PLATE XIII.—PART OF THE MILKY WAY.
From a study made during the years 1874, 1875 and 1876
By beginning at its northernmost part, represented at the upper part of Plate XIII., situated in "the chair" of the constellation Cassiopeia, and descending southwardly, and continuing in the same direction until the whole circle is completed, the course of the Milky-way through the constellations may be briefly described as follows: From Cassiopeia's chair, the Galaxy, forming two streams, descends south, passing partly through Lacerta on the left, and Cepheus on the right; at this last point it approaches nearest to the polar star. Then it enters Cygnus, where it becomes very complicated and bright, and where several large cloudy masses are seen terminating its left branch, which passes to the right, near the bright star Deneb, the leader of this constellation. Below Deneb, the Galaxy is apparently disconnected and separated from the northern part by a narrow, irregular dark gap. From this rupture, the Milky-way divides into two great streams separated by an irregular dark rift. An immense branch extends to the right, which, after having formed an important luminous mass between the stars γ and β, continues its southward progress through parts of Lyra, Vulpecula, Hercules, Aquila and Ophiuchus, where it gradually terminates a few degrees south of the equator. The main stream on the left, after having formed a bright mass around ε Cygni, passes through Vulpecula and then Aquila, where it crosses the equinoctial just below the star η after having involved in its nebulosity the bright star Altair, the leader of Aquila. In the southern hemisphere the Galaxy becomes very complicated and forms a succession of very bright, irregular masses, the upper one being in Scutum Sobieskii, while the others are respectively situated in Sagittarius and in Scorpio; the last, just a little above our horizon, being always considerably dimmed by vapors. From Scutum Sobieskii, the Galaxy expands considerably on the right, and sends a branch into Scorpio, in which the fiery red star Antares is somewhat involved.
Continuing its course below our horizon, the Milky-way enters Ara and Norma, and then, passing partly through Circinus, Centaurus and Musca, it reaches the Southern Cross, after having been divided by the large dark pear-shaped spot known to navigators as the "Coal-Sack." In Ara and Crux the Milky-way attains its maximum of brightness, which there surpasses its brightest parts in Cygnus. In Musca, it makes its nearest approach to the south pole of the heavens. It then enters Carina and Vela, where it spreads out like a fan, and terminates in this last constellation, before reaching λ, being once more interrupted by a dark and very irregular gap, on a line with the two stars γ and λ. It is noteworthy that this second rupture of continuity of the Galaxy in Vela is very nearly opposite, or at about 18o° from the break near Deneb in Cygnus.
Continuing its course on the other side of the break, the Milky-way again spreads out into the shape of a fan, grows narrower in entering Puppis, where it is longitudinally divided by darkish channels. It then passes above our southern horizon, becoming visible to us, passing through part of Canis Major, where its border just grazes the brilliant star Sirius. But from Puppis it gradually diminishes in brightness and complication, becoming faint and uniform. It enters Monoceros and Orion, where it again crosses the equator a little above δ, the northernmost of the three bright stars in the belt of Orion. Continuing its northward course it passes through Gemini, extending as far as Castor and Pollux, and then entering Auriga, where it begins to increase in brightness and in complication of structure. It passes partly through Camelopardus and into Perseus, where an important branch proceeds from its southern border.
This branch beginning near the star θ, advances towards the celebrated variable star Algol, around which it is quite bright and complicated. Continuing its course in the same direction, the branch rapidly loses its brightness, becoming very faint a little below Algol, and passing through ζ Persei, it enters Taurus, leaving the Pleiades on its extreme southern margin; and after having passed through ε where it branches off, it rapidly curves towards the main stream, which it joins near ζ Tauri, thus forming an immense loop. The ramification projecting near ε Tauri involves in its nebulosity the ruddy star Aldebaran and the scattered group of the Hyades. It then advances towards the three bright stars δ, ε and ζ of the belt of Orion, which, together with the sextuple star θ Orionis, are involved in its faint nebulosity, and joins the main stream on the equinoctial, having thus formed a second loop, whose interior part is comparatively free from nebulosity, and contains the fine stars Betelgeuse and Bellatrix.
That portion of the main galactic stream which is comprised between the star Deneb in Cygnus, and Capella in Auriga, is divided longitudinally by a very irregular, narrow, darkish cleft, comparatively devoid of nebulosity, which, however, is interrupted at some points. This dark gap sends short branches north and south, the most important of which are situated near ζ Cephei and β Cassiopeiæ. Another branch runs from γ beyond ε of the constellation last mentioned. The main stream of the Galaxy after leaving Perseus, enters Cassiopeia, and sending short branches into Andromeda, it completes its immense circle in Cassiopeia's chair, where this description was begun.
When examined through the telescope, the appearance of the Milky-way completely changes, and its nebulous light is resolved into an immense number of stars, too faint to be individually seen with the naked eye. When Galileo first directed the telescope to the galactic belt, its nebulous, cloud-like masses were at once resolved into stars, even by the feeble magnifying power of his instrument. When, much later, Sir William Herschel undertook his celebrated star-gaugings of the Galaxy, millions of stars blazed out in his powerful telescopes. The stars composing this great nebulous belt are so numerous that it is impossible to arrive at any definite idea as to their number. From his soundings Herschel estimated at 116,000 the number of stars which, on one occasion, passed through the field of his telescope in 15 minutes, by the simple effect of the diurnal motion of the heavens; and on another occasion, a number estimated at 250,000 crossed the field in 41 minutes. In a space of 50, comprised between β and γ Cygni, shown on Plate XIII., he found no less than 331,000 stars. Prof. Struve has estimated at 20,500,000 the number of stars seen in the Milky-way through the twenty-foot telescope employed by Herschel in his star-gaugings. Great as this number may seem, it is yet far below the truth; as the great modern telescopes, according to Professor Newcomb, would very probably double the number of stars seen through Herschel's largest telescope, and detect from thirty to fifty millions of stars in the Milky-way.
Although the telescope resolves the Galaxy into millions of stars, yet the largest instruments fail to penetrate its immense depths. The forty-foot telescope of Herschel, and even the giant telescope of Lord Rosse, have failed to resolve the Milky-way entirely into stars, the most distant ones appearing in them as nebulosities upon which the nearer stars are seen projected, the galactic stratum being unfathomable by the largest telescopes yet made.
The stars composing the Milky-way are very unevenly distributed, as might easily be supposed from the cloud-like appearance of this belt. In some regions they are loosely scattered, forming long rows or streams of various figures, while in others they congregate into star groups and clusters having all imaginable forms, some being compressed into very dense globular masses. The intervals left between the clustering masses are poorer in stars, and indeed some of them are even totally devoid of stars or nebulosity. Such are the great and small "coal-sacks" in the southern Galaxy. I have myself detected such a dark space devoid of stars and nebulosity in one of the brightest parts of the Milky-way, in the constellation Sagittarius, in about 17h. 45m. right ascension, and 27° 35' south declination. It is a small miniature coal-sack or opening in the Galaxy, through which the sight penetrates beyond this great assemblage of stars. Close to this, is another narrow opening near a small, loose cluster.