Chapter XV: The Great Nebula in Orion (8)

The grandest and most remarkable of the annular nebulæ is found in the constellation Lyra, about midway between the two stars _β_ and _γ_. It is slightly elliptical in form, and according to Prof. E. S. Holden, its major axis is 77".3 and the minor 58". From a study and several drawings which I have made of this object, with instruments of various apertures, I have found it decidedly brighter towards its outer border, at the extremities of its minor axis, than at the ends of the major axis. On very favorable occasions, some of its brightest parts have appeared decidedly, but very faintly mottled, and I have recognized three small centres of condensation. Its interior, in which Professor Holden has detected a very faint star, is quite strongly nebulous. In Lord Rosse's telescope, this nebula is completely surrounded by wisps and appendages of all sorts of forms, which I have failed to trace, however, both with the refractor of the Harvard College Observatory and with that of the Naval Observatory at Washington; Rosse, Secchi and Chacornac, have seen this nebula glittering as if it were a "heap of star dust," although its spectrum indicates that it is gaseous.

The nebula No. 1,541, in Camelopardus, of which I have also made a study and a drawing, is closely allied to the class of annular nebulæ. This object, which is quite bright, has a remarkable appearance. It consists principally of somewhat more than half of an oval ring, surrounding a bright, nebulous mass which condenses around a star; this mass being separated from the imperfect ring by a dark interval. Upon the bright portion of the ring, and on opposite points, are found two bright stars, between which lies the star occupying the central mass. The central mass extends at some distance outside of the ring on its open side. Several stars are involved in this object.

The Spiral nebulæ are very curious and complicated objects, but they are visible only in the largest telescopes. Prominent above all is the double spiral nebula No. 3,572, in Canes Venatici, which is not far from _η_ Ursa Majoris. In Lord Rosse's telescope, this object presents a wonderful spiral disposition, looking somewhat like one of the fire-works called pin-wheels, and forming long, curved wisps, diverging from two bright centres. The spectrum of this object, however, is not that of a gas. In the constellation Virgo, Rosse has detected another such nebula. In Cepheus, Triangulum, and Ursa Major, are found other spiral nebulæ of smaller size. Lord Rosse has recognized 40 spiral nebulæ and suspected a similar structure in 30 others.

The class of the Irregular nebulæ, which will be now considered, differs greatly in character from the others, and includes the largest, the brightest and the most extraordinary nebulæ in the heavens. The nebulæ of this class differ from those belonging to the other classes by a want of symmetry in their form and in the distribution of their light, as well as by their capricious shapes, and their very complicated structure. Another and perhaps the principal difference between them and the objects above described, consists in the remarkable fact already stated, that they are not, except in rare cases, to be found in the regions where the other nebulæ abound. On the contrary, they are found in or very near the Milky-way, precisely where the other nebulæ are the most rare. This fact, recognized by Sir J. Herschel, led him to consider them as "outlying, very distant, and as it were detached fragments of the great stratum of the Galaxy." It seems very probable that the reason why these objects differ so greatly from the other nebulæ in size, brightness and complication of structure, is simply because they are much nearer to us than are most of the others. They are perhaps nebulous members of our Galaxy. The same remark which has been made of star-clusters may be applied to nebulæ. The nearer they are to us, the larger, the brighter and the more complicated they will appear, while the farther they are removed, the more simple and regular and round they will appear, only their brightest and deepest parts being then visible.

The Crab Nebula of Lord Rosse, near _ζ_ Tauri, No. 1,157, is one of the interesting objects of this class. It has curious appendages streaming off from an oval, luminous mass, which give it a distant resemblance to the animal from which it derives its name. The Bifid nebula in Cygnus, Nos. 4,400 and 4,616, is another object of this class. It consists of a long, narrow, crooked streak, forking out at several places, and passing through _χ_ Cygni. Observers, having failed to recognize the connection existing between its different centres of brightness, have made distinct nebulæ of this extended object.

The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and curious object, with a general resemblance to the instrument from which it derives its name. Lord Rosse's telescope has shown many stars in it, projected on a nebulous background, and Prof. Bond seems to have thought that it showed traces of resolvability, although in the study which I made of this nebula with the same instrument used by the latter observer, I failed to perceive any such traces. Dr. Huggins finds its spectrum gaseous.

