IV. THE STRUCTURE OF THE STELLAR UNIVERSE.

391. Sir William Herschel's View.—Sir William Herschel assumed that the stars are distributed with tolerable uniformity throughout the space occupied by our stellar system. He accounted for the increase in the number of stars in the field of view as he approached the plane of the Milky-Way, not by the supposition that the stars are really closer together in and about this plane, but by the supposition that our stellar system is in the form of a flat disk cloven at one side, and with our sun near its centre. A section of this disk is shown in Fig. 457.

Fig. 457.

An observer near S, with his telescope pointed in the direction of S b, would see comparatively few stars within the field of view, because looking through a comparatively thin stratum of stars. With his telescope pointed in the direction S a, he would see many more stars within his field of view, even though the stars were really no nearer together, because he would be looking through a thicker stratum of stars. As he directed his telescope more and more nearly in the direction S f, he would be looking through a thicker and thicker stratum of stars, and hence he would see a greater and greater number of them in the field of view, though they were everywhere in the disk distributed at uniform distances. He assumed, also, that the stars are all tolerably uniform in size, and that certain stars appear smaller than others, only because they are farther off. He supposed the faint stars of the Milky-Way to be merely the most distant stars of the stellar disk; that they are really as large as the other stars, but appear small owing to their great distance. The disk was assumed to be cloven on one side, to account for the division of the Milky-Way through nearly half of its course. This theory of the structure of the stellar universe is often referred to as the cloven disk theory.

Fig. 458.

392. The Cloven Ring Theory.—According to Mädler, the stars of the Milky-Way are entirely separated from the other stars of our system, belonging to an outlying ring, or system of rings. To account for the division of the Milky-Way, the ring is supposed to be cloven on one side: hence this theory is often referred to as the cloven ring theory. According to this hypothesis, the stellar system viewed from without would present an appearance somewhat like that in Fig. 458. The outlying ring cloven on one side would represent the stars of the Milky-Way; and the luminous mass at the centre, the remaining stars of the system.

393. Proctor's View.—According to Proctor, the Milky-Way is composed of an irregular spiral stream of minute stars lying in and among the larger stars of our system, as represented in Fig. 459. The spiral stream is shown in the inner circle as it really exists among the stars, and in the outer circle as it is seen projected upon the sky. According to this view, the stars of the Milky-Way appear faint, not because they are distant, but because they are really small.

Fig. 459.

394. Newcomb's View.—According to Newcomb, the stars of our system are all situated in a comparatively thin zone lying in the plane of the Milky-Way, while there is a zone of nebulæ lying on each side of the stellar zone. He believes that so much is certain with reference to the structure of our stellar universe; but he considers that we are as yet comparatively ignorant of the internal structure of either the stellar or the nebular zones. The structure of the stellar universe, according to this view, is shown in Fig. 460.

Fig. 460.

INDEX

Transcriber's Note:

Missing or obscured punctuation was corrected.

Typographical errors were silently corrected.