Chapter XV: The Great Nebula in Orion (7)

Seen with the naked eye, the Galaxy appears as an irregular, narrow, nebulous belt, apparently composed of cloud-like luminous masses of different forms and sizes, separated by comparatively dark intervals. These cloud-like masses vary much in luminous intensity, and while some among them are very bright and conspicuous, others are so faint that they are hard to recognize. In general, the brightest parts of the Milky-way are situated along the middle of its belt, while its borders, which are usually very faint, gradually vanish in the sky. Some parts of the Galaxy, however, show very little of the cloudy structure so characteristic of other parts, being almost uniform throughout, except towards the borders, which are always fainter. These parts showing greater uniformity are also the faintest.

Such is the general appearance of the Milky-way on ordinary nights, but on rare occasions, when the atmosphere is particularly pure, it presents one of the grandest sights that can be imagined. At such favorable moments I have seen the Galaxy gleaming with light, and appearing as if composed of star-dust or of precious stones. The strange belt then appeared all mottled over and fleecy, its large cloud-like masses being subdivided into numerous small, irregular cloudlets of great brilliancy, which appeared projected upon a soft luminous background.

The width of the Galaxy is far from being uniform; while in some places it is only 4° or 5°, in others it is 15° and even more. In some places it appears wavy in outline, at others quite straight; then it contracts, to expand a few degrees distant; while at other places it sends off branches and loops, varying in form, size and direction, some of which are quite prominent, while others are very faint.

Although very irregular in form, the general appearance of the galactic belt is that of a regular curve occupying one of the great circles of the celestial sphere. The Milky-way completely encircles the heavens, but, of course, only one-half is visible at any one moment, since our globe prevents the other half from being seen. If, for a moment, we imagine ourselves left in space, our globe having vanished from under our feet, we should then see the whole Galaxy forming a continuous belt in the heavens, at the centre of which we should apparently be situated.

While only one-half of the galactic belt can be seen at once from any point on the Earth, yet, according to the position of the observer, a larger or smaller portion of the whole can be seen at different times. In high northern or southern latitudes but little more than half can be seen even by continuous observations; but as we approach the equatorial regions, more and more of it becomes visible, until the whole may be seen at different hours and seasons. In the latitudes of the northern states, about two-thirds of the Galaxy is visible, the rest remaining hidden below the horizon; but from the southern states very nearly the whole can be seen. The half of the Milky-way visible at any one time from any latitude on the Earth never entirely sets below the horizon, although in some places it may be so near the horizon as to be rendered invisible by vapors. In the latitude of Cambridge, when in its lowest position, the summit of its arc is still about 12° or 15° above the northern horizon. The great circle of the celestial sphere, occupied by the galactic belt, is inclined at an angle of about 63° to the celestial equator, and intersects this great circle on one side in the constellation Monoceros in 6h. 47m., and on the opposite side in the constellations Aquila and Ophiuchus in 18h. 47m. of right ascension; so that its northern pole is situated in the constellation Coma Berenices in R. A. 12h. 47m., declination N. 27°, and the southern in the constellation Cetus in R. A. 0h. 47m., declination S. 27°.

According to the seasons and to the hours of the night at which it is observed, the galactic arch presents different inclinations in the sky. Owing to its inclination to the equator of the celestial sphere, its opposite parts exhibit opposite inclinations when they pass the meridian of a place. That part of the Galaxy which is represented on Plate XIII., and which intersects the celestial equator in the constellation Aquila, is inclined to the left or towards the east, when it is on the meridian; while the opposite part, situated in Monoceros, is inclined to the right, or towards the west, when it reaches the meridian. The former passes the meridian in the evening in the summer and autumn months; the latter, in the winter and spring months.

[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 180° 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.

Although lacking the optical resources which now enable us to recognize the structure of the Milky-way, some of the ancient philosophers had succeeded tolerably well in their speculations regarding its nature. It was the opinion of Democritus, Pythagoras and Manilius, that the Galaxy was nothing else but a vast and confused assemblage of stars, whose faint light was the true cause of its milky appearance.

Before the invention of the telescope, no well-founded theory in regard to the structure of the Milky-way could, of course, be attempted. Although Kepler entertained different ideas in regard to the structure of this great belt from those now generally admitted, yet in them may be found the starting point of the modern conception of the structure of the Galaxy and of the visible universe. In the view of this great mind, the Milky-way, with all its stars, formed a vast system, the centre of which, and of the universe, was occupied by our Sun. Kepler reasoned that the place of the Sun must be near the centre of the galactic belt, from the fact this last object appears very nearly as a great circle of the celestial sphere, and that its luminous intensity is about the same in all its parts.

