Popular Science Monthly/Volume 5/July 1874/Telescopic Research on the Nebula of Orion
By EDWARD S. HOLDEN,
PROFESSOR IN THE UNITED STATES NAVAL OBSERVATORY AT WASHINGTON.
It is well occasionally to cast our eyes back over a series of labors, in order that we may rightly judge of our progress.
Rough comparisons alone will not tell us: it is easy to contrast Galileo's piece of lead-tubing, having a lens in each end of it, with the gigantic telescopes of to-day; but we hardly learn much from such a comparison. The extremes of the series are too far removed; to get a just idea of the terms between, the comparisons must be made at shorter intervals.
If we can select some celestial object, which has been telescopically studied for a long period, we may, by contrasting the results obtained at different times, gain some accurate notion of the progress made, and of the way in which it was made.
This latter idea is of some importance when we consider that the day of startling discoveries is over. When Galileo found the four moons of Jupiter, the whole world of men of learning was astounded; and still more wonderful was his discovery of the ring of Saturn—that is to say, of the existence of a ring of Saturn—a portion of which, showing on each side of the planet, gave to Saturn the "tri-corporate" aspect which Galileo describes in a letter to his friend Kepler
Even the discovery of Uranus by Sir William Herschel, in 1789, was by no means an extraordinary event, although it was received with enthusiasm in all Europe, for Herschel's examination of the heavens was done with the aid of an instrument which could not fail to show Uranus with a sensible disk, if the planet should enter the field of his telescope.
The discovery of the first asteroid, Ceres, on the 1st day of January, 1801, by Piazzi, was received with some surprise, but these small bodies have lately become quite common (there being now 135 of them known), and certainly we in America, who have Peters and Watson among us, have no cause to wonder at such discoveries.
Neptune was discovered first by two theoretical astronomers in their studies, and the delight with which the news of the actual discovery was received was a tribute to the power of pure analysis, and in nowise contributed to the glory of telescopic research.
The startling discoveries, as we have seen, were reserved for the early astronomers, who first found the new country, leaving their successors to accurately map it out. The lesson of patience which can be learned from the labors of these successors is no mean one. To this patience, supplemented by a skill which usually must be of a high order, we owe the later discoveries of the telescope, such as the finding of the eighth satellite of Saturn (by Bond and Lassell), and of the two interior moons of Uranus (Lassell, 1847 and 1851), and of the satellite of Neptune.
There is hardly an object in the whole heavens—planets, of course, excepted—which has been so thoroughly and faithfully studied as the Great Nebula in Orion. And this nebula has a history which will well repay a study somewhat in detail. We shall, in comparing the different work already done upon it, arrive at a very good idea of the progress of telescopic astronomy itself, since, for over 200 years, the details of this nebula have been a subject of solicitude to a great number of eminently skillful astronomers, aided by the best telescopes of their time.
The nebula was discovered by Huyghens in 1656, and in our own century it has been studied by the great reflectors of Herschel and Rosse, and by the refractors of Cambridge, Pulkova, and Rome, in the hands of Bond, Struve, and Secchi.
The place of this nebula in the heavens is easily to be found by any one tolerably familiar with the aspect of our winter sky.
The constellation of Orion is a well-known and brilliant asterism, and very conspicuous among the other stars of the group are the three stars which constitute the "belt." Below these are three others, in nearly a straight line, and these are known as "the sword:" the northern star of these is C Orionis; the middle one is? Ο (Theta) Orionis; and the southern is? ι (Iota) Orionis; it is of the nebula surrounding? Orionis that we wish to speak.
Flamsteed, astronomer-royal of England, marked Orionis in his catalogue of stars, as of the fourth magnitude, and to the naked eye it so appears.
But, on examination with a telescope, this star is seen to be not single, but multiple. When Huyghens, in 1656, turned his newly-constructed telescope to it, he saw three stars, and these were surrounded by a cloud-like mass—the nebula.
The figure which he printed in his "Systema Saturnium" is given (Fig. 1, p. 259), together with the figure of the French astronomer, Le Gentil, which was drawn in 1758, and which may be found in "Histoire de l'Académie," 1759. This is Fig. 2.
The fourth star was, however, seen by Domenique Cassini before Le Gentil's drawing was made.