The star-cluster, No. 4,400, in Scutum Sobieskii, which is described by Sir J. Herschel as a loose cluster of at least 100 stars, I have found to be involved in an extensive, although not very bright, nebula, which would seem to have escaped his scrutiny. In a study and drawing of this nebula made in 1876, its general form is that of an open fan, with the exception that the handle is wanting, with deeply indented branches on the preceding side, where the brightest stars of the cluster are grouped. From its peculiar form, this object might appropriately be called the Fan nebula.

The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of which I have made a study and two drawings, one with a refractor 6⅓ inches in aperture, and the other conjointly with Prof. Holden, with the great telescope of the Naval Observatory, is a bright and very complicated object. Its general appearance in small instruments, with low power, is that of a long, narrow pisciform mass of light, from which proceeds on the preceding side, the great double loop from which it derives its name. But in the great Washington refractor its structure becomes very complicated, forming various bright nebulous masses and wisps of great extension. Prof. Holden, who has made a careful, comparative study of the published drawings of this object, thinks there are reasons to believe that its western branch has moved relatively to the stars found within its loop. The spectrum of this nebula is gaseous.

The Trifid nebula, No. 4,355, in the same constellation, is also a very remarkable object, although it is not so bright as the last. This nebula, which I have studied with the refractor of the Cambridge Observatory, consists of four principal masses of light, separated by a wide and irregular gap branching out in several places. These masses, which are brighter along the dark gap, gradually fade away externally. A group of stars, two of which are quite bright, is found near the centre of the nebula, on the inner edge of the following mass, and close to the principal branch of the dark channel. A little to the north, and apparently forming a part of this nebula, is a globular-looking nebula, having a pale yellow star at its centre. Prof. Holden's studies on this nebula show that the triple star, which was centrally situated in the dark gap from 1784 to 1833, was found involved in the border of the nebulous mass following it, from 1839 to 1877; the change, he thinks, is attributable either to the proper motion of the group of stars or to that of the nebula itself.

In the same vicinity is found the splendid and very extensive nebula No. 4,361, in which is involved a loose, but very brilliant star-cluster. This nebula and cluster, which I have studied and drawn with a 6⅓ inch telescope, is very complicated in structure, and divided by a dark irregular gap into three principal masses of light, condensing at one point around a star, and at others forming long, bright, gently-curved branches, which give to this object a strong resemblance to the wings of a bird when extended upwards in the action of flying. From this peculiarity this object might appropriately be called the Winged nebula. Its spectrum is that of a gas.

The variable star _η_ Argus is completely surrounded by the great nebula of the same name, No. 2,197, first delineated by Sir J. Herschel, during his residence at the Cape of Good Hope, in 1838. This object, which covers more than ⁴⁄₇ of a square degree, is divided into three unequal masses, separated by dark oval spots, comparatively free from nebulosity, and is suspected to have undergone changes since Herschel's time.

In the same field with the double star, _ζ_ Orionis, the most easterly of the three bright stars in the belt of Orion, is found another irregular nebula of the Trifid type. From the drawings which I have made of this object, it appears to be composed of three principal unequal masses, separated by a wide, irregular, dark channel, two of the masses being quite complicated in structure, and forming curved, nebulous streams of considerable length and breadth. This nebula, like the next to be described, seems to be connected with the Galaxy by the great galactic loop described in another section.

By far the most conspicuous irregular nebula visible from our northern States, is the great nebula in Orion, No. 1,179, represented on Plate XV. This object, visible to the naked eye, is the brightest and the most wonderful nebula in the heavens. It is situated a little to the south of the three bright stars in the belt of Orion, and may be readily detected surrounding the star _θ_, situated between and in a line with two faint stars, the three being in a straight line which points directly towards _ε_, the middle star of the three in Orion's belt. The area occupied by this nebula is about equal to that occupied by the Moon.