Half a century later, another attempt to explain the Milky-way was made by Wright, of Durham, who rejected the idea of an accidental and confused distribution of the stars as inconsistent with the appearance of the Galaxy, and regarded them as arranged along a fundamental plane corresponding to that of the Milky-way. These ideas which were subsequently developed and enlarged by Kant, and then by Lambert, constitute what is now known as Kant's theory. According to this theory, the stars composing the Galaxy are conceived as being uniformly arranged between two flat planes of considerable extension, but which are comparatively near together, the Sun occupying a place not very far from the centre of this immense starry stratum. As we view this system crosswise through its thinnest parts, the stars composing it appear scattered and comparatively few in number, but when we view it lengthwiser through its most extended parts, they appear condensed and extremely numerous, thus giving the impression of a luminous belt encircling the heavens. In the conception of Kant, each star was a sun, forming the centre of a planetary system. These systems are not independent, but are kept together by the bonds of universal gravitation. The Galaxy itself is one of these great systems, its principal plane being the equivalent of the zodiac in our planetary system, while a preponderant body, which might be Sirius, is the equivalent of our Sun, and keeps the galactic system together. In the universe there are other galaxies, but as they are too distant to be resolved into stars, they appear as elliptical nebulæ. Such are, in brief, the grand speculations of Kant and Lambert on the Milky-way, and the structure of the universe.

Kant's theory rested more on conjectures than on observed facts, and needed therefore the sanction of direct observations to be established on a firm basis. With this view, Sir William Herschel investigated the subject, by a long and laborious series of observations. His plan, which was that of "star-gauging," consisted in counting all the stars visible in his twenty-foot telescope, comprised in a wide belt cutting the Galaxy at right angles, and extending from one of its sides to the opposite one, thus embracing 180° of the celestial sphere. In this belt he executed 3,400 telescopic star-gaugings of a quarter of a degree each, from which he obtained 683 mean gaugings giving the stellar density of the corresponding regions.

The general result derived from this immense labor was that the stars are fewest in regions the most distant from the galactic belt; while from these regions, which correspond to the pole of the Galaxy, they gradually increase in number in approaching the Milky-way. The star density was found to be extremely variable, and while some of the telescopic gaugings detected either no star at all, or only one or two, other gaugings gave 500 stars and even more. The average number of stars in a field of view of his telescope, obtained for the six zones, each of 15°, into which Herschel divided up the portion of his observing belt, extending from the Galaxy to its pole, is as follows: In the first zone, commencing at 90° from the galactic belt and extending towards it, 4 stars per telescopic field were found; 5 in the second; 8 in the third; 14 in the fourth; 24 in the fifth and 53 in the sixth, which terminated in the Galaxy itself. Very nearly similar results were afterwards found by Sir John Herschel, for corresponding regions in the southern hemisphere.

From these studies, Herschel concluded that the stellar system is of the general form supposed by the Kantian theory, and that its diameter must be five times as extended in the direction of the galactic plane, as it is in a direction perpendicular to it. To explain the great branch sent out by the Galaxy in Cygnus, he supposed a great cleft dividing the system edgewise, about half way from its circumference to its centre. From suppositions founded on the apparent magnitude and arrangement of stars, he estimated that it would take light about 7,000 years to reach us from the extremities of the Galaxy, and therefore 14,000 years to travel across the system, from one border to the opposite one.

But Herschel's theory concerning the Milky-way rested on the erroneous assumption that the stars are uniformly distributed in space, and also that his telescopes penetrated through the entire depth of the Galaxy. Further study showed him that his telescope of twenty feet, and even his great forty-foot telescope, which was estimated to penetrate to a distance 2,300 times that of stars of the first magnitude, failed to resolve some parts of the Galaxy into stars. Meanwhile, the structure of the Milky-way being better known, the irregular condensation of its stars became apparent, while the mutual relation existing between binary and multiple systems of stars, as also between the stars which form clusters, was recognized, as showing evidence of closer association between certain groups of stars than between the stars in general. Herschel's system, which rested on the assumption of the uniform distribution of the stars in space, and on the supposition that the telescopes used for his gauges penetrated through the greater depths of the Galaxy, being thus found to contradict the facts, was gradually abandoned by its author, who adopted another method of estimating the relative distances of the stars observed in his gaugings.

