THE MILKY WAY.—Richard A. Proctor
To those who rightly appreciate its meaning the Milky Way is the most magnificent of all astronomical phenomena. However opinions may vary as to the configuration of the star-streams composing this object, no doubt now exists among astronomers that the Milky Way consists really of suns, some doubtless falling short of our own sun in brilliancy, but many probably surpassing it. Around these suns, we may fairly conceive, there revolve systems of dependent orbs, each supporting its myriads of living creatures. We have afforded to us a noble theme for contemplation in the consideration of the endless diversities of structure, and of arrangement, which must prevail throughout this immensity of systems.
The Galaxy traverses the constellation Cassiopeia. Thence it throws off a branch toward Alpha Persei (Mirfak), prolonged faintly toward the Pleiades. The main stream, here faint, passes on through Auriga, between the feet of Gemini and the Bull’s horns, over Orion’s club to the neck of Monoceros. Thence, growing gradually brighter, the stream passes over the head of Canis Major, in a uniform stream, until it enters the prow of Argo, where it subdivides. One stream continues to Gamma Argus, the other diffuses itself broadly, forming a fan-like expanse of interlacing branches, which terminate abruptly on a line through Lambda and Gamma Argus. Here there is a gap beyond which the Milky Way commences in a similar fan-shaped grouping, converging on the brilliant (and in other respects remarkable) star Eta Argus. Thence, it enters the Cross by a narrow neck, and then directly expands into a broad, bright mass, extending almost to Alpha Centauri. Within this mass is a singular cavity known as the Coal-Sack. At Alpha Centauri the Milky Way again subdivides, a branch running off at an angle of 20°, and losing itself in a narrow streamlet. The main stream increases in breadth, until, “making an abrupt elbow,” it subdivides into one continuous but irregular stream, and a complicated system of interlacing streams covering the region around the tail and following claw of Scorpio. A wide interval separates this part of the Galaxy from the great branch on the northern side, terminating close on Beta Ophiuchi.
The main stream, after exhibiting several very remarkable condensations, passes through Aquila, Sagitta, and Vulpecula to Cygnus. In Cygnus there is a “confused and patchy” region marked by a broad vacancy, not unlike the Coal-Sack. From this region there is thrown off the offset to Beta Ophiuchi, already mentioned; the main stream is continued to Cassiopeia.
Fig. 16.—The Midnight Sky, with Milky Way
There only remains to be noticed “a considerable offset or protuberant appendage,” thrown from the head of Cepheus directly toward the pole. Galileo was the first to prove, though earlier astronomers had entertained the notion, that the Milky Way was composed of a vast number of stars crowded closely together. But no attempt was made to offer a theory of its structure until, in 1754 Thomas Wright, in his Theory of the Universe, propounded views closely according with those entertained later by Sir W. Herschel. Wright, having examined a portion of the Galaxy with a reflecting telescope, only one foot in focal length, came to the conclusion that our sun is in the midst of a vast stratum of stars; that it is when we look along the direction in which this stratum extends that we see the zone of light constituting the Milky Way; and that as the line of sight is inclined at a greater and greater angle to the mean plane of the stratum, the apparent density of the star-grouping gradually diminishes.
But it is to Sir W. Herschel, and the supplementary labors of Sir J. Herschel, that we owe the more definite views now commonly entertained respecting the Via Lactea. The elder Herschel, whose nobly speculative views of nature were accompanied by practical common-sense, and a wonderful power of patient observation, applied to the heavens his celebrated method of gauging. He assumed as a first principle, to be modified by the results of observation, that there is a tolerable uniformity in the distribution of stars through space. Directing his twenty-foot reflector successively toward different parts of the heavens, he counted the number of stars which were visible at any single view. The field of view of this reflector was fifteen minutes in diameter, so that the portion of the sky included in any one view was less than one-fourth of that covered by the moon. He found the number of stars visible in different parts of the heavens in a field of view of this size to be very variable. Sometimes there were but two or three stars in the field;[20] indeed, on one occasion he counted only three stars in four fields. In other parts of the heavens the whole field was crowded with stars. In the richer parts of the Galaxy as many as four hundred or five hundred stars would be visible at once, and on one occasion he saw as many as five hundred and eighty-eight. He calculated that in one-quarter of an hour 116,000 stars traversed the field of his telescope, when the richest part of the Galaxy was under observation. Now, on the assumption above named, the number of stars visible when the telescope was pointed in any given direction was a criterion of the depth of the bed of stars in that direction. Thus, by combining a large number of observations, a conception—rough, indeed, but instructive—might be formed of the figure of that stratum of stars within which our sun is situated.
