Popular Science Monthly/Volume 12/December 1877/Star or Star-Mist

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A REMARKABLE discovery has been made by the astronomers of Lord Lindsay's observatory at Dunecht—a discovery the true meaning of which is not as yet fully perceived. It may be remembered that some nine months ago a new star, as it was called, made its appearance in the constellation Cygnus[1]. This object shone out where before no star had been known to astronomers—not merely, be it noticed, where there was no visible star, but where none was recorded even in lists like Argelander's "Durchmusterung," containing hundreds of thousands of telescopic stars. It was not, however, altogether impossible that some small star within moderate telescopic range had existed in the spot where the new star shone out, and that in some way this small star had escaped observation. This seemed the more likely because the new star had appeared in a part of the heavens very rich indeed in telescopic stars; at any rate, astronomers had reason to believe that they would be readily able to determine the question with a high degree of probability by watching the star as it gradually faded out of view. For a "new star" which had shone out in the constellation of the Northern Crown in May, 1866, and had been identified with a tenth-magnitude star in Argelander's list, had gradually faded out of view, and, growing yet fainter, had sunk through one telescopic magnitude after another until it shone again as a tenth-magnitude star only. Since that star had resumed its former lustre, or rather its former faintness, it seemed not unreasonable to conclude that so also would the star in Cygnus. We shall presently see how far this expectation was from being fulfilled.

During its time of greatest observed brilliancy the new star in the Swan was very carefully watched by spectroscopists. The results were in many respects interesting. The star in the Crown had shown the bright lines of hydrogen, superposed upon a faint rainbow-tinted spectrum, which was understood to signify that around a real, though probably a small, sun, some outburst of glowing hydrogen had taken place, the chief part of the star's new light being due to this outburst. The same bright hydrogen lines were seen also in the case of the star in Cygnus. But in addition to them other bright lines were seen, which seemed to be identical with those belonging to the solar sierra (or, as many astronomers unclassically call it, the chromosphere) and corona. This, at least, was the opinion of M. Cornu, of the Paris Observatory. Herr Vogel, who began his observations on December 5th, when the star was between the fourth and fifth magnitude, and continued them to March 10th, when the star had sunk below the eighth magnitude, does not agree on this point with M. Cornu, since aline not agreeing with any known line in the spectrum of the sun's sierra was clearly visible from the beginning in the spectrum of the new star. But the most interesting point in connection with Vogel's observations, confirmed also by Mr. Copeland, at the Dunecht Observatory, and by Mr. Backhouse, of Sunderland, was this: that, as the new star died out, not only did the rainbow-tinted background of the spectrum fade gradually out of view, but the relative lightness of the bright lines steadily changed. At last, on March 10th, very little was left of the spectrum which Cornu and Vogel had seen in December. The blue and violet portion of the spectrum had faded entirely from view, a dark gap had appeared in the green, and a very broad, dark band in the blue. Of the bright lines two only remained. One, the F line of hydrogen, in the green-blue, which had been singularly conspicuous last December, was now faint. The other, in the green, which had been faint in December, was now very bright—in fact, nearly the whole light of the star seemed at this time to come from this bright line.

Now, the changes which had thus far taken place were altogether unlike those which had been noticed in the case of the new star in the Northern Crown. As that star 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 Cygnus the part of the spectrum corresponding to stellar light—that is to say, the rainbow-tinted streak crossed by dark lines—faded gradually from view, and bright lines only were left, at least as conspicuous parts of the star's spectrum. This body, then, did not seem to be returning to the stellar condition at all, but actually fading out into a nebula. Not only so, but the lines which still remained conspicuous last March were lines known to belong to the so-called gaseous nebulae. One of them, that which had been the faintest, but was now the brightest, corresponded to the nitrogen line of the nebular spectrum; the other, which was still conspicuous, though faint, corresponded to the hydrogen line of nebulæ.

