[Illustration: 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.