This method, founded on photometric principles, consisted in judging the penetrating power of his telescope by the brightness of the stars, and not, as formerly, by the number which they brought into view. He then studied by this new method the structure of the Milky-way and the probable distance of the clustering masses of which it is formed, concluding that the portion of the Galaxy traversing the constellation Orion is the nearest to us. This last result seems indicated by the fact that this portion of the Milky-way is the faintest and the most uniform of all the galactic belt.

More recently Otto Struve investigated the same subject, and arrived at very nearly similar conclusions, which may be briefly stated as follows: The galactic system is composed of a countless number of stars, spreading out on all sides along a very extended plane. These stars, which are very unevenly distributed, show a decided tendency to cluster together into individual groups of different sizes and forms, separated by comparatively vacant spaces. This layer where the stars congregate in such vast numbers may be conceived as a very irregular flat disk, sending many branches in various directions, and having a diameter eight or ten times its thickness. The size of this starry disk cannot be determined, since it is unfathomable in some directions, even when examined with the largest telescopes. The Sun, with its attending planets, is involved in this immense congregation of suns, of which it forms but a small particle, occupying a position at some distance from the principal plane of the Galaxy. According to Struve, this distance is approximately equal to 208,000 times the radius of the Earth's orbit. The Milky-way is mainly composed of star-clusters, two-thirds, perhaps, of the whole number visible in the heavens being involved in this great belt. In conclusion, our Sun is only one of the individual stars which constitute the galactic system, and each of these stars itself is a sun similar to our Sun. These individual suns are not independent, but are associated in groups varying in number from a few to several thousands, the Galaxy itself being nothing but an immense aggregation of such clusters, whose whole number of individual suns probably ranges between thirty and fifty millions. In this vast system our globe is so insignificant that it cannot even be regarded as one of its members. According to Dr. Gould, there are reasons to believe that our Sun is a member of a small, flattened, bifid cluster, composed of more than 400 stars, ranging between the first and seventh magnitude, its position in this small system being eccentric, but not very far from the galactic plane.

The study of the Milky-way, of which Plate XIII. is only a part, was undertaken to answer a friendly appeal made by Mr. A. Marth, in the Monthly Notices of the Royal Astronomical Society, in 1872. I take pleasure in offering him my thanks for the suggestion, and for the facility afforded me in this study by his "List of Co-ordinates of Stars within and near the Milky-way," which was published with it.

THE STAR-CLUSTERS

PLATE XIV

It is a well-known fact that the stars visible to the naked eye are very unequally distributed in the heavens, and that while they are loosely scattered in some regions, in others they are comparatively numerous, sometimes forming groups in which they appear quite close together.

In our northern sky are found a few such agglomerations of stars, which are familiar objects to all observers of celestial objects. In the constellation Coma Berenices, the stars are small, but quite condensed, and form a loosely scattered, faint group. In Taurus, the Hyades and the Pleiades, visible during our winter nights, are conspicuous and familiar objects which cannot fail to be recognized. In the last group, six stars may be easily detected by ordinary eyes on any clear night, but more can sometimes be seen; on rare occasions, when the sky was especially favorable, I have detected eleven clearly and suspected several others. The six stars ordinarily visible, are in order of decreasing brightness, as follows: Alcyone, Electra, Atlas, Maia, Taygeta and Merope. Glimpses of Celano and Pleione are sometimes obtained.

When the sky is examined with some attention on any clear, moonless night, small, hazy, luminous patches, having a cometary aspect, are visible here and there to the naked eye. In the constellation Cancer is found one of the most conspicuous, called Præsepe, which forms a small triangle with the two stars _γ_ and _δ_. In Perseus, and involved in the Milky-way, is found another luminous cloud, situated in the sword-handle, and almost in a line with the two stars _γ_ and _δ_ of Cassiopeia's Chair. In the constellation Hercules, another nebulous mass of light, but fainter, is also visible between the stars _η_ and _ζ_ where it appears as a faint comet, in the depths of space. In Ophiuchus and Monoceros are likewise found hazy, luminous patches. In the southern sky, several such objects are also visible to the naked eye, being found in Sagittarius, in Canis Major and in Puppis; but the most conspicuous are those in Centaurus and Toucan. That in Centaurus involves the star _ω_ in its pale diffused nebulosity, and that in Toucan is involved in the lesser Magellanic cloud.

When the telescope is directed to these nebulous objects, their hazy, ill-defined aspect disappears, and they are found to consist of individual stars of different magnitudes, which being more or less closely grouped together, apparently form a system of their own. These groups, which are so well adapted to give us an insight into the structure and the vastness of the stellar universe, are called _Star-clusters_.