In its brightest parts the nebula in Orion appears as a luminous cloud of a pentagonal form, from which issue many luminous appendages of various shapes and lengths. This principal mass is divided into secondary masses, separated by darkish, irregular intervals. These secondary masses in their turn appear mottled and fleecy. Towards the lower part of the pentagonal mass is found a roundish dark space, comparatively devoid of nebulosity, in which are involved four bright stars forming a trapezium, and several fainter ones. The four bright stars of the trapezium constitute the quadruple star _θ_ Orionis, from which the nebula has received its name. The cloud-like pentagonal form is brightest on the north-west of the trapezium, and is surrounded on three sides by long, soft, curved wisps, fading insensibly into the outer nebulous mass in which they are involved. On the east a broad, wavy wing spreads out, and sends an important branch southward. South-east of the trapezium are found several curious dark spaces, comparatively devoid of nebulosity, especially those on the east, which give to this nebula a singular character. Close to the north-eastern part of the nebula, or rather in contact with it, is found a small, curiously-shaped nebula, condensing around a bright star into a blazing nucleus. From this centre it continues northward in a narrow diffused stream, which spreads out in passing over the stars _c^1_ and _c^2_; and after having sent short branches northward, it curves back to the south and joins the main nebula on the west of its starting point, having thus formed a great loop which is not shown on the Plate. The nebula also forms a loop towards the south, which is partly shown on Plate XV., a small branch of which, passing through _τ_ Orionis, the nebulous star shown at the top of the Plate, and extending southward, is not here represented.

On ordinary nights the nebula in Orion is a splendid object, and inspires the observer with amazement; but this is as nothing compared with the grand and magnificent sight which it presents during the very rare moments when our atmosphere is perfectly clear and steady. I have seen this nebula but once under these favorable circumstances, and I was surprised by the grandeur of the scene. Then could be detected features to be seen at no other time, and its fleecy, floculent, cloud-like masses glittered with such intensity that it seemed as if thousands of stars were going to blaze out the next moment. Although I observed the nebula under such favorable conditions, and with the fifteen-inch refractor of the Cambridge Observatory, yet I was disappointed in my expectations, and distinguished no new stars or points of light, and nothing more than a very bright mass, finely divided into minute blazing cloudlets. Although I failed to resolve this nebula into stars, yet Lord Rosse, Bond and Secchi thought they had caught glimpses of star dust. Its spectrum, however, proves to be mainly that of incandescent gases, probably hydrogen and nitrogen. In the curved wisps found in this nebula, Lord Rosse and others saw indications of a spiral structure.

Several bright stars are found scattered over this nebula, and besides those forming the trapezium, there are three in a row, a little to the south-east of that group, which are quite bright and remarkable. Among the stars involved in this nebula, few show signs of having a physical connection with it, although it seems probable that the group of the trapezium is so connected. Some of these stars are variable. The small stars represented on this Plate, as on others of the series, are somewhat exaggerated in size, as was unavoidable with any process of reproduction which could be adopted.

In 1811, W. Herschel was led to suspect that some changes had occurred in this nebula, but changes in such complicated and delicate objects are not easily ascertained, since, for the most part, we have for comparison with our later observations only coarse drawings made by hands unskilled in delineation.

Although comparatively rare, double and multiple nebulæ may be found in the sky. When this occurs, their constituents most commonly belong to the class of spherical nebulæ. Sometimes the components are separated and distinct, at other times one of them is projected upon the other, either really or by the effect of perspective. Sometimes one is round and the other elongated. It is probable that while some of these nebulæ are physically associated and form a system, others appear to be so only because they happen to be almost in a line with the observer. A double nebula in Draco, Nos. 4,127 and 4,128, which I have drawn, is a fair type of those which are separated. The first is a globular nebula, and the last an oval one, with a star at its centre. The double nebula, Nos. 858 and 859, in Taurus, which I have also studied, is a type of the cases in which one nebula is partly projected on another. In this instance both the nebulæ are globular.

The nebulæ in general show very little color in their light, which is ordinarily whitish and pale. Some, however, present a decided bluish or greenish tint. The great nebula in Orion has a greenish cast, and we have seen that some planetary nebulæ are bluish.

It has been a question whether nebulæ are changing. It has already been stated that Prof. Holden believes there is ground to suspect that the Trifid and Horse-shoe nebulæ have undergone some changes. A nebula near _ε_ Tauri has been lost and found again several times. Two other nebulæ in the same constellation have presented curious variations. One, near a star of the tenth magnitude, exhibited variations of brightness like those of the star itself, and for a time disappeared. The other, near _ζ_ Tauri, increased in brightness for three months, after which it disappeared. In 1859 Tempel discovered a nebula in the Pleiades, which has shown some fluctuations. In 1875 I made a long study of this object, and drew it carefully a dozen times, but I was not able to see any changes in it within the two or three months during which my observations were continued. But on Nov. 24, 1876, it was found of a different color, being purplish and very faint. On Dec. 23, 1880, it was found just as bright and visible as when I drew it in 1875, and on Oct. 20, 1881, it appeared faint and purplish again, as in 1876. On this last night, and on those which followed it, it was impossible for me to trace the nebulosity as far as in 1875. I consider this as due to a variation in the light of this object, which in 1875 was bright enough to be well seen while the Moon after her First Quarter was within ten or fifteen degrees from the Pleiades.