Sir J. Herschel, during his residence at the Cape of Good Hope, carried out an extensive series of observations of the southern heavens. Applying his father’s methods of gauging with a telescope of equal power, he obtained a result agreeing, in a most remarkable manner, with those obtained by Sir William Herschel. It appeared, however, that the Southern Hemisphere is somewhat richer in stars than the Northern—a result which has been accepted as indicating that our system is probably somewhat nearer the southern than the northern part of the galactic nebula. Moreover, Sir J. Herschel was led to believe that the sidereal system forms a cloven flat ring rather than a disk.
I think no one who has attentively examined the glories of Orion, the richly jeweled Taurus, the singular festoon of stars in Perseus, and the closely set stars of Cassiopeia, but must have felt that the association of splendor along this streak of the heavens is not wholly accidental. The stars here seem to form a system, and a system which one can hardly conceive to be wholly unconnected with the neighboring stream of the Milky Way. But in the southern portion the arrangement is yet more remarkable and significant. From Scorpio, over the feet of the Centaur, over the keel of Argo, to Canis Major, there is a clustering of brilliant stars, which it seems wholly impossible not to connect with the background of nebulous light. It is noteworthy, also, that this stream of stars merges into the stream commencing with the group of Orion, already noticed. Nor is this all. It is impossible not to be struck by the marked absence of bright stars in the region of the heavens between Algol, Crux, and Corvus. One has the impression that the stars have been attracted toward the region of the stream indicated, so as to leave this space comparatively bare.
Now, this last circumstance would appear less remarkable if the paucity of stars here noticed were common also in parts of the heavens far removed from the Milky Way. But this is not the case. Beyond this very region, which we find so bare of stars, we come to a region in which stars are clustered in considerable density, a region including Crater, Corvus, and Virgo, with the conspicuous stars Algores, Alkes, and Spica. But what is very remarkable, while we can trace a connection between the stream of bright stars over the Milky Way and the stream of nebulous light in the background, it is obvious that the two streams are not absolutely coincident in direction.
The stream lies on one side of the Milky Way near Scorpio, crosses it in the neighborhood of Crux, and passes to the other side along Canis Major, Orion, and Taurus. Does the stream return to the Milky Way? It seems to me that there is clear evidence of a separation near Aldebaran, one branch curving through Auriga, Perseus, and Cassiopeia, the other proceeding (more nearly in the direction originally observed) through Aries (throwing out an outlier along the band of Pisces), over the Square of Pegasus, and along the streams which the ancients compared to water from the urn of Aquarius (but which in our modern maps are divided between Aquarius and Grus). The stream-formation here is very marked, as is evident from the phenomenon having attracted the notice of astronomers so long ago. But modern travels have brought within our ken the continuation of the stream over Toucan, Hydrus, and Reticulum (the two latter names being doubtless suggested by the convolutions of the stream in this neighborhood). Here the stream seems to end in a sort of double loop, and it is not a little remarkable that the Nubecula Major lies within one loop, the Nubecula Minor within the other. It is also noteworthy that from the foot of Orion there is another remarkable stream of stars, recognized by the ancients under the name of the River Eridanus, which proceeds in a sinuous course toward this same region of the Nubeculæ.