Star-clusters are found of all degrees of aggregation, and while in some of them, such as in the Pleiades, in Præsepe and in Perseus, the stars are so loosely scattered that an opera glass, and even the naked eye, will resolve them; in others, such as in those situated in Hercules, Aquarius, Toucan and Centaurus, they are so greatly compressed that even in the largest telescopes they appear as a confused mass of blazing dust, in which comparatively few individual stars can be distinctly recognized. Although only about a dozen Star-clusters can be seen in the sky with the naked eye, yet nearly eleven hundred such objects visible through the telescope, have been catalogued by astronomers.

The stars composing the different clusters visible in the heavens vary greatly in number, and while in some clusters there are only a few, in others they are so numerous and crowded that it would be idle to try to count them, their number amounting to several thousands. It has been calculated by Herschel that some clusters are so closely condensed, that in an area not more than ⅒ part of that covered by the Moon, at least 5,000 stars are agglomerated.

When the group in the Pleiades is seen through the telescope it appears more important than it does to the naked eye, and several hundreds of stars are found in it. In a study of Tempel's nebula, which is involved in the Pleiades, I have mapped out 250 stars, mostly comprised within this nebula, with the telescope of 6⅓ inches aperture, which I have used for this study.

As a type of a loose, coarse cluster, that in Perseus is one of the finest of its class. It appears to the naked eye as a single object, but in the telescope it has two centres of condensation, around which cluster a great number of bright stars, forming various curves and festoons of great beauty. Among its components are found several yellow and red stars, which give a most beautiful contrast of colors in this gorgeous and sparkling region. In a study which I have made of this twin cluster, I have mapped out 664 stars belonging to it, among which are two yellow and five red stars.

While some clusters, like those just described, are very easily resolvable into stars with the smallest instruments, others yield with the greatest difficulty, even to the largest telescopes, in which their starry nature is barely suspected. Owing to this peculiarity, star-clusters are usually divided into two principal classes. In the first class are comprised all the clusters which have been plainly resolved into stars, and in the second all those which, although not plainly resolvable with the largest instruments now at our disposal, show a decided tendency to resolvability, and convey the impression that an increase of power in telescopes is the only thing needed to resolve them into stars. Of course this classification, which depends on the power of telescopes to decide the nature of these objects, is arbitrary, and a classification based on spectrum analysis is now substituted for it.

The star-clusters are also divided into globular and irregular clusters, according to their general form and appearance. The globular clusters, which are the most numerous, are usually well-defined objects, more or less circular in their general outlines. The rapid increase of brightness towards their centres, where the stars composing them are greatly condensed, readily conveys the impression that the general form of these sparkling masses is globular. The irregular clusters are not so rich in stars as the former. Usually their stars are less condensed towards the centre, and are, for the most part, so loosely and irregularly distributed, that it is impossible to recognize the outlines of these clusters or to decide where they terminate. The globular clusters are usually quite easily resolvable into stars, either partly or wholly, although some among them do not show the least traces of resolvability, even in the largest instruments. This may result from different causes, and may be attributed either to the minuteness of their components or to their great distance from the Earth, many star-clusters being at such immense distances that they are beyond our means of measurement.

As has been shown in the preceding section, the star-clusters are found in great number in the Galaxy; indeed, it is in this region and in its vicinity that the greater portion of them are found. In other regions, with the exception of the Magellanic clouds, where they are found in great number and in every stage of resolution, the clusters are few and scattered.

[PLATE XIV.--STAR-CLUSTER IN HERCULES.

From a study made in June, 1877]