Not a little is to be learned from these figures, which we must try to keep in mind, as the region to which they refer is the central and most interesting portion of the great nebula. We can see, first, that
Nebula Orionis. (Huyghens, 1656.)
there is an entire absence of shading in the portion of the nebula drawn; the outline of nebulous matter is simply filled in by shading of a uniform tint. It is much to be regretted that it is impossible to print here the exquisite engraving of this nebula, made from drawings by Prof. G. P. Bond, director of Harvard College Observatory, in 1865. In this drawing one hardly knows which to admire most, the accuracy and painstaking skill of the astronomer, or the exquisite tact of the engraver. There are very few nights so fine, and there are very few instruments so perfect, as to give a view of this nebula as good and as detailed as may be had from this engraving alone.
But we must be content with referring to the plate ("Annals of Harvard College Observatory," vol, v.) for the details of the nebulous portions of this region, and we must content ourselves with reproducing only the stars of the surrounding space. It is true that especial attention has been paid by some astronomers to the smaller stars in this nebula; while some, as Herschel the younger, have evidently bestowed more care upon the grades of light in the nebula itself; but, on the whole, we shall find that the number and faintness of the small stars, seen by any astronomer in any one limited region, are a very fair test of his own assiduity, and of the excellence of his telescope.
It will be seen that Le Gentil has made of the star θ Orionis four stars, adding one to the number seen by Huyghens. These four are the celebrated trapezium of Orion, and they constitute one of the most remarkable multiple stars of the whole heavens.
Nebula Orionis. (Le Gentil, 1758.)
Messier, who examined the nebula carefully, in 1771, gives these four stars, and a very good drawing of the nebula, in "Histoire de l'Académie," 1771. This work was extremely creditable, considering his instrumental means, and the comparatively short time spent upon it.
This trapezium was destined to become famous, for, in 1826, the elder Struve found, close to one of the large stars of the trapezium, a small companion—the "fifth star." This was for a long time considered as a delicate test for an instrument; but, in December, 1832, Herschel found yet another small star—the "sixth star." The trapezium now was seen (with suitable means) as in Fig. 3.
Let us follow the history of the whole nebula for a time, and we shall see what additions have been made to our knowledge of the space near this remarkable group.
It may be here remarked that the grouping of these six stars is in all probability not accidental, but that they are physically connected: Struve has shown that the chances are 9,000 to one against any two stars from the first to the seventh magnitude, inclusive, falling within 4" of each other if the stars were fortuitously scattered over the whole heavens to begin with. And, further, he has shown that the chances are 170,000 to one against any such stars, if fortuitously scattered, falling within 32" of a third so as to form a triple star. The chances against any such grouping as that seen in the trapezium of Orion are enormously greater than the numbers given above, and the inference is irresistible that these stars are in some way physically connected. We shall see, too, that it is also probable that these stars are in some unknown way related to the surrounding nebula.
Central and most brilliant portion of the Great Nebula in the Sword-handle of Orion, as observed by Sir John Herschel in his 20-foot Reflector at Feldhausen. Cape of Good Hope (1834 to 1837).
In 1811 Sir William Herschel was led to suspect changes in the form and brightness itself: an inference which he probably drew from a comparison of the early drawings with the appearance of the nebula as he saw it in his own reflectors; and in 1824 the younger Herschel made a drawing of the nebula and the stars immersed in it, as shown by his 20-foot reflector.
This drawing and a description of the nebula are given in "Memoirs of the Royal Astronomical Society," vol. ii., p. 489, and a copy of the drawing, so far as the small stars immediately about the trapezium are concerned, is given in Fig. 4.
The recession of the nebula from the stars of the trapezium is here noted for the first time, so far as I know: this recession is a real phenomenon, and is thus spoken of by Sir John Herschel: "The nebula which is very bright in the parts surrounding the trapezium, seems (whether by the effect of contrast with the dazzling light of these stars, or from a real deficiency in nebulous matter) to have retreated from immediate contact with them, so that they appear in some degree insulated and with a darkness about them. This would agree with the idea of a subsidence of the nebula into the stars by gravitation; but it is probably only a deception."
Nebula Orionis. (Herschel, 1824.)
Sir John likewise expresses an opinion as to the resolvability of the nebula, and describes its appearance in a very graphic way. He says: "I know not how to describe it better than by comparing it to a curdling liquid, or to a surface strewed over with flocks of wool, or to the breaking up of a mackerel sky when the clouds of which it consists begin to assume a cirrous appearance. ..." It presents, however, "no appearance of being composed of small stars, and its aspect is altogether different from that of resolvable nebulæ. In the latter we fancy by glimpses that we see stars, or that, could we strain our sight a little more, we should see them. But the nebula suggests no idea of stars, but rather of something quite distinct from them."
