The Galaxy is familiar to all readers, and although visible all the year round, is perceived more plainly in August, September, and October, or at the beginning and ending of that period. This zone of stars was of course well known to the ancients, but it is to Galileo that we owe the first important information about the Galaxy; he decided that it was formed of stars. Sir John Herschel investigated the subject very closely, and to him much of the information concerning the Milky Way is due.
It is not very distinct in the north, but as it advances from Cepheus southwards by the Unicorn, it gets clearer, and opens out in Argo, and descends still south, becoming brighter near the Southern Cross. It then passes northward again, dividing into two branches, one of which dies out, and then over Sagittarius, and so on to Cygnus, then to Casseopeia and the starting-point. The number of stars in the Galaxy is about 18,000,000.
Fig. 628.—The Milky Way.
In this wonderful zone of stars the centre of our system, the sun, is placed. It was supposed to be divided as in the diagram above; the inner portion being the stars seen in their thickness, and the outer ring representing the stars viewed in the direction of the length and breadth. But afterwards, Herschel modified his opinions respecting the Milky Way, and since his death many astronomers—and Mr. Proctor more particularly—have devoted considerable time to an examination of this wonderful zone of stars; which, it must be remembered, is not a continuous stream; it is a series of luminous patches. On this point Professor Nichol says:—
“It is only to the most careless glance that the Milky Way appears a continuous zone. Let the naked eye rest thoughtfully on any part of it, and if circumstances are favourable, it will stand out rather as an accumulation of patches and streams of light in every conceivable variety of form and brightness; now side by side, now heaped on each other, again spanning across dark spaces ... and at other times darting off into the neighbouring skies in branches of capricious length and shape, which gradually thin away and disappear.”
The Milky Way has its greatest breadth in the “Swan,” and in the “Eagle” constellation it divides itself. In the “Southern Triangle” the zone is brightest, and in the “Southern Cross” the hole or space, termed by sailors the “Coal Sack,” is very distinct. It then contracts and expands, and there is in Argo another gap. Then it is lost for a space, then it branches out, and soon crosses the Equator, dilates, contracts, opens out again, and so returns to the “Swan” again.
Philosophers have frequently discoursed upon this phenomenon, but all statements must remain more or less speculative. From Kepler’s to the present time astronomers have been considering the Milky Way, and when the Nebular theory was given up, when the Galaxy was found to be composed of stars, there was, as we have noticed, the idea of the ring and the cloven disc. Mr. R. Proctor has likened the Galaxy to a coiled serpent, and considers the openings in the Milky Way as evidence that the stratum of stars is limited, and that here we can see beyond it. In fact, it would appear that it is a very complicated question; and as the zone itself is complicated “with outlying branches beyond the range of our most powerful telescopes,” so an actual knowledge of the Milky Way is beyond us at present. It is composed of most extraordinary aggregations of stars, which appear not only impossible to count, but each one to be independent of the other. Thus we must conclude our rapid survey of the Milky Way, and close with Mr. Proctor’s remark in his “Universe of Stars.” “The sidereal system,” he says, “is altogether more complicated, altogether more varied in structure than has hitherto been supposed. Within one and the same region co-exist stars of many orders of real magnitude, the greatest being thousands of times larger than the least. All the Nebulæ hitherto discovered, whether gaseous or stellar, irregular, planetary, ring-formed, or elliptic, exist within the limits of the sidereal system. They all form part and parcel of that wonderful system, whose nearer and brighter parts constitute the glories of our nocturnal heavens.”
And a little reflection will show how true this is. Not very long ago in the world’s life the solar system was supposed to consist of one sun with a few planets wandering around him. Then some more were found, and they were called “satellites.” For a long time man fancied he had reached the “ultima thule” of astronomy in these depths; but the whole idea was changed when it was discovered that beyond Mars there lie the asteroids and the host of bodies in this solar system which we cannot do more than allude to. Then when we consider that this “sun” of ours, which we think so enormous, and which keeps in subjection so many heavenly bodies, and illuminates them; when we reflect that there are in space, and visible, stars many times larger than our ruling star, each a sun, and that our sun would, if put where the great Sirius glows, be but a speck in the firmament, and his system invisible to our eyes, we may well wonder at the magnitude of the subject, and the Infinite Wisdom and Power “that telleth the number of the stars, and calleth them all by their names.”
How to read the Sky.