The star-cluster in the constellation Hercules, designated as No. 4,230 in Sir J. Herschel's catalogue, and which is represented on Plate XIV., is one of the brightest and most condensed in the northern hemisphere, although it is not so extended as several others, its angular diameter being only 7' or 8'. This object, which was discovered by Halley in 1714, is one of the most beautiful of its class in the heavens. According to Herschel, it is composed of thousands of stars between the tenth and fifteenth magnitudes. Undoubtedly the stars composing this group are very numerous, although those which can be distinctly seen as individual stars, and whose position can be determined, are not so many as a superficial look at the object would lead us to suppose. From a long study of this cluster, which I have made with instruments of various apertures, I have not been able to identify more stars than are represented on the plate, although the nebulosity of which this object mainly consists, and especially the region situated towards its centre, appeared at times granular and blazing with countless points of light, too faint and too flickering to be individually recognized. Towards its centre there is quite an extended region, whose luminous intensity is very great, and which irresistibly conveys the impression of the globular structure of this cluster. Besides several outlying appendages, formed by its nebulosity, the larger stars recognized in this cluster are scattered and distributed in such a way that they form various branches, corresponding with those formed by the irresolvable nebulosity. At least six or seven of these branches and wings are recognized, some of which are curved and bent in various ways, thus giving this object a distant resemblance to some crustacean forms. Although I have looked for it with care, I have failed to recognize the spiral structure attributed to this object by several observers. Among the six appendages which I have recognized, some are slightly curved; but their curves are sometimes in opposite directions, and two branches of the upper portion make so short a bend that they resemble a claw rather than a spiral wing. The spectrum of this cluster, like that of many objects of its class, is continuous, with the red end deficient.

A little to the north-east of this object is found the cluster No. 4,294, which, although smaller and less bright than the preceding, is still quite interesting. It appears as a distinctly globular cluster without wings, and much condensed towards its centre. The stars individually recognized in it, although less bright than those of the other cluster, are so very curiously distributed in curved lines that they give a peculiar appearance to this condensed region.

A little to the north of _γ_ Centauri may be found the great _ω_ Centauri cluster, No. 3,531, already referred to above. This magnificent object, which appears as a blazing globe 20' in diameter, is, according to Herschel, the richest in the sky, and is resolved into a countless number of stars from the twelfth to the fifteenth magnitude, which are greatly compressed towards the centre. The larger stars are so arranged as to form a sort of net-work, with two dark spaces in the middle.

The great globular cluster No. 52, involved in the lesser Magellanic cloud, in the constellation Toucan, is a beautiful and remarkable object. It is composed of three distinct, concentric layers of stars, varying in brightness and in degree of condensation in each layer. The central mass, which is the largest and most brilliant, is composed of an immense number of stars greatly compressed, whose reddish color gives to this blazing circle a splendid appearance. Around the sparkling centre is a broad circle, composed of less compressed stars, this circle being itself involved in another circular layer, where the stars are fainter and more scattered and gradually fade away.

Many other great globular clusters are found in various parts of the heavens, among which may be mentioned the cluster No. 4,678, in Aquarius. This object is composed of several thousand stars of the fifteenth magnitude, greatly condensed towards the centre, and, as remarked by Sir J. Herschel, since the brightness of this cluster does not exceed that of a star of the sixth magnitude, it follows that in this case several thousand stars of the fifteenth magnitude equal only a star of the sixth magnitude. In the constellation Serpens the globular clusters No. 4,083 and No. 4,118 are both conspicuous objects, also No. 4,687 in Capricornus. In Scutum Sobieskii the cluster No. 4,437 is one of the most remarkable of this region. The stars composing it, which are quite large and easily made out separately, form various figures, in which the square predominates.

Among the loose irregular clusters, some are very remarkable for the curious arrangement of their stars. In the constellation Gemini the cluster No. 1,360, which is visible to the naked eye, is a magnificent object seen through the telescope, in which its sparkling stars form curves and festoons of great elegance. The cluster No. 1,467, of the same constellation, is remarkable for its triangular form. In the constellation Ara the cluster No. 4,233, composed of loosely scattered stars, forming various lines and curves, is enclosed on three sides by nearly straight single lines of stars. In Scorpio the cluster No. 4,224 is still more curious, being composed of a continuous ring of loosely scattered stars, inside of which is a round, loose cluster, which is divided into four parts by a dark cross-shaped gap, in which no stars are visible.

Among the 1,034 objects which are now classified as clusters more or less resolvable, 565 have been absolutely resolved into stars, and 469 have been only partly resolved, but are considered as belonging to this class of objects. In Sir J. Herschel's catalogue there are 102 clusters which are Considered as being globular; among them 30 have been positively resolved into stars.

