Chapter V: Part 5

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.

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_.”

FOOTNOTES:

[20] Field means the actual space covered by the lens.—E. S.

[21] Sometimes a singular regularity of curvature is noticed, and a spiral is formed closely resembling in configuration some of the great spiral nebulæ, as drawn by Lord Rosse, so that one is tempted to see in the centrifugal tendency of the disturbed water, and the centripetal effects caused by reflection from the basin’s surface, causes which may in some sense illustrate the laws operating in wider domains of space.

THE MAGELLANIC CLOUDS—ZODIACAL LIGHT—STAR GROUPS.—AMÉDÉE GUILLEMIN

When we look on the region of the celestial vault which surrounds the South Pole, we can not help being struck with the contrast presented by the small quantity of stars which it contains, with the brilliant zone which borders the Milky Way, from Orion and Argo to the Centaur, passing by the Southern Cross. One solitary star of the first magnitude, Achernar, more distant from the pole than are the beautiful stars of the Centaur and of the Cross, shines in this part of the sky.

But even this circumstance renders the singular aspect of the two nebulous spots, which seem two detached pieces of the great galactic zone, still more striking. These half-stellar, half-nebulous systems, unequal in magnitude and brightness, but easily seen with the naked eye on a clear, moonless night, are situated, one, the larger and more brilliant, between the pole and Canopus, in the constellation of Doradus; the other, the smaller and less brilliant, ordinarily visible during the full moon, in Hydrus, between Achernar and the pole.

Both are known by astronomers and navigators under the name of “Cape Clouds,” or again, “Magellanic Clouds.” And, to distinguish them, we have again the Great Cloud (_Nebecula Major_) and the Small Cloud (_Nebecula Minor_).

The Clouds of Magellan are distinguished from all other nebulæ by their great apparent dimensions, and by their physical structure; this last character distinguishes them from most of the branches and offshoots of the Milky Way, with which, we may also add, they do not appear connected in any way.

The Great Cloud extends over a space which embraces not less than forty-two square degrees—about two hundred times the apparent surface of the lunar disk. The Small Cloud occupies in extent four times less than the other; according to Humboldt, it is surrounded “with a kind of desert,” where, it is true, shines the magnificent stellar cluster of Toucan. If the exterior aspect of these two remarkable nebulæ, and their situation in a celestial region poor in stars, give to the southern sky a peculiar appearance, their real structure makes them one of the wonders of the heavens.

In the Great Cloud, Herschel has counted 582 single stars, among which one only is of the fifth magnitude; six others are of the order immediately inferior, and would doubtless be visible to the naked eye if their light were not effaced by the general glare.

In the Small Cloud, the single stars are proportionally more numerous, since 200 have been counted, among which three are of the sixth magnitude, while it only includes thirty-seven of the nebulæ and seven star-clusters. These immense aggregations, the elements of which are themselves swarms of suns, remind us of the largest, in appearance at least, of all the clusters which the eye contemplates in the depths of the sky—the Milky Way.

In the evenings, about the time of the vernal equinox—in March and April, when in our climate the twilight is of short duration—if we examine the horizon toward the west, a little after sunset, we may perceive a faint light that rises in the form of a cone among the starry constellations.

This is what astronomers call the Zodiacal Light. Those unfamiliar with it, or little accustomed to the ordinary aspect of the sky, might confuse the glimmering either with the Milky Way or with the ordinary twilight, or even with an aurora. But, with a little attention, it is impossible to mistake it.

The triangular form of this luminous cone, its elevation and its inclined position to the horizon, make it a thing apart, and one eminently deserving particular mention.

As the days lengthen, and with them the duration of twilight, the Zodiacal Light disappears; it becomes invisible, at least in our climate. But it may again be seen in the morning, in the east, about the time of the autumnal equinox, in September and October, when the dawn has an equally short duration—again, however, to disappear during the period of long nights and long twilights.

It is needless to add that the sky must be clear and the night moonless for observations of the Zodiacal Light to be possible.

Among the explanations that have been given, the most probable one is that which likens the Zodiacal Light to a flattened nebulous ring surrounding the sun at some distance. It is to be remarked that the direction of the axis of the cone, or of the pyramid, prolonged below the horizon, always passes through the sun.