Having thus met with evidence—striking at least, if not decisive—of a tendency to aggregation into streams, let us consider if, in any other parts of the heavens, similar traces may not be observable. We traced a stream from Scorpio toward Orion, and so round in a spiral to the Nubeculæ. Let us now return to Scorpio, and trace the stream (if any appear) in the contrary direction. Now, although over the Northern Hemisphere star-streams are not nearly so marked as over the Southern, yet there appears a decided indication of stream-formation along Serpens and Corona over the group on the left hand of Boötes to the Great Bear. A branch of this stream, starting from Corona, traverses the body of Boötes, Berenice’s Hair, the Sickle in Leo, the Beehive in Cancer, passing over Castor and Pollux in Gemini, toward Capella. A branch from the feet of Gemini passes over Canis Minor, along Hydra (so named doubtless from the obvious tendency to stream-formation along the length of this constellation), and so to the right claw of Scorpio.
One other remarkable congeries of stars is to be mentioned. From the northern part of the Milky Way there will be noticed a projection toward the North Pole from the head of Cepheus. This projection seems to merge itself in a complex convolution of stars, forming the ancient constellation Draco, which doubtless included the ancient (but probably less ancient) constellation Ursa Minor. After following the convolutions of Draco, we reach the bright stars Alwaid and Etanin (Beta and Gamma) of this constellation, and thence the stream passes to Lyra, where it seems to divide into two, one passing through Hercules, the other along Aquila, curving into the remarkable group Delphinus.
The streams here considered include every conspicuous star in the heavens. But the question will at once suggest itself, whether we have not been following a merely fanciful scheme, whether all these apparent streams might not very well be supposed to result from mere accident. Now, from experiments I have made, I am inclined to believe that in any chance distribution of points over a surface, the chance against the occurrence of a single stream as marked as that which lies (in part) along the back of Grus, or as the curved stream of bright stars along Scorpio, is very great indeed; I am certain that the occurrence of many such streams is altogether improbable. And wherever one observes a tendency to stream-formation in objects apparently distributed wholly by chance, one is led to suspect, and thence often to detect, the operation of law. I will take an illustration, very homely perhaps, but which will serve admirably to explain my meaning. In soapy water, left in a basin after washing, there will often be noticed a tendency to the formation of spiral whorls on the surface. In other cases there may be no definite spirality, but still a tendency to stream-formation. Now, in this case, it is easy to see that the curved bottom of the basin has assisted to generate streams in the water, either circulating in one direction or opposing and modifying each other’s effects, according to the accidental character of the disturbance given to the water in the process of washing.[21] Here, of course, there can be no doubt of the cause of the observed phenomena; and I believe that in every case in which even a single marked stream is seen in any congeries of spots or points, a little consideration will suggest a regulating cause to which the peculiarity may be referred.
It is hardly necessary to say that, if the stream-formation I have indicated is considered to be really referable to systematic distribution, the theory of a stratum of stars distributed with any approach to uniformity, either as respects magnitude or distance, must be abandoned. It seems to me to be also quite clear that the immense extent of the Galaxy, as compared with the distances of the lucid stars from us, could no longer be maintained. On this last point we have other evidence, which I will briefly consider.
First, there is the evidence afforded by clusterings in the Milky Way. I will select one which is well known to every telescopist, namely, the magnificent cluster on the sword-hand of Perseus. No doubt can be entertained that this cluster belongs to the galactic system, that is, that it is not an external cluster: the evidence from the configuration of the spot and from the position it occupies is conclusive on this point. Now, within this spot, which shows no stars to the naked eye, a telescope of moderate power reveals a multitude of brilliant stars, the brightest of which are of about the seventh magnitude. Around these there still appears a milky unresolved light. If a telescope of higher power be applied, more stars are seen, and around these there still remains a nebulous light. Increase power until the whole field blazes with almost unbearable light, yet still there remains an unresolved background. “The illustrious Herschel,” says Professor Nichol, “penetrated, on one occasion, into this spot, until he found himself among depths whose light could not have reached him in much less than 4,000 years; no marvel that he withdrew from the pursuit, conceiving that such abysses must be endless.” It is precisely this view that I wish to controvert. And I think it is no difficult matter to show at least a probability against the supposition that the milky light in the spot is removed at a vast distance behind the stars of the seventh magnitude seen in the same field.