The agglomeration of thousands of stars into a globular cluster cannot be conceived, of course, to be simply the result of chance. This globular form seems clearly to indicate the existence of some bond of union, some general attractive force acting between the different members of these systems, which keeps them together, and condenses them towards the centre. Herschel regards the loose, irregular clusters as systems in a less advanced stage of condensation, but gradually concentrating by their mutual attraction into the globular form. Although the stars of some globular clusters appear very close together, they are not necessarily so, and may be separated by great intervals of space. It has been shown that the clusters are agglomeration of suns, and that our Sun itself is a member of a cluster composed of several hundreds of suns, although, from our point of observation, these do not seem very close together. So far as known, the nearest star to us is a Centauri, but its distance from the Earth equals 221,000 times the distance of the Sun from our globe, a distance which cannot be traversed by light in less than three years and five months. It seems very probable that if the suns composing the globular clusters appear so near together, it is because, in the first place, they are at immense distances from us, and in the second, because they appear nearly in a line with other suns, which are at a still greater distance from us, and on which they accordingly are nearly projected. If one should imagine himself placed at the centre of the cluster in Hercules, for instance, the stars, which from our Earth seems to be so closely grouped, would then quite likely appear very loosely scattered around him in the sky, and would resemble the fixed stars as seen from our terrestrial station.

Judging by their loose and irregular distribution, the easily resolvable clusters would appear, in general, to be the nearer to us. It is probable that the globular clusters do not possess, to a very great degree, the regular form which they ordinarily present to us. It seems rather more natural to infer that they are irregular, and composed of many wings and branches, such as are observed in the cluster in Hercules; but as these appendages would necessarily be much poorer in stars than the central portions, they would be likely to become invisible at a great distance, and therefore the object would appear more or less globular; the globular form being simply given by the close grouping of the stars in the central portion. It would seem, then, that in general, the most loosely scattered and irregular clusters are the nearest to us, while the smallest globular clusters and those resolvable with most difficulty are the most distant.

In accordance with the theory that the clusters are composed of stars, the spectrum of these objects is in general continuous; although, in many cases, the red end of the spectrum is either very faint or altogether wanting. Many objects presenting in a very high degree the principal characteristics exhibited by the true star-clusters, namely, a circular or oval mass, whose luminous intensity is greatly condensed toward the centre, have not yielded, however, to the resolving power of the largest telescopes, although their continuous spectrum is in close agreement with their general resemblance to the star-clusters. Although such objects may remain irresolvable forever, yet it is highly probable that they do not materially differ from the resolvable and partly resolvable clusters, except by their enormous distance from us, which probably reaches the extreme boundary of our visible universe.

THE NEBULÆ

PLATE XV

Besides the foggy, luminous patches which have just been described, a few hazy spots of a different kind are also visible to the naked eye on any clear, moonless night. These objects mainly differ from the former in this particular, that when viewed through the largest telescopes in existence they are not resolved into stars, but still retain the same cloudy appearance which they present to the unassisted eye. On account of the misty and vaporous appearance which they exhibit, these objects have been called _Nebulæ_.

Of the 26 nebulous objects visible to the naked eye in the whole heavens, 19 belong to the class of star-clusters, and 7 to the class of nebulæ. Among the most conspicuous nebulæ visible to the unassisted eye, are those in the constellations Argo Navis, Andromeda and Orion.

Besides the seven nebulæ visible to the naked eye, a great number of similar objects are visible through the telescope. In Sir John Herschel's catalogue of nebulæ and clusters, are found 4,053 irresolvable nebulæ, and with every increase of the aperture of telescopes, new nebulæ, invisible in smaller instruments, are found. Notwithstanding their irresolvability it is probable, however, that many among them have a stellar structure, which their immense distance prevents us from recognizing, and are not therefore true nebulæ. The giant telescope of Lord Rosse has shown nebulæ so remote that it has been estimated that it takes their light 30 million years to reach the Earth.

The nebulæ are very far from being uniformly distributed in space. In some regions they are rare, while in others they are numerous and crowded together, forming many small, irregular groups, differing in size and in richness of aggregation. The grouping of the nebulæ does not occur at random in any part of the heavens, as might naturally be supposed, but, on the contrary, it is chiefly confined to certain regions. Outside of these regions nebulæ are rare and are separated from each other by immense intervals; so that these isolated objects appear as if they were lost wanderers from the great nebulous systems.

The regions where the nebulæ congregate in great number are very extensive, and in a general view there are two vast systems of nebular agglomeration, occupying almost opposite points of the heavens, whose centres are not very distant from the poles of the Milky-way. In the northern hemisphere, the nebulous system is much richer and more condensed than in the southern hemisphere. The northern nebulæ are principally contained in the constellations Ursa Minor and Major, in Draco, Canes Venatici, Bootes, Leo Major and Minor, Coma Berenices, and Virgo. In this region, which occupies about ⅛ of the whole surface of the heavens, ⅓ of the known nebulæ are assembled. The southern nebulæ are more evenly distributed and less numerous, with the exception of two comparatively small, but very remarkable centres of condensation which, together with many star-clusters, constitute the Magellanic clouds.

