GENERAL MOVEMENT OF THE STARS. 231 



present this appearance in a still iAore striking- manner than the 

 northern. The Milky Way is itself composed of several concentric 

 rings, situated one behind the other, forming circular zones rich in 

 stars, comprised between others which are less so. 



At the centre of this great system exists a rich group, forming in 

 the whole a considerable and well-defined mass, whose diameter, 

 nearly equal to the distance from our Sun to the 61st of Cygnus, is 

 about 600,000 radii of the earth's orbit. It has already been said that 

 the zone in which our Sun is now found is poor in stars; and it is to 

 this situation that we may chiefly attribute the fact that the mean 

 distances of the stars so little correspond for us with their apparent 

 magnitude. It may be, also, that what we term richness in stars is 

 referable to a greater luminous power, increasing or decreasing by 

 alternating zones. 



The double stars, and the most considerable groups of stars, occur 

 in regions poor in stars as well as in those which are rich. Thus the 

 two stars considered the nearest to us are double, and situated in the 

 same zone with the Sun, which is placed not far from the middle of 

 the interval comprised between them. 



This constitution by alternative zones is not, as M. Masdler re- 

 marks, so different from that of our planetary system as might appear 

 at the first glance, for around our Sun there is first a void space of 

 0.38 of a radius of the earth's orbit; then we find a zone occupied 

 by four very dense planets of mean size, to which succeeds that of 

 the asteroids of very slight mass, and then that of the great planets, 

 attended by numerous satellites. 



The extent of the rings of the Milky Way may be determined up 

 to a certain point. In effect, the shortest distance from Alcyone to 

 the middle of that belt is 21°, and the mean divergence of the Milky 

 Way from a great circle, as has been said above, is three degrees and 

 a half. If we draw right lines between the Sun S, the group of the 

 Pleiades P, and the points M and M', the nearest and the most dis- 

 tant from the Milky Way, we shall have two triangles having a com- 

 mon side PS, supposed to be known from what precedes. M. Masdler 

 deduces therefrom the following values: 



The half-diameter of the Milky Way corresponds to a distance which 

 the light would require 3,648 years to traverse. The distance from 

 the. Sun to the nearest point of that belt would be traversed by the 

 light in 3, 166 years, and its distance to the most remote in 4, 140 years. 



But as the belt is double, and for the points in question the two 

 portions, by an effect of perspective, appear blended, the interior 

 ring ought to be a little less distant, and the exterior one, on the 

 contrary, considerably more so. It had already been supposed that 

 the more remote regions of the Milky Way would correspond to a 

 distance which light would occupy nearly 4,000 years in passing over. 



From this it results that the orbit described by our Sun in space is 

 very nearly to that of the circumference of the Milky Way in the 

 proportion of the orbit of Jupiter to that of Neptune; and, pursuing 

 the analogy, it may be remarked that the portion of stars compara- 

 tively the smallest is on the interior, and the largest on the exterior. 

 When the stars which surround the Pleiades to a distance of 25 to 



