of clear sky, appeared to "consist in great part of bright lines, and hence probably to be the result of an incandescent vapor or gas." Here it will be seen, is a first point of resemblance to the variable star of Corona Borealis.
A second study, made by Cornu on December 4th, enabled him to define the bright lines of the spectrum. Three of them are the lines C, F, and 434 of hydrogen; a fourth line seemed to correspond to the line D of sodium; another to the characteristic line b of magnesium. Finally, two lines with the wave-lengths 531 and 451 seem to coincide, the one with the famous line 1,474 (Kirchhoff's scale) observed in the solar corona during eclipse, the other with a line of the chromosphere. These results possess so high an interest that we will here transcribe a portion of the text of M. Cornu's note:
"The spectrum of this star," he writes, "consists of a certain number of detached bright lines on a sort of luminous ground, almost entirely interrupted between the green and the indigo, so that the spectrum appears as though made up of a number of separate parts. In order to study it qualitatively, I adopted a spectroscopic eye-piece of special construction, which utilizes most of the light, and allows the observer to vary its concentration. In measuring I used a direct vision Duboscq spectroscope. I observed only bright lines; the dark lines, if any there are, must be very minute, and escaped my observation, owing to the faintness of the star's light."
In this respect the spectrum of the star in the Swan differs from that of the star T of Corona Borealis, as may be seen from the figure here copied from Huggins.
We omit the details of the measurements of the positions of the lines, and pass to the results, which are stated in the following table. The bright lines, ranged in the order of their brightness, α, δ, γ, β, ζ, η, θ, ε, are eight in number, and their wave-lengths are here given in millionths of a millimetre:
From this table it is seen that there is almost perfect coincidence as regards the lines-α, η, and ε, with three hydrogen-lines; as regards β, with the line b of magnesium; as regards δ, with the line of sodium, or, perhaps, as Cornu suggests, with the bright line D3 of the chromosphere. The γ corresponds with a bright line also belonging to the chromosphere and the solar corona; and, finally, θ corresponds with a line of the chromosphere. The line ζ alone stands unidentified with any known line.
"If this interpretation were correct, the bright lines of the spectrum of the star would comprise exclusively the brightest and most frequent lines, of the chromosphere."
In the extreme violet there is a line, the fourth of hydrogen, also noted by Dr. Young, as one of the most frequent in the spectrum of the chromosphere, the wave-length of which is 410. Cornu thinks he has often perceived this line in the spectrum of the star, but he has not been able to measure it. The distinguished observer closes his account of these interesting researches as follows:
"To sum up, the light of the star seems to have precisely the same composition as the light of the sun's envelope, the chromosphere. Though the temptation is strong to draw from this fact conclusions as to the physical condition of this new star, its temperature, the chemical reactions it must exhibit, I will make no comments, nor offer any hypotheses. We lack, I think, the data necessary for reaching a profitable conclusion, or at least a conclusion that can be verified. However attractive such hypotheses may be, we must not forget that they He outside of the domain of science, and that, instead of being of service to science, they are likely to hinder its progress."
The readers of La Nature will permit us to add a few reflections of our own to those offered by our learned fellow-countryman. His reserve we acknowledge to be very wise, but he has expressed himself a little too strongly. Who, after perusing M. Cornu's analysis and the conclusions he has drawn, would not make the short step that here intervenes between fact and hypothesis, and assert the similarity, if not the identity, of the light of the star with that of the chromosphere? True, we cannot with certainty affirm that the apparently continuous faint spectrum in which are seen the bright lines was also its spectrum before it became visible to us, but that such is the case is highly probable. We cannot say what was the cause of this sudden development of gases, whose existence and incandescence are revealed by the star's spectrum, just as we are as yet unable to assign the cause of the sun's hydrogen protuberances. But is it nothing to class together phenomena, the only difference between which seems to be one of degree alone? The hypotheses of Huggins and Miller, as to the causes of the apparition of the star in the Corona Borealis, can of course never be verified; but no more can we verify the current hypotheses held by astronomers and physicists with regard to the presence of various chemical elements in the sun; we have here only probabilities. Such hypotheses, far from being of injury to science, are indispensable for its progress: they stimulate the observer's mind, constantly suggest to him fresh observations, and become a hinderance only when they are held to be demonstrated truths, and when men refuse to give them up after they have been proved to be erroneous, or after they have served their purpose.—La Nature.