These two vast nebular groups are by no means regular in outline, and send various branches toward each other. They are separated by a wide and very irregular belt, comparatively free from nebulæ, which encircles the celestial sphere, and whose medial line approximately coincides with that of the galactic belt. The Milky-way, so rich in star-clusters, is very barren in nebulæ; but it is a very remarkable fact, nevertheless, that almost all the brightest, largest, and most complicated nebulæ of the heavens are situated either within it, or in its immediate vicinity. Such are the great nebulæ in Orion and Andromeda; the nebula of _ζ_ Orionis; the Ring nebula in Lyra; the bifurcate nebula in Cygnus; the Dumb-bell nebula in Vulpecula; the Fan, Horse-shoe, Trifid and Winged nebulæ in Sagittarius; the great nebula around _η_ Argus Navis, and the Crab nebula in Taurus.

Aside from the discovery of some of the largest nebulæ by different observers, and their subsequent arrangement in catalogues by Lacaille and Messier, very little had been done towards the study of these objects before 1779, when Sir W. Herschel began to observe them with the earnestness of purpose which was one of the distinctive points of the character of this great man. He successively published three catalogues in 1786, 1789, and 1802, in which the position of 2,500 nebulous objects was given. This number was more than doubled before 1864, when Sir John Herschel published his catalogue of 5,079 nebulæ and star-clusters. To this long list must be added several hundred similar objects, since discovered by D'Arrest, Stephan, Gould and others. But, as has been shown above, among the so-called nebulæ are many star-clusters which do not properly belong to the same class of objects, it being sometimes impossible in the present state of our knowledge to know whether a nebulous object belongs to one class or to the other.

The nebulæ exhibit a great variety of forms and appearances, and, in accordance with their most typical characters, they are usually divided into several classes, which are: the Nebulous stars, the Circular, or Planetary, the Elliptical, the Annular, the Spiral and Irregular nebulæ.

[PLATE XV.--THE GREAT NEBULA IN ORION.

From a study made in the years 1875-76]

The so-called nebulous stars consist of a faint nebulosity, usually circular, surrounding a bright and sharp star, which generally occupies its centre. The nebulosity surrounding these stars varies in brightness as well as in extent, and while, in general, its light gradually fades away, it sometimes terminates quite suddenly. Such nebulosities are usually brighter and more condensed towards the central star. The stars thus surrounded do not seem, however, to be distinguished from others by any additional peculiarity. Some nebulæ of this kind are round, with one star in the centre; others are oval and have two stars, one at each of their foci. The nebulous star, _τ_ Orionis, represented at the upper part of Plate XV., above the great nebula, has a bright star at its centre and two smaller ones on the side. The association of double stars with nebulæ is very remarkable, and may in some cases indicate a mutual relation between them.

The so-called planetary nebulæ derive their name from their likeness to the planets, which they resemble in a more or less equable distribution of light and in their round or slightly oval form. While some of them have edges comparatively sharp and well defined, the outlines of others are more hazy and diffused. These nebulæ, which are frequently of a bluish tint, are comparatively rare objects, and most of those known belong to the southern hemisphere. When seen through large telescopes, however, they present a different aspect, and their apparent uniformity changes. The largest of these objects, No. 2,343 of the General Catalogue, is situated in the Great Bear, close to the star _β_. Its apparent diameter is, according to Sir J. Herschel, 2', 40", and "its light is equable, except at the edge, where it is a little hazy." In a study which I made of this object in 1876, with a refractor 6⅓ inches in aperture, I found it decidedly brighter on the preceding side, where the brightest part is crescent shaped. In Lord Rosse's telescope its disk is transformed into a luminous ring with a fringed border, and two small star-like condensations are found within. Another planetary nebula, near _χ_ Andromedæ, has also shown an annular structure in Rosse's telescope.

The elliptical nebulæ, as their name implies, are elongated, elliptical objects; but while some of them are only slightly elongated ovals, others form ellipses whose eccentricity is so great that they appear almost linear. In all these objects the light is more or less condensed towards the centre; but while in some of them the condensation is gradual and slight, in others it is so great and sudden that the centre of the nebula appears as a large diffused star, somewhat resembling the nucleus of a comet. From the general appearance of these objects, it is not unlikely that some of them are either flattish, nebulous disks, like the planetary nebulæ, or nebulous rings, seen more or less sidewise. The condensation of light at their centres does not appear to be stellar, but nebulous like the rest, and it is a remarkable fact that very few, if any, of these objects are resolvable into stars.