That however, was by no means the closing chapter of this singular history. Vogel seems to have ceased from observing the star's spectrum, strangely enough, at the very time when the most remarkable part of the process of change seemed to be approaching. At the Dunecht Observatory also, through pressure of work relating to Mars, no observations were made for nearly half a year. But, on September 3d, Lord Lindsay's 15-inch refractor was turned on the star. In the telescope a star was still shining, but with a faint blue color, utterly unlike that of the orb which had shone out so conspicuously last November. Under spectroscopic examination, however, the blue star was found to be no star at all, if we are to regard those orbs only as stars which present a spectrum in some degree analogous to that of our own sun. We regard Sirius as a sun, though in his spectrum the lines of hydrogen are abnormally strong; and, passing over the class of stars more closely resembling our sun, we regard as a true star the orange orb, Betelgeux, though the lines of hydrogen are wanting in its spectrum; nor do we reject from among the suns those stars which, like Gamma of Cassiopeia, show the lines of hydrogen bright upon a fainter rainbow-tinted spectrum. There is yet another order of stars—those whose spectrum presents bright bands with faintly lustrous internals, which, again, we regard as true suns, though they differ doubtless notably from our own. But we have been in the habit of regarding all the star cloudlets, whether consisting of multitudinous stars, like the clusters, or of luminous star-mist, as differing toto coelo from the sun and from all his fellow-stars. The clusters, indeed, save a spectrum resembling the sun's, and we regard them as different only because of their clustering condition. But the nebulæ which Sir W. Herschel regarded as consisting entirely of luminous vapor, and which spectroscopic analysis has proved to be so constituted, we have regarded not merely as different because of the structure and arrangement of their component parts, but as differing altogether in constitution. Now, the object seen as a faint blue star showed the same spectrum as these gaseous nebulæ, or rather as the very faintest of these nebulæ. For most of them show three bright lines, and one or two even show four bright lines; only the faintest shine with absolutely monochromatic or one-tint light. The star in Cygnus now shines like these faintest of the gaseous nebulæ—that is, with a light which, under spectroscopic analysis, presents only one bright line.

The words in which Lord Lindsay announced this remarkable discovery are these: "There is little doubt but that this star has changed into a planetary nebula of small angular diameter," though, he goes on to say, "such a result is in direct opposition to the nebular hypothesis." On this last point I venture to express dissent from Lord Lindsay's opinion, which is in any case a somewhat bold inference from a single observation. Assuredly the discovery just made is in direct opposition to a certain argument, derived from the gaseity of nebulæ, in favor of the gaseous hypothesis of Laplace—an argument which had always appeared to the present writer insufficiently established. But the nebular hypothesis, regarded not merely in the form suggested by Laplace (in which form it was utterly inconsistent with physical facts now known), but in the wider sense which would simply present our solar system in the remote past as in a nebular state, without defining its nebulosity as due either to gaseity on the one hand, or to a mixed meteoric and cometic constitution on the other, has most certainly not received a shock, but rather receives strong support, from Mr. Copeland's observation. A theory of the evolution of the solar system, advocated by me during the last seven years, according to which the solar system had its origin in meteoric and cometic aggregation, requires that during the long ages through which the process of development continued there should be occasional outbursts of light and heat in moderate degree from the rest of the system, even to its outskirts. That intense heat imagined by Laplace as pervading the entire gaseous mass, extending originally far beyond the path of the remotest planet of our system, would require, indeed (if it were a physical possibility in other ways), that the spectrum of a developing solar system should be uniformly that of gaseity for millions on millions of years. If it had been found or could be proved that the gaseous nebulæ are in a state of intense heat, Laplace's gaseous hypothesis would have had one powerful argument in its favor. This argument has been strongly urged by those who have taken that special view of the gaseous nebulæ which the recent discovery shows to be erroneous. But those who have maintained, as I have, that in the gaseous nebulæ we probably "see vast systems of comets traveling in extensive orbits around nuclear stars," will find confirmation, not disproof, in the discovery lately made, especially when considered in combination with the circumstance that Prof. Wright, of Yale, has found the cometic spectrum to be emitted by meteoric masses exposed to moderate heat; while, under slight changes of condition, the cometic spectrum of bright carbon bands appears to give place to the nebular bright-line spectrum.

However, speculation apart, we have in the discovery just made a most important fact for our guidance—the fact, namely, that a body which to ordinary observation has been in all respects like the star in the Crown, and even under spectroscopic observation shone for a while with true stellar light, has dwindled into a nebula giving the spectrum which has heretofore been regarded as indicative of ordinary gaseity.—English Mechanic.

  1. See Popular Science Monthly, vol. xi., p. 59.