In the beginning of 1834 Sir John Herschel went to the Cape of Good Hope for the purpose of completing a survey of the whole heavens, which had been commenced in England: he took with him his 20-foot reflector (aperture 18 1/4 inches), and devoted himself during his stay there to astronomical work. The nebula of Orion of course attracted much of his attention, and on account of its favorable situation for observation he was enabled to examine it to much better advantatage than in England. In the "Astronomical Observations at the Cape of Good Hope," Herschel has given a figure of the nebula as he saw it (see Fig. 3), and has added an account of its appearance, a portion of which account we shall quote.
Fig. 5 is a representation of the small stars immediately in the vicinity of the trapezium as seen by Herschel.
Nebula Orionis. (Herschel, 1834.)
It is easily seen that the circumstances at the Cape of Good Hope were much more favorable than they had been in England: this is witnessed in the figures here given by the greater number of stars shown, and in the original drawing by the amount of detail in the features of the nebula itself. The nebulosity surrounding θ Orionis was traced by Herschel over half a square degree of space, and numerous points of doubt were settled. In his account Herschel states that he discovered the sixth star of the trapezium in 1832, and he expresses his surprise that Struve, who had examined the stars in this neighborhood frequently, should have overlooked it.
Da Vico, of the Roman College, had in 1839 declared that he had seen the stars within the trapezium (an assertion which has been repeated since by various observers, good and bad), and Herschel takes the opportunity to declare that not only had he seen no stars there, but that the absence of any trace of star or even of nebulosity was one of its most striking features. (Herschel's remarks in this regard are fully confirmed by the best observers.)
But, as we may see by a reference to Fig. 5, Herschel had discovered quite a number of small stars, some of them extremely faint and difficult, notably one marked 78 in the figure (No. 78 of Herschel's Catalogue).
Later researches on the stars in this group have added largely to their number, but hardly any have been found more faint than this star H. 78.
Bond, of Cambridge, and O. Struve, of Pulkova, with the fifteen-inch refractors of those observatories, both observed it and found it extremely difficult, and both of these observers supposed it to be variable in magnitude. It seems almost impossible that Herschel should have seen H. 78 and that he should not have seen others seen by both Bond and Struve, if the star was as faint in 1834 as it is in 1874; and the inference seems hard to avoid, that this very faint star was perceptibly brighter in 1834 than at present.
Herschel discusses the evidences of change in the nebula at some length, and, although his own drawings of 1824 and 1837 differ as much as any two drawings which we have, he is strongly of belief that these differences, although great, "are not more so than I am disposed to attribute to inexperience in such delineations," and to various other causes, such as the favorable situation of the nebula in the southern sky, etc. One part of the nebula he does regard as probably variable, and, to prove this, he compares his two drawings. It may be said that, so far as our knowledge now goes, this suspicion of Herschel's is not confirmed.
In 1852, Mr. Lassell, already famous as the discoverer of various satellites of the major planets, took his magnificent twenty-foot reflector to Valetta, to use it to good advantage in the serene atmosphere there. While his telescope was mounted at Valetta, Mr. Lassell made careful observations of the Orion nebula, and he even had a painting in oil made from his own drawings, and from the nebula itself, by an artist-friend. When we consider the immense difficulty of. drawing even the form of so complex an object as this is at the telescope, from which the eye must be removed every moment to add a new line, or to verify one already drawn, and when we further consider what added pains must be taken in order to get an approach to accuracy of light and shade, we must admit that the attempt to represent not only form, and light and dark, but also color and tint, is almost a vain one. Accordingly, we find Mr. Lassell's drawing of the nebula itself to be strikingly different from Herschel's or Rosse's (made in 1867), but we ought not on that account assume any change in the nebula itself. Any one who has made such drawings will know what strong and direct evidence of change must be had to establish it as a fact.
Mr. Lassell's drawing (Fig. 6) exhibits the same characteristics as Herschel's in one respect. He has noted some very faint stars, particularly one, of which we will speak further, but he has omitted others much brighter (at least, brighter in 1874, and also brighter when mapped by Bond in 1865). We must attribute this to a desire to depict the form of the nebula itself, and a neglect of the stars in comparison; and yet this is difficult to do, since Lassell has given us a map of new stars which he found, some of which have never been seen by any observer since, and presumably do not exist, or have vanished. Lassell, again, finds no trace of resolvability in this nebula.
Nebula Orionis. (Lassell, 1852.)
It will be noticed that Lassell has a large number of small stars above and to the left of the trapezium. These are put in the map by eye-estimates of their position, and it is somewhat difficult to identify them with Bond's stars in this place, but I have no doubt that all of them are real. Lassell's b and g have never been seen by any later observer, and probably they do not exist, a' of Lassell's map was not even noted by him as a new discovery, but it remained unseen even by the keen vision of Bond and Struve, until the mounting of the great Alvan Clark refractor (18 1/2 inches aperture) in 1862, when Alvan Clark, Jr., found this star by the aid of that instrument.