It was believed at first that this direction precisely coincided with the solar equator; but it seems more certain that it coincides with the plane of the earth’s orbit, or the ecliptic.

Now, what is the nature of this luminous mass? Must it be considered as a zone of vapors thrown off by the sun, when in the process of consolidation, when our central star passed from a nebulous state to that of a condensed fluid sphere? This was the opinion of Laplace.

Another hypothesis, also connected with the first, is that the Zodiacal Light is formed of myriads of solid particles, analogous to the aerolites, possessing a general movement, but traveling separately around the focus of our solar world. The light of the ring would be thus produced by the accumulation of this multitude of brilliant points, reflecting toward us the light borrowed by each of them from the sun.

This explanation accounts for the intensity of the Zodiacal Light at different epochs; it would suffice to admit that the condensation of the particles or the density of the ring is not the same throughout its extent, and that its movement of circulation round the sun presents successively different parts to the earth. In this case, it becomes a question whether this lenticular ring of matter is distinct from the zone of aerolites.

Lastly, some astronomers regard the Zodiacal Light as a vaporous ring which belongs to the earth, surrounding it at some distance. But this is an opinion which appears somewhat wild, and is utterly at variance with observation.

Are the stars that are visible to the naked eye spread orderless on the celestial vault? or is there not between those apparently most closely connected some real or physical connection which requires us to rank them in natural groups?

These questions have been already partly solved by what is known of the double and multiple star systems. Soon, exploring the regions of the sky visible by means of the telescope, we shall have to pass in review a multitude of stellar associations, in which suns are found so compact and so numerous, and the form of the groups so regular, that it is impossible to deny their reciprocal dependence.

But long before the discovery of these islands, these archipelagos as worlds, scattered with such astonishing profusion over the infinite, the naked eye had already distinguished a certain number of groups, the stars composing which were so near together that it was impossible to doubt their physical connection.

Such, for example, is the group of the Pleiades. Such, again, are the groups known under the names of the Hyades, of Præsepe, and of Berenice’s Hair. All are visible to the naked eye, and good eyes distinguish without difficulty the principal stars of the first-named groups. The Pleiades are situated in the constellation of the Bull, which we can distinguish so easily to the northwest of Orion and Aldebaran.

Of about eighty stars which form the group of the Pleiades, six are visible without the help of telescopes. Formerly, the Latin poet tells us, seven were counted, which may be held to prove that one of them is variable, and has diminished in brightness, or else has disappeared.

The most brilliant, Alcyone, is of the third magnitude; Electra and Atlas are of the fourth; Merope, Maïa, and Taygete of the fifth. Three others again have received particular names, although they are below the limit of ordinary vision; these are Pleione, Celeno, and Asterope, from the sixth to the eighth magnitude. All the others are only visible by the aid of a telescope; but with an ordinary glass it is possible to distinguish a large number. The Pleiades are known under the name of the Hen-coop, doubtless because Alcyone appears in the group as a hen surrounded with her chickens.

The Hyades, which are near the Pleiades, form a less numerous and more scattered group. The bright light of Aldebaran, which is, as is known, of the first magnitude, renders them more difficult to distinguish with the naked eye.

They appear in the rainy season. Hence their name of Hyades, from the Greek word which signifies to rain.

The connection of the stars which compose this group is not so striking as in the case of the Pleiades. Nevertheless, it seems difficult to admit that they are quite independent of each other’s attraction. In examining the position of these two groups in the vicinity of the Milky Way, and observing that both are situated in the prolongation of a branch of the great zone, we are almost entitled to consider them as two clusters of stars, belonging to the immense stellar stratum which surrounds us, and in the midst of which the sun himself is placed.

In Berenice’s Hair, most of the stars are visible to the naked eye, and are perfectly distinguished in the sky, a little to the east of the Lion. No very brilliant star in the vicinity inconveniences the eye by effacing their light.

The next group is situated in the Crab, and is known under the name of Præsepe: it is visible to the unassisted sight; but it is impossible to distinguish the separate stars without the help of a telescope. Nevertheless, an instrument of moderate power easily separates them.

