I have observed further that certain types abound in certain parts of the heavens, and that the stars of the same kind are generally grouped together. Thus the white stars are found in the Pleiades, the Bear, the Lyre, etc.; the yellow in the Whale, Eridan, etc. The constellation of Orion deserves particular attention; it abounds in stars of a green color, reminding us of the nebula which is found in the same region of the sky. This small number of types, and the grouping of which I have spoken, constitute an unforeseen fact, the importance of which is considerable from a cosmological point of view. We should not, however, be hasty in drawing conclusions from it.
A curious fact has been established with regard to one of the white stars in Cassiopeia. Its appearance is directly the opposite of that which is presented by stars of the same color, for, in place of black lines, it shows some brilliant lines. This phenomenon has appeared to me so extraordinary, that I am anxious whether it is an isolated fact. I have observed more than five hundred stars, selecting some of the largest, and I have found only one, [Beta] of the Lyre, which possesses the same peculiarity. M. Wolff says that among the small stars of the Swan he has found some examples of the same kind. A most remarkable fact is, that these brilliant lines were found in a transient star which glittered for a time in the Crown in May, 1866.
These observations upset the theories which had been prematurely built upon facts formerly known. Still, there is nothing inexplicable here. You have seen that sodium burning gives a line of a very lively yellow, while the line becomes black if the sodium is increased to a considerable quantity. Might not the same thing happen with the hydrogen, which produces the brilliant lines of which I have spoken to you? Might not a small quantity act by radiation, while the action would be one of absorption should the mass be greater?
After having examined the stars, it was impossible to resist the temptation of observing the nebulae. You know that we designate by this name the kind of white clouds which are found spread in the heavens, and of which the nature is not perfectly known. Herschel has assured us that many of them, by means of the telescope, may be resolved into a multitude of small stars approaching very closely to each other. We infer from this that the greater part are composed in the same manner, and that the feebleness of our instruments is the only thing that prevents us from proving it. It is, however, admitted that many of these nebulae are formed of cosmical matter in a state of vapor not condensed. Everybody knows the nebulae which compose the Milky Way. But besides those which are visible to the naked eye, there is a vast number whose existence the telescope has revealed to us. One, of the most celebrated is that which is found in the magnificent constellation of Orion: we have carefully drawn it at the Roman College, and you see at this moment a sketch of it on the screen. The nebulae possess a very feeble light, and we had our doubts of success in seeking to apply the spectral analysis to them. We have, however, succeeded beyond our hopes. The appearances obtained in these observations are very singular. They reduce themselves constantly to luminous streaks, all the other colors failing; it is, in another way, that which happens when we burn an alcoholic solution with marine salt; the flame, analyzed by the spectroscope, gives simply a yellow streak. In the nebulae we find two green lines and a blue one. Such is the result which we obtained in examining the large nebulae of Orion, and that of the Milky Way in Sagittarius. Such is that, also, which furnishes the little nebulae called planetaries, on account of their form, which resembles that of the planets. These facts have been established for the first time by M. Huggins.
As I have just told you, the nebulas present generally but three lines; one belongs to azote, another to hydrogen, and the third is unknown. This result, which was not known before, is of the highest importance; for it teaches us that the nebulae are composed of gas and of vapors far removed from their point of saturation and condensation. These appearances, with luminous lines, distinctly isolated and separated from one another, appertain essentially to gas, and, we ought to add, to gas raised to a very high temperature. Thus we have made a discovery by the aid of the prism, for which the most powerful glasses had failed us.
The nebulae, notwithstanding their shining points, are not in general a collection of stars, but masses of cosmical matter in a state of dissociation under the action of an extremely elevated temperature. The collections of stars are perfectly distinguishable by the continuity of their appearances, as we see in the nebulae of Andromeda, and in some others which are well known. The discovery opens a vast field of investigation, and will be an epoch in science.
We have wandered far into the depths of space, very far from the point from which we started. This is of no consequence, however, for between the sun, the stars, and the nebulae there is a close relation. The sun is simply a star approaching nearer to us than others. According to a bold hypothesis, its entire mass was at one period in a state of dissociation, which a great part of it still actually preserves. The only thing that makes it differ from the nebulae, and causes us to rank it among the stars, is its superficial stratum of inconsiderable thickness.
What mysteries do we not discover in nature, when we investigate it by the aid of those principles and instruments with which modern science has furnished us! And in the presence of the wonders, what an exalted idea ought we to form of the splendors of the universe and the power of its Creator!