The supposition amounts, in fact, to the highly improbable view that we are looking here at a range of stars extending in a cylindrical stratum directly from the eye—a stratum whose section is so very minute in comparison with its breadth that, whereas the whole field within which the spot is included is but small, the distance separating the nearest parts of the group from the furthest is equivalent to the immense distance supposed to separate the sphere of seventh magnitude stars from the extreme limits of our Galaxy. And the great improbability of this view is yet further increased when it is observed that within this spot there is to be seen a very marked tendency to the formation of minor streams, around which the milky light seems to cling. It seems, therefore, wholly improbable that the cluster really has that indefinite longitudinal extension suggested by Professor Nichol. In fact, it becomes practically certain that the milky light comes from orbs really smaller than the seventh magnitude stars in the same field, and clustering round these stars in reality as well as in appearance.
The observations applied to this spot may be extended to all clusters of globular form; and where a cluster is not globular in form, but exhibits, on examination, either (1) any tendency within its bounds to stream-formation, or (2) a uniform increase in density as we proceed from any part of the circumference toward the centre, it appears wholly inconceivable that the apparent cluster is not really a cluster, but a long range of stars extending to an enormous distance directly from the eye of the observer. When, in such a case, many stars of the higher magnitudes appear within the cluster, we seem compelled to admit the probability that they belong to it; and, in any case, we can not assign to the furthest parts of the cluster a distance greatly exceeding (proportionally) that of the nearest parts.
Of a like character is the evidence afforded by narrow streams and necks within the Galaxy itself. If we consider the convolutions over Scorpio, it will seem highly improbable that in each of these we see, not a real convolution or stream, but the edge of a roll of stars. For instance, if a spiral roll of paper be viewed from any point taken at random, the chances are thousands to one against its appearing as a spiral curve, and, of course, the chance against several such rolls so appearing is very much greater. The fact that we are assumed to be not very far from the supposed mean plane of the Milky Way would partly remove the difficulty here considered, if it were not that the thickness and extent of the stratum, as compared with the distances of the lucid stars, must necessarily be supposed very great, on the assumption of any approach to uniformity of distribution.
Evidence pointing the same way is afforded by circular apertures in the Galaxy, or indeed by apertures of other forms. Another peculiarity of these cavities is also noticeable; whereas on the borders of every one there are many lucid stars, or in some cases two or three very bright stars, within the cavity there is a marked paucity of stars. This phenomenon seems to indicate a much closer connection between the brighter stars and the milky light beyond than is supposed on the stratum theory. One can hardly conceive the phenomenon to be wholly accidental.
There are some other points on which I fain would dwell, but space will not permit me. I will merely note that there are peculiarities in the distribution of red double and multiple stars, in the position in which temporary stars have made their appearance, and in the distribution of nebulæ, which seem very worthy of notice.
One point, however, immediately connected with my subject remains to be mentioned. I have traced streams of stars more conspicuous than those forming the Milky Way. We have also evidence of streams of light yet more delicate and evanescent than the light of our own Galaxy. In Sir John Herschel’s great work on the southern skies, he notes the frequent recurrence of “an exceedingly delicate and uniform dotting, or stippling, of the field of view by points of light too small to admit of any one being steadily or fully examined, and too numerous for counting, were it possible so to view them.” In thirty-seven places he detected this remarkable and significant phenomenon; a phenomenon so faint that he says, “The idea of illusion has continually arisen subsequently”; an idea well befitting the modesty of the philosophic observer, but which those who appreciate Sir John Herschel’s skill as an observer will be very unwilling to accept. As Professor Nichol remarks, “It is enough to read from Herschel’s notebook—‘I feel satisfied the stippling is no illusion, for its dark mottling moves with the stars as I move the tube to and fro’—to feel convinced that the phenomenon is real.” Now a remarkable fact connected with those observations is, that when Sir J. Herschel marked down in a star-chart the places in which he had detected this nebulous appearance, he found that, “with the exception of three which appeared outlying and disconnected, they formed several distinct but continuous streams.”