The groups which we have just described form a transition between the stars scattered over the celestial vault and the more condensed clusters, the undefined aspect of which caused them formerly to be designated under the general name of nebulæ.

Doubtless, if we could place ourselves in space, and contemplate from a sufficiently distant standpoint the whole of the stars which appear to us isolated, we should see them condensed into one or several distinct groups, analogous to those of the Pleiades; while, were we to penetrate into the midst of one of those compact clusters, we should see the stars of which it is formed separated and scattered over the celestial vault in such a way as to give it the aspect of our own heavens.

THE NEBULÆ AND SWARMS OF SUNS.—J. E. GORE

We will now consider the nebulæ, properly so called, that is to say, objects which the spectroscope shows to consist of glowing gas. These are sometimes large and irregular in form, like the great nebula in the “Sword” of Orion, sometimes with spiral convolutions, and sometimes of a definite shape, like the planetary and annular nebulæ.

Of the large and irregular nebulæ, one of the most remarkable is that known as “the great nebula in Orion.” It surrounds the multiple star, Theta Orionis. It is a curious fact that it escaped the searching eye of Galileo, although he gave special attention to the constellation of Orion, for even with a good opera-glass a nebulous gleam is distinctly visible round the central star of the “Sword.” The nebula seems to have been discovered by Cysat, a Swiss astronomer, in the year 1618, and it was sketched by Huygens in 1656. It has been called the “fish-mouth” nebula, from the fancied resemblance of the centre portion to the mouth of a fish. A number of small stars are visible over the surface of the nebula, and at one time Lord Rosse thought it showed indications of resolution into stars when examined with his giant telescope; but this is now known to have been a mistake, for Dr. Huggins finds, with the spectroscope, that it consists of nothing but glowing gas.

The brightest line in the nebular spectrum—the “chief nebular line,” as it is called—has not yet been identified with that of any terrestrial substance.

Mr. W. H. Pickering and Dr. Max Wolf have photographed another nebula surrounding the star Zeta Orionis—the southern star of the “Belt,” which seems to be connected with the nebula in the “Sword”; and Professor Barnard, using the “lens of a cheap oil lantern” of 1½ inches aperture and 3½ inches focal length, has photographed “an enormous curved nebulosity” stretching over nearly the whole of the constellation of Orion, and involving the “great nebula.”

Professor Keeler found, with the spectroscope, that the Orion nebula is apparently receding from the earth at the rate of nearly eleven miles a second, but this motion may be, in part at least, due to the sun’s motion in space in the opposite direction. Professor Pickering considers that the parallax of the nebula is probably not more than 0.″003, which corresponds to a thousand years’ journey for light!

In the southern constellation, Argo is a magnificent nebula, somewhat similar in appearance to the great nebula in Orion. It surrounds the famous variable star Eta Argûs. It is sometimes spoken of as the “keyhole” nebula, owing to a curious opening of that shape near its centre. It was carefully drawn by Sir John Herschel at the Cape of Good Hope in the years 1834-38. It lies in a very brilliant portion of the Milky Way, and Sir John Herschel thus describes it: “It is not easy for language to convey a full impression of the beauty and sublimity of the spectacle which the nebula offers as it enters the field of view of a telescope, fixed in right ascension, by the diurnal motion, ushered in as it is by so glorious and innumerable a procession of stars, to which it forms a sort of climax, and in a part of the heavens otherwise full of interest,” and he adds: “In no part of its extent does this nebula show any appearance of resolvability into stars, being, in this respect, analogous to the nebula of Orion. It has, therefore, nothing in common with the Milky Way, on the ground of which we see it projected, and may therefore be, and not improbably is, placed at an immeasurable distance behind that stratum.” Sir John Herschel’s conclusion as to its physical constitution has been fully confirmed by the spectroscope, which shows it to consist of luminous gas. As in the Orion nebula, there are numerous stars scattered over it. Some of these may possibly have a physical connection with the nebula, while others may belong to the Milky Way. The nebula is of great extent, covering an apparent space about five times the area of the full moon, and its real dimensions must be enormous. It was photographed by Mr. Russell, director of the Sydney Observatory, in July, 1890, and the photograph shows that “one of the brightest and most conspicuous parts of the nebula”—the swan-shaped form near the centre of Herschel’s drawing—has “wholly disappeared,” and its place is now occupied by “a great, dark oval.” Mr. Russell first missed the vanished portion of the nebula in the year 1871, while examining it with a telescope of 11½ inches aperture, and the photograph now confirms the disappearance, which is very remarkable, and shows that changes are actually in progress in these wonderful nebulæ, changes which may be detected after a comparatively short interval of time.

