It is clear that the Milky Way may have been formed through the collision of two immense nebulous gas-aggregations in the manner just described. Subsequently and by virtue of the magnitude of the Galaxy great quantities of wandering cosmic matter and minor stellar bodies have been gathered in occasionally accompanied by larger clusterings such as the planetary nebulæ referred to.

How well justified we are in looking upon the Milky Way as a spiral nebula is apparent from the picture ([Fig. 5]) reproducing a photograph of the familiar regular nebula in the Dogs (Canes Venatici). It shows a wealth of detail hitherto not surmised. The feat was accomplished in the Carnegie Observatory on Mount Wilson in Southern California with the help of optical resources vastly superior to all earlier means. The Milky Way has previously been compared to this nebula, but due to deficient enlargement their striking similarity has not been fully recognized until now.

Assume the Sun located in the point marked “S” in [Fig. 5] and some distance above the plane of the picture, then the nebula if viewed from this point would appear somewhat as the Milky Way appears to us. In the middle we behold the substantial nucleus and on its left side a cleft between the two branches of the inner spiral. Farther to the left, we see only the outer spiral, first broadening toward the left where it approaches “S,” then narrowing only to spread again on account of the great clustering in the lower right part of the spiral. The axis of the nebula corresponds to the densest parts of the Galaxy in Cygnus (the Swan), the loop in the inner spiral again to the empty space between Cepheus and Cassiopeia, the narrow part of the outer spiral branch resembles the constriction at Algenib, the following diffusion corresponds to the broad section in Auriga (Charioteer) and Monoceros (Unicorn). At the subsequent narrow place we see the outer nebula-clump corresponding in certain respects to the Magellanic clouds on our firmament although these are farther removed, and apparently not indigenous to the Milky Way. There follows in the nebula a massive section in our system represented by the well known, far less compact, yet brightly luminous tract containing the Southern Cross. Here, from the star Alpha in Centaurus—the nearest bright fixed star[3] to our Sun, “only” 4.5 light years or about 25 trillion[4] miles distant—commences a bi-section of the spiral and strangely enough the nebula is similarly forked. Now the outer spiral stretching in a faint line upward from the “clump” begins to show as a weak band, while the inner spiral stands forth powerfully above “S” corresponding to the brilliant section of the Galaxy in Scutum (Shield) and Aquila (Eagle). The partition in the nebula between these two branches is the counterpart of the 110° long “prong” in the Milky Way between the constellations Norma and Lyra. Numerous faint bridges join the two branches in the nebula as well as in the Galaxy, according to Wolf.

[3] Up to a short time ago Alpha Centauri was considered the fixed star nearest to the Sun. By comparing old photographs of the firmament with such of recent date the renowned astronomer Barnard found that a very small fixed star—of the magnitude 10.5 and therefore far from visible to the naked eye—in the constellation Ophiuchus (Right ascension 17 h. 58 m. 44 s., North declination 4° 27´.4 January 1, 1917) possesses a very large proper motion. It traverses in a year 10.3 seconds on the firmament. The distance to this star, which has the largest proper motion so far known, was later determined to be 3.3 light years or 3/4 of the distance from Alpha Centauri to our Sun. Hence its velocity at right angle to the line of vision is computed to be 49 km. (32 miles) per second. Spectroscopic measurements show that it approaches us with a velocity of 91 km. (56.5 miles) per second along the line of vision. The combined velocity, therefore, is 103 km. (63 miles) per second, an unusually high value. The value of 3.3 light years used in this calculation was determined by the French astronomer Gonnessiat, who found it by the study of old photographs from Algeria. He also calculated the parallax of this star to be 1 second. According to later measurements, given in the Harvard Bulletins 616 and 617, its parallax is only 0.7 seconds and consequently its distance 4.6 light years and its speed perpendicularly to the line of vision 70 km. (43.5 miles) per second. Campbell, in the Lick Observatory, had determined its radial velocity and found that it approaches the Sun at the rate of 128 km. (79 miles) per second. Its total velocity is, according to these two last determinations, 146 km. (91 miles) per second.

It is by no means improbable that similar discoveries will be made in the future, so that the Sun will be found to have more stars in its “immediate” vicinity than previously assumed.

[4] American and French numeration; the Swedish and English usage is billion.

The correspondence is indeed surprisingly good. Proportions are, of course, somewhat different—in particular is the central part of the Galaxy not so predominating, which fact has been troublesome to the adherents of the nebula theory. Probably it was originally denser but has become attenuated through star-formation, explaining, for instance, the great gap between the constellations Lyra and Vulpecula.

To give a better idea of the structure of the Milky Way, two photographs are here reproduced as taken by Wolf, the German astronomer in Heidelberg, who has done particularly meritorious work in this department. One ([Fig. 6]) shows a section of the Galaxy in Cygnus (the Swan) with the star Deneb in the centre and to the left the “Northamerican-nebula” so named from its shape. Above Deneb is the dark “hole” in Cygnus and below another chasm not quite so black. Left of the “hole” is the winding canal enclosing the so-called Cocoon nebula. (See Worlds in the Making, page 172.)

The following picture ([Fig. 7]) contains, in the upper left, the bright star Altair in Aquila (the Eagle) located close to the powerful arm of the Galaxy in this constellation. Farther to the right is the fainter arm in Ophiuchus (the Serpent-holder). The lower half contains the most brilliant part of the Milky Way in the constellations Scutum (the Shield) and Sagittarius (the Archer). Bright stars are infrequent but the fainter ones are innumerable: “They are crowded into dense clusters and between them the most delicate star dust is scattered.”—“We behold how the star-ribbon dissolves into detached tufts which intertwine into the strangest patterns. These clouds of stars reach their greatest splendour in the lower part of the map.”