From the observations of M. Laugier, it appears that some nebulæ have a proper motion, comparable to that of stars. From the displacement of the lines of their spectra by their motion in the line of sight, Dr. Huggins found that no nebula observed by him has a proper motion surpassing 25 miles per second. The Ring nebula in Lyra appears to move from us at the rate of 3 miles per second, and that in Orion recedes about 17 miles per second.

The important question arises, are all the irresolvable nebulæ in the heavens to be considered as so many star-clusters, differing only from them by the minuteness of their components, or their immense distance from us; or are they cosmical clouds, composed of luminous vapors, similar to the matter composing the heads and tails of comets? Originally, W. Herschel, with many astronomers, thought that all these objects were stellar aggregations, too distant to be resolved into stars; but he subsequently modified his opinion, and accepted the idea that some of them are of a gaseous nature.

No direct proof that the nebulæ are gaseous could be obtained, however, before the spectroscope was known. The attempt to analyze the light of the nebulæ with this instrument was made in 1864, by Dr. Huggins, who directed his spectroscope to the planetary nebula, No. 4,373, in Draco. Its spectrum was found to consist of three bright, distinct lines, the brightest of which corresponded with the strongest nitrogen line, and the feeblest with the hydrogen C line. Besides these lines, it gave also a very faint, continuous spectrum, apparently due to a central point of condensation. By this observation, the gaseous nature of a nebula was for the first time demonstrated. Dr. Huggins thus analyzed 70 nebulæ, of which one-third gave a gaseous spectrum, consisting of several bright lines, the brightest of which invariably corresponded with the lines of nitrogen. The others gave a continuous spectrum, with the red end usually deficient. These results indicate that if some of the so-called nebulæ are due to an aggregation of stars, either too minute or too remote in space to be individually resolved, others are in a gaseous state. Yet the faint, continuous spectrum, given by some nebulæ, in addition to their gaseous spectra, seems to show that these nebulæ have some stars or matter in a different state, either involved in them or projected on their surface.

The idea of diffused matter distributed here and there in space, and gradually condensing into stars, is by no means new. As early as 1572, Tycho Brahé proposed such an hypothesis, to explain the sudden apparition of a new star in Cassiopeia, which he considered as formed by the recent agglomeration of the "celestial matter" diffused in space. Kepler adopted the same idea to explain the new star which appeared in Ophiuchus, in 1604. Halley, Lacaille, Mairan and others, entertained the same opinion. The hypothesis of a self-luminous, nebulous matter diffused in space, and forming here and there immense masses, has been proposed from the origin of the telescope, and was adopted by Sir William Herschel, who in his grand speculations on the universe considered the nebulæ as immense masses of phosphorescent vapors, gradually condensing around one or several centres into stars or clusters of stars. The evidence afforded by the spectroscope seems to be in favor of such an hypothesis, and shows us that gaseous agglomerations exist in space.

According to our modern conception, the visible universe is but an infinitely small portion of the infinite universe perceived by our mind. The great blazing centre around which our little, non-luminous globe pursues its endless journey, is only an humble member of a cluster comprising four hundred equally powerful suns, as they are believed to be, although they appear to us as little twinkling stars. The nearest of these stars is 221,000 times as far from the Sun as the Sun is from the Earth, and yet this entire cluster is only one among the several hundred Star-clusters composing the great galactic nebula in which we are involved, comprising thirty or fifty millions of such suns. Among the 4,000 irresolvable nebulæ in the sky, perhaps over one-half are supposed to be galaxies, like our own galaxy, composed of star-clusters, and millions of stars. Besides these remote galaxies, vast agglomerations of yet uncondensed, nebulous matter exist in space, and form the nebulæ proper, in which the genesis of suns is slowly elaborated. Although the visible universe is limited by the penetrating power of our instruments, yet we see in imagination the infinite universe stretching farther and farther; but we know not whether this invisible universe is totally devoid of matter, or whether it also is filled with millions and millions of suns and galaxies.