Smaller than the nebula in Argo, but somewhat similar in general appearance, is that known as 30 Doradus, which forms one of the numerous and diverse objects which together constitute the greater Magellanic Cloud. Sir John Herschel drew it carefully at the Cape of Good Hope, and describes it as “one of the most singular and extraordinary objects which the heavens present,” and he says “it is unique even in the system to which it belongs, there being no other object in either nubecula to which it bears the least resemblance.” It is sometimes called the “looped nebula,” from the curious openings it contains. One of these is somewhat similar to the “key-hole” opening in the Argo nebula. Near its centre is a small cluster of stars, and scattered over the nebula are many faint stars, of which Sir John Herschel gives a catalogue of 105, ranging from the ninth to the seventeenth magnitude. I do not know whether this nebula has been examined with the spectroscope, but its appearance would suggest that it is gaseous. It is remarkable as being the only object of its class which is found outside the zone of the Milky Way.

Among the nebulæ of irregular shape, although its spectrum is said to be not gaseous, may be mentioned that known as the “trifid nebula,” or 20 Messier. It lies closely north of the star 4 Sagittarii in a magnificent region of the heavens. In the drawing made by Sir John Herschel at the Cape of Good Hope, the principal portion consists of three masses of nebulous matter separated by dark “lanes” or “rifts.” Near the junction of the three “rifts” is a triple star. A beautiful drawing of this nebula has also been made by Trouvelot. It agrees fairly well with that of Sir John Herschel, but shows more detail.

Among other gaseous nebulæ may be mentioned that called by Sir John Herschel the “dumb-bell” nebula. It lies a little south of the sixth magnitude star 14 Vulpeculæ, and was discovered by Messier in 1779, while observing Bode’s comet of that year. In small telescopes it has the appearance of a dumb-bell, or hour-glass, but in larger telescopes the outline is filled in with fainter nebulous light, giving to the whole an elliptical form. Several faint stars have been seen in it, but these probably belong to the Milky Way, as Dr. Huggins finds the spectrum gaseous. Dr. Roberts has photographed it, and he thinks that “the nebula is probably a globular mass of nebular matter which is undergoing the process of condensation into stars, and the faint protrusions of nebulosity in the _south following_ and _north preceding_ ends are the projections of a broad ring of nebulosity which surrounds the globular mass. This ring, not being sufficiently dense to obscure the light of the central region of the globular mass, is dense enough to obscure those parts of it that are hidden by the increased thickness of the nebulosity, thus producing the ‘dumb-bell’ appearance. If these inferences are true, we may proceed yet a step, or a series of steps, further, and predict that the consummation of the life-history of this nebula will be its reduction to a globular cluster of stars.”

