The reader is no doubt aware that when the light of a star is analysed into its component colours by the instrument called the spectroscope, it is found that all the colours of the rainbow are present, as in the case of solar light, but (also in the sun's case) not all the tints of these colours. Certain dark lines athwart the rainbow-tinted streak, called the spectrum of the star, indicate the presence of absorbing vapours in the star's atmosphere. This general statement is true of every fixed star, though the dark lines of some stars differ in number and position from the dark lines of others, showing that other absorbing vapours are present. In the case of the new star in the Crown, the usual stellar spectrum was shewn,—a rainbow-tinted streak crossed by a number of dark lines. But besides these, there were seen four very bright lines,—lines so bright that the rainbow-tinted streak appeared as a dark background. The meaning of this is well understood by spectroscopists. It signifies that besides the vapours which, being cooler than the star, absorbed a portion of its light, and produced the dark lines, some vapours were present in the star's atmosphere which were a great deal hotter than the star, and so produced bright lines. Now two of the lines corresponded in position with two of the well known lines of the gas hydrogen, showing that this was one of the gases which had been raised to an unusual degree of heat.
It was inferred that there had been some tremendous disturbance in that remote star, by which the hydrogen and some other vapours present in its atmosphere had been intensely heated. But astronomers were unable to decide whether the disturbance was of the nature of a conflagration, the hydrogen actually burning, or whether the heat was occasioned in some other way, as by the downfall of some immense mass upon that remote sun. For burning hydrogen and glowing hydrogen, though either could give the observed bright lines, are very different things. In the former case a chemical change is taking place, as in the case of burning wood or coal; the latter case resembles that of redhot iron, which is not burning itself (not changing into a different form as everything does which burns), though it will burn other things,—in the ordinary, and incorrect, use of the expression.
The general belief was that there had been a downfall of matter on the star in the Crown, by which the whole globe of that sun had been excited to an intense degree of heat, especially at the surface, near which lies the hydrogen atmosphere of the star.
I must leave, however, to the next part, the further consideration of the strange thoughts suggested by the outburst of this star. I wish to use the small space remaining at present to indicate the place where another new star burst forth last November, so that any readers of these pages who have telescopes may know where to look for a sun which is now dying out, but was shining a few weeks ago as a third magnitude star. [Fig. 8] presents a portion of the well-known constellation Cygnus or the Swan. Any star atlas will indicate the place of the lettered stars shown in the figure. The constellation itself does not show at all well at this season of the year.[8] The part shown in the figure is close to the horizon, and directly under the pole-star, at about half-past ten in the middle of February; but a little higher up, between north and north-east, at midnight. Professor Schmidt, of the Athens Observatory, noticed a new star, in the place shown, on November 24th last. It must have shone out suddenly, for Schmidt had been observing in that region on the night of November 22nd (the last preceding clear night). It has since gradually faded, until now a small telescope is required to show it, shining as a seventh magnitude star, with a well-marked orange tint.
Fig. 8.—Part of Cygnus, showing the place of new star (November 24, 1876).
We have now to consider the history of this star, and discuss the general questions suggested by the sudden blazing out of suns which had for many years, and probably for many centuries, shone continuously with a far feebler lustre. It is clear that we have good reason to be interested in these questions, seeing that, for aught we know, our sun may be one of those exposed to sudden great increase of lustre.
It seems certain, in the first place, that this star leapt very suddenly to its full splendour. Schmidt had been observing the same regions of the heavens only two evenings before, and is sure the star was not then shining visibly to the naked eye. Again, astronomy is now studied by so many persons, and so many more who are not students of astronomy are now well acquainted with the constellations, that it is very difficult for a new star to shine many hours without being detected. For example, the new star in the Crown, which appeared in May, 1866, though not so well placed for observation, was detected by many observers at widely distant stations within a few hours of each other. It is probable that the star acquired its full lustre in a few hours at the utmost, and quite possible that, had any one been watching the place where the star appeared, he would have been able to see the star grow into full brightness by visible change of lustre, just as the lustre of a revolving light in a distant lighthouse visibly waxes and wanes. It may be, of course, that the increase of the star from its ordinary lustre, up to the stage when first it was visible to the naked eye, occupied many days, or even many months or years; but it seems more likely that as the later stages of increase were rapid, so also was the entire development of the new lustre. In that case, if there were inhabited worlds circling around that remote sun, they had but brief warning of the fate in store for them, as presently to be described.
Like the star in the Northern Crown, the new star in Cygnus was subjected to the searching scrutiny of the spectroscope. The results, though similar in general respects, were even more interesting than in the case of the brighter new star. In the interval between 1866 and 1876 spectroscopic analysis has developed largely. It has thus become possible to analyse more completely the light even of faint stars than the light of bright stars could be analysed a decade of years since.