His observation has been verified by the great Clark refractor, at Washington (26 inches aperture).
In 1848, Mr. W. C. Bond, Director of Harvard College Observatory, made, by the aid of the 15-inch refractor, a map and a drawing of the nebula and its contained stars. There was a large number of errors in this catalogue, and Boud's work was sharply criticised by O. Struve, whose "Memoir on the Nebula of Orion" appeared in 1862. Struve's work, which was a revision and an extension of the work of Liapanoff (done at Kazan with a 9-inch retractor), was executed very carefully with the 15-inch telescope of Pulkova; and some of his strictures on the elder Bond's work were so severe as to induce G. P. Bond, his son, then Director of Harvard College Observatory, to take up his father's work, to complete and amend it. This he has done in a most admirable monograph, which is a model of its kind. We have already spoken of his engraving of the nebula, and its excellence is only commensurate with the completeness of the whole of the memoir.
Fig. 7 represents the small stars in the now familiar ground near the trapezium.
Nebula Orionis. (G. P. Bond, 1865.)
It will be seen how much fuller this map is than Lassell's, which contains more small stars probably than any of the preceding ones.
Lord Rosse's great reflecting telescope of 6-feet aperture was employed at various times, between 1848 and 1867, in making drawings of the Orion nebula; and we have, as the results of the work, two great engravings, upon which much care has been spent.
These certainly differ in many important points from the preceding drawings which were made by reflectors (and experience will show us that it is not easy, critically, to compare drawings made by reflectors and by refractors); but this is simply a proof that the drawings of different instruments are hardly comparable. Indeed, drawings by different observers, with the same instrument, and made at the same time, are often as different from each other as any of the previous drawings from Lord Rosse's.
The resolvability of the nebula is claimed by Lord Rosse, but the testimony of the spectroscope, as far as that goes, is against that inference, and the testimony of large telescopes, at least equal to Lord Rosse's in their defining power, is likewise unanimously contrary.
Any one, who will critically study the drawings named above, will, it is believed, arrive at the conclusion that no traces of resolvability have been fairly made out. Changes of form, although the evidence of the various drawings is seemingly in favor of such changes, are not probable, from a comparison of all the data. The drawing of Bond is confirmed, we believe, by Safford, of Chicago, by aid of the 18 1/2-inch refractor, and by the great refractor of the Naval Observatory at Washington, so far as an examination has been made, and
Nebula Orionis. (Rosse, 1860-'67.)
after a careful collation of all drawings; the only inference it seems possible now to draw is as to the enormous personal differences of the artists. It should be remembered that the difficulty of getting a correct drawing engraved correctly is itself immense, and not often to be overcome, especially if the engraver has not himself studied the nebula which he is to represent.
Fig. 8 is a part of Rosse's drawing made with the six-foot telescope.
It will be seen that the number of small stars above and west of the trapezium is very small, but a reference to the original drawing would show that the nebula was traced very far away from the central star: we see from these facts that the famous Rosse telescope is surpassed by several other telescopes in definition, but its enormous surface, when in good condition, is a great light-gatherer, and a faint nebulosity is distinguished in it more easily than in most other instruments—Lassell's reflector, for example.
Bond, however, has surpassed, by the aid of the 15-inch refractor at Cambridge, the work of Lord Rosse's assistants in every way, even in regard to the very points for which the reflector was best fitted. We must, however, attribute this, in no small degree, to the skill and assiduity of Bond himself.
We have thus traced the history of the nebula of Orion, as developed by the telescope alone, from its discovery to the present time. This has been done only imperfectly, on account of the impossibility of presenting complete engravings of the drawings made by the different observers, to whose work reference has been made. Several other astronomers of note have worked on this subject—notably Lamont and Secchi—but the results of their telescopic work are included in the preceding account.
The necessary conclusions as to the resolvability and change of the nebula have been indicated, as well as the strong probability that the nebula and the stars in the neighborhood are physically connected, which inference is mainly drawn from the recession of the nebula spoken of by Herschel.
The progress of telescopic research is well shown by the additions made year by year to our knowledge of this beautiful object, and we have no reason to be ashamed of it. The spectroscope has been busy on this nebula, as with others; but of its work it is not our purpose to speak. The desideratum in the study of these faint objects seems to be the extension of photography to their delineation. Until that is accomplished, we may rest content with such work as Bond and Struve have left us.