Among the gaseous nebulæ may also be included those known as “annular nebulæ.” These are very rare objects, only a few being known in the whole heavens. The most remarkable is that known as 57 Messier, which lies between the stars Beta and Gamma Lyræ, south of the bright star Vega. It was discovered by Darquier, at Toulouse, in 1779, while following Bode’s comet of that year. Lord Rosse thought it resolvable into stars, and so did Chacornac and Secchi, but no stars are perceptible with the great American telescopes, and Dr. Huggins finds it to be gaseous. The central portion is not absolutely dark, but contains some faint nebulous light. Examined with the great telescope of the Lick Observatory, Professor Barnard finds that the opening of the ring is filled in with fainter light “about midway in brightness between the brightness of the ring and the darkness of the adjacent sky. The aperture was more nearly circular than the outer boundary of the nebula, so that the ends of the ring were thicker than the sides.” The entire nebula was of a milky color. A central star, noticed by some observers, was usually seen by Professor Barnard, but was never a conspicuous object. He found the extreme dimensions of the nebula about 81″ in length by about 59″ in width, or more than double the apparent area of Jupiter’s disk. It has been beautifully photographed by Dr. Roberts, and he says “the photograph shows the nebula and the interior of the ring more elliptical than the drawings and descriptions indicate; and the star of the _following_ side is nearer to the ring than the distance given. The nebulosity on the _preceding_ and _following_ ends of the ring protrudes a little, and is less dense than on the _north_ and _south_ sides. This probably suggested the filamentous appearance which Lord Rosse shows. Some photographs of the nebula have been taken between 1887 and 1891, and the central star is strongly shown on some of them, but on others it is scarcely visible, which points to the star being variable.” On a photograph taken by MM. Androyer and Montaugerand of the Toulouse Observatory, with an exposure of nine hours (in multiple exposures), about 4,800 stars are visible on and near the nebula in an area of three square degrees.

Another object of the annular class will be found a little to the southwest of the star Lambda Scorpii. It is thus described by Sir John Herschel: “A delicate, extremely faint, but perfectly well defined, annulus. The field crowded with stars, two of which are on the nebula. A beautiful, delicate ring of a faint, ghost-like appearance, about 40″ in diameter in a field of about 150 stars, eleven and twelve magnitude and under.”

Near the stars 44 and 51 Ophiuchi is another object of the annular class, which Sir John Herschel describes as “exactly round, pretty faint, 12″ diameter, well terminated, but a little cottony at the edge, and with a decided darkness in the middle, equal to a tenth magnitude star at the most. Few stars in the field, a beautiful specimen of the planetary annular class of nebula.”

The Planetary Nebulæ form an interesting class. They were so named by Sir William Herschel from their resemblance to the disks of the planets, but, of course, much fainter. They are generally of uniform brightness, without any nucleus or brighter part in the centre. There are numerous examples of this class, one of the most remarkable being that known as 97 Messier, which is situated about two degrees southeast of Beta Ursæ Majoris—the southern of the two “pointers” in the Plow. It is of considerable apparent size, and even supposing its distance to be not greater than that of 61 Cygni, its real dimensions must be enormous. Lord Rosse observed two openings in the centre with a star in each opening, and from this appearance he called it the “owl nebula.” One of the stars seems to have disappeared since 1850, and a photograph recently taken by Dr. Roberts confirms the disappearance.

Another fine object of the planetary class is one which lies close to the pole of the ecliptic. Webb saw it “like a considerable star out of focus.” Smyth found it pale blue in color. Dr. Huggins finds a gaseous spectrum, the first discovery of the kind made. Professor Holden, observing it with the great Lick telescope, finds its structure extraordinary. He says it “is apparently composed of rings overlying each other, and it is difficult to resist the conviction that these are arranged in space in the form of a true helix,” and he ranks it in a new class which he calls “helical nebulæ.”

A somewhat similar nebula lies a little to the west of the star Nu Aquarii. Secchi believed it to be in reality a cluster of small stars, but Dr. Huggins finds its spectrum gaseous. A small nebula on each side gives it an appearance somewhat similar to the planet Saturn, with the rings seen edgewise. The great Lick telescope shows it as a wonderful object—“a central ring lies upon an oval of much fainter nebulosity.” Professor Holden says “the color is a pale blue,” and he compares the appearance of the central ring “to that of a footprint left in the wet sand on a sea beach.”

About two degrees south of the star Mu Hydræ is another planetary nebula, which Smyth describes as resembling the planet Jupiter in “size, equable light and color.” Webb saw it of “a steady, pale blue light,” and Sir John Herschel, at the Cape of Good Hope, speaks of its color as “a decided blue—at all events, a good sky-blue,” a color which seems characteristic of these curious objects. Although Sir William Herschel, with his large telescopes, failed to resolve it into stars, Secchi thought he saw it breaking up into stars with a “sparkling ring.” Dr. Huggins, however, finds the spectrum to be gaseous, so that the luminous points seen by Secchi could not have been stellar.