Several elliptical nebulæ are remarkable for having a star at or near each of their foci, or at each of their extremities. Such are the elliptical nebulæ in Draco, Centaurus, and Sagittarius, Nos. 4,419, 3,706 and 4,395 of the General Catalogue, the last of which is in the vicinity of the triple star _μ_ Sagitarii. Each of these nebulæ has a star at each of its foci, while No. 1, in Cetus, has a star at each of its extremities.

Among the most remarkable elliptic nebulæ may be mentioned Nos. 1,861 and 2,373 of Sir J. Herschel's catalogue, both situated in the constellation Leo. The first is one of Lord Rosse's spiral nebulæ, and the last, which is a very elongated object, is formed of concentric oval rings, which are especially visible towards its central part. The constellation Draco is particularly remarkable for the number of elliptical nebulæ found within its boundaries. Among them are Nos. 3,939, 4,058, 4,064, 4,087, 4,415, etc., which are quite remarkable objects of their class. No. 4,058, of which I have made a study, is bright, and has a decided lenticular form with a condensation in the centre. Its following edge is better defined than the preceding. In Lord Rosse's telescope this object exhibits a narrow, dark, longitudinal, gap in its interior.

By far the largest and the finest object of this class is the great nebula in Andromeda. Although this object belongs rather to the class of irregular nebulæ, yet it is generally considered as an elliptic nebula, since its complicated structure, being less prominent, was not recognized until 1848, when it was perceived by George P. Bond, Director of the Harvard College Observatory. This, the first nebula discovered, was found in 1612 by Simon Marius. It is situated in the constellation Andromeda, in the vicinity of the star _ν_, and almost in a line with the stars _μ_ and _β_ of the same constellation. It is visible to the naked eye, and appears as a faint comet-like object. It is represented at the upper left hand corner of Plate XIII., on the border of the Milky-way, as it appears to the naked eye.

The nebula in Andromeda is one of the brightest in the heavens, and is closely attended by two smaller nebulæ. Perhaps it would be rather more correct to say that it has three centres of condensation, as the two small nebulæ referred to are entirely involved in the same faint and extensive nebulosity. Its general form is that of an irregular oval, upwards of one degree in breadth and two and a half degrees in length. Its brightest and most prominent part, which alone was seen by the earlier observers, consists in a very elongated lenticular mass, which gradually condenses towards its centre into a blazing, star-like nucleus, surrounded by a brilliant nebulous mass. At a little distance to the south of this central condensation is found one of the lesser centres of condensation noted above, which is globular in appearance, with a bright, star-like nucleus like the former. The other centre of condensation is found to the north-west of the centre of the principal mass, and is quite elongated, with a centre of condensation towards its southern extremity, but it is not so bright as the others. Close to the western edge of the bright lenticular mass first described, and making a very slight angle with its longer axis, are found two narrow and nearly rectilinear dark rifts, running almost parallel to each other, and both terminating in a slender point in the south. These dark rifts, which are almost totally devoid of nebulous matter, are quite rare in nebulæ, and afford a good opportunity to watch the changes which this part of the nebulæ may undergo.

This nebula has never been positively resolved into stars, although Prof. Geo. Bond and others have strongly suspected its resolvability. In a study which I have made of it, with the same instrument employed by Bond, and also with the great Washington telescope, I detected a decided mottled appearance in several places, which might be attributable to a beginning of resolvability; but I do not consider this a conclusive indication that the nebula is resolvable. The continuous spectrum given by this nebula, showing that it is not in the gaseous state which its appearance seems to indicate, warrants the conclusion, however, that it will ultimately be found to be resolvable. This object, being situated on the edge of the Galaxy and involved in its diffused light, has a great number of small stars belonging to this belt projected upon it. During my observations I have mapped out 1,323 of these stars, none of which seems to be in physical connection with the nebula.

Among the circular and elliptical nebulæ a few exhibit a very remarkable structure, being apparently perforated, and forming either round, slightly oval, or elongated rings of great beauty. These Annular nebulæ are among the rarest objects in the heavens. In Scorpio, two such nebulæ are found involved in the Milky-way, and also one in Cygnus. One of those in Scorpio has two stars involved within its ring, at the extremities of its smallest interior diameter. A very elongated nebula in the vicinity of the fine triple star _γ_ Andromedæ is also annular, and has two stars symmetrically placed at the extremities of its greatest interior axis. Another elongated annular nebula is also found north of _η_ Pegasi.