Sir John Herschel, in his _Cape Observations_, describes a planetary nebula which lies between the stars Pi Centauri and Delta Crucis. He says it is “perfectly round, very planetary, color fine blue ... very like Uranus, only about half as large again, and blue.... It is of the most decided independent blue color when in the field by itself, and with no lamplight and no bright star. About 10′ north of it is an orange-colored star, eighth magnitude. When this is brought into view, the blue color of the nebula becomes intense ... color, a beautiful rich blue, between Prussian blue and verditer green.”

There are some rare objects called “nebulous stars.” The star Epsilon Orionis—the centre star of Orion’s Belt—is involved in a great nebulous atmosphere. The triple star Iota Orionis is surrounded by a nebulous haze. The star Beta in Canes Venatici is a 4½ magnitude star surrounded by a nebulous atmosphere.

The term elliptical nebulæ has been applied to those of an elliptical or elongated shape. This form is probably due in many cases to the effect of perspective, their real shape being circular, or nearly so. Perhaps the most remarkable object of this class is the well-known “nebula in Andromeda,” known to astronomers as 31 Messier. It can be just seen with the naked eye, on a clear moonless night, as a hazy spot of light near the star Nu Andromedæ, and it is curious that it is not mentioned by the ancients, although it must have been very visible to their keen eyesight in the clear Eastern skies. It was, however, certainly seen so far back as 905 A. D., and it is referred to as a familiar object by the Persian astronomer, Al-Sûfi, who wrote a description of the heavens about the middle of the Tenth Century. Tycho Brahe and Bayer failed to notice it, but Simon Marius saw it in December, 1612, and described it “as a light seen from a great distance through half-transparent horn plates.” It was also observed by Bullialdus, in 1664, while following the comet of that year. It has frequently been mistaken for a comet by amateur observers in recent years. Closely northwest of the great nebula is a smaller one discovered by Le Gentil in 1749, and another to the south, detected by Miss Caroline Herschel in 1783. The great nebula is of an elliptical shape and considerable apparent size. The American astronomer, Bond, using a telescope of 15 inches aperture, traced it to a length of about four degrees, and a width of two and a half degrees. A beautiful photograph taken by Dr. Roberts in December, 1888, shows an extension of nearly two degrees in length, and about half a degree in width, or considerably larger than the apparent size of the full moon. Bond could not see any symptom of resolution into stars, but noticed two dark rifts or channels running nearly parallel to the length of the nebula. In Dr. Roberts’s photograph these rifts are seen to be really dark intervals between consecutive nebulous rings into which the nebula is divided. Dr. Roberts says: “A photograph which I took with the 20-inch reflector on October 10, 1887, revealed for the first time the true character of the great nebula, and one of the features exhibited was that the dark bands, referred to by Bond, formed parts of divisions between symmetrical rings of nebulous matter surrounding the large diffuse centre of the nebula. Other photographs were taken in 1887, November 15; 1888, October 1; 1888, October 2; 1888, December 29; besides several others taken since, upon all of which the rings of nebulosity are identically shown, and thus the photographs confirm the accuracy of each other, and the objective reality of the details shown of the structure of the nebula.” Dr. Roberts adds: “These photographs throw a strong light on the probable truth of the _Nebular Hypothesis_, for they show what appears to be the progressive evolution of a gigantic stellar system.”

The largest telescopes have hitherto completely failed to resolve this wonderful object into stars. Dr. Huggins, however, finds that the spectrum is _not_ gaseous, so that if the nebula really consists of stellar points, they must be of very small dimensions.

The question may be asked, What is the probable size and distance of this wonderful nebula? and could it be an external universe?

The temporary star which appeared near the nucleus of the nebula in August, 1885, was of the seventh magnitude. I find that our sun, if placed at the distance indicated by a parallax of 1/200th of a second, would be reduced to a star of about the eleventh magnitude, or four magnitudes fainter than the temporary star appeared to us. That is to say, the star would have been—with the assumed distance—about forty times brighter than the sun. With any greater distance, the star would have been proportionately brighter, compared with the sun. This seems improbable, and tends to the conclusion that the nebula is _not_ an external galaxy, but a member of our own sidereal system, a system which probably includes all the stars and nebulæ visible in our largest telescopes. Dr. Common, indeed, suggests that it may be comparatively near our system. He says: “It is difficult to imagine that such an enormous object, as the Andromeda nebula must be, is not very near to us; perhaps it may be found to be the nearest celestial object of all beyond the Solar System. It is one that offers the best chance of the detection of parallax, as it seems to be projected on a crowd of stars, and there are well-defined points that might be taken as fiducial points for measurement,” and he adds: “Apart from the great promise this nebula seems to give of determining parallax, there is a fair presumption that in the course of time the rotation of the outer portion may perhaps be detected by observation of the positions of the two outer detached portions in relation to the neighboring stars.”

The spiral nebulæ are wonderful objects, and were discovered by the late Lord Rosse with his great six-foot telescope. Their character has been fully confirmed by photographs taken by Dr. Roberts. One of the most remarkable of these extraordinary objects is that known as 51 Messier. It lies about three degrees southwest of the bright star Eta Ursæ Majoris—the star at the end of the Great Bear’s tail. It was discovered by Messier while comet-hunting on October 13, 1773. Telescopes of moderate power merely show two nebulæ nearly in contact, but Lord Rosse saw it as a wonderful spiral, and his drawing agrees fairly well with a photograph taken by Dr. Roberts in April, 1889. The nebula has also been photographed by Dr. Common. Dr. Roberts says: “The photograph shows both nuclei of the nebula to be stellar, surrounded by dense nebulosity, and the convolutions of the spiral in this as in other spiral nebulæ are broken up into star-like condensations with nebulosity around them. Those stars that do not conform to the trends of the spiral have nebulous trails attached to them, and seem as if they had broken away from the spirals.” A tendency to a spiral structure in the smaller nebula is also visible on the original negative. Dr. Huggins finds that the spectrum is _not_ gaseous.

The nebula known as 99 Messier is of the spiral form. It lies on the borders of Virgo and Coma Berenices, near the star 6 Comæ. In large telescopes it somewhat resembles a “Catherine wheel.” D’Arrest and Key thought it resolvable into stars. It has been photographed by M. Von Gothard.

Among the clusters and nebulæ, we may class the Magellanic Clouds, or Nubeculæ in the Southern Hemisphere, as they consist of stars, clusters, and nebulæ.

Among the so-called nebulæ are many objects which, when examined with telescopes of adequate power, are seen to be resolved into myriads of small stars; their comparative isolation from surrounding objects impresses us forcibly with the idea that they form, as it were, families of stars connected by some physical bond of union. Of these clusters, as they are called, we have naked-eye examples in the Pleiades and the “Bee-Hive” in Cancer. Others may be partially seen with a good opera-glass or binocular, but most of them require telescopes of considerable power to view them to advantage. They are of various forms and of all degrees of condensation. Some are comparatively large and irregular, others small and compressed, with the component stars densely crowded. Many are of such uniform shape as to have received the name of globular clusters. These have been aptly termed “balls of stars,” and are among the most interesting objects in the stellar heavens.

The most remarkable object of this class visible in the Northern Hemisphere is that known as 13 Messier. It lies between the tolerably bright stars Zeta and Eta Herculis, nearer the latter star. It may be seen with an opera-glass as a hazy-looking star of about the sixth magnitude, with a star on each side of it. Examined with a powerful telescope, it is resolved into numerous small stars. Sir William Herschel estimated them at 14,000, but the real number is probably much less. Assuming the average magnitude of the components at twelve and a half, I find that an aggregation of 14,000 stars of this brightness would shine as a star of about the second magnitude, or a little fainter.

Another object of the globular class, but less resolvable, is that known as 92 Messier, which lies between the stars Eta and Iota in Hercules, nearer the latter. Sir William Herschel’s telescopes showed it as seven or eight minutes of arc in diameter. It is considerably brighter at the centre. The larger components are easily visible in moderate-sized telescopes, but even Lord Rosse’s giant instrument failed to resolve the central blaze. There is no doubt, however, that it consists wholly of small stars, as the unerring eye of the spectroscope shows a stellar spectrum, similar to that of the neighboring 13 Messier.