Popular Science Monthly/Volume 8/January 1876/The Horseshoe Nebula in Sagittarius

by Edward Singleton Holden

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THE HORSESHOE NEBULA IN SAGITTARIUS.

By EDWARD S. HOLDEN

PROFESSOR IN THE UNITED STATES NAVAL OBSERVATORY, WASHINGTON.

IN the number of The Popular Science Monthly for July, 1874, I gave a brief account of the successive observations of the great nebula of Orion, from 1656 to 1874, and I pointed out how instructive such an historical review was in its bearing upon the improvement of our means of observation and as an example of how the standard of such work has been gradually raised. It will be interesting to trace in the same way the history of the Horseshoe Nebula in Sagittarius, which, next to the great nebulosities of Orion and Andromeda, is the most curious of these objects, and which perhaps as much as any other deserves careful study.

Its discovery dates back about a hundred years to the time of Messier, the assiduous astronomer of the Observatoire de la Marine at Paris; it is No. 17 of his list, which comprises most of the brighter and more remarkable nebulæ of the northern sky. It was at this time that Sir William Herschel, the famous astronomer of England, with instruments far superior in power to those of Messier, was forming his great catalogues of the nebulæ discovered in his "sweeps." Messier wisely used his smaller instrument in the endeavor to obtain accurate positions for those found by him, and he has left us monographic studies of the Orion and the Andromeda nebula ("Mémoires de l'Académie des Sciences," 1771 and 1807), which are almost the first trustworthy works of the kind, and which are the beginnings from which sprang the elaborate drawings of Lassell, Rosse, Struve and Bond.

Fig. 1.—J. Herschel, 1833.

From the time of Messier to 1826, when Sir John Herschel published his first figure of the Orion nebula, almost nothing was done in this line of research; but in 1833 a study of the Horseshoe Nebula was published by Sir John Herschel, together with many other similar drawings, in the "Philosophical Transactions" (see Fig. 1). This was the first considerable and systematic attempt to accurately figure the nebulae, and it doubtless turned the attention of astronomers generally to this branch, the importance of which was manifest. If so many of the fixed stars changed in brilliancy and in position, why should not the same thing occur among the nebulæ? And if such changes were once established, would not an important increase of our knowledge accrue, concerning these objects of which almost nothing was known? It was one of the avowed objects of Sir John Herschel's celebrated journey to the Cape of Good Hope to figure the nebulae of the southern sky, and, while there, the drawing given in Fig. 2 was made, although it was not published until 1847.

Fig. 2.—J. Herschel, 1837.

As we have said, Herschel's paper of 1833 created a wide-spread interest among astronomers, and about 1836 two monographic studies of the Horseshoe Nebula were begun, under circumstances so different as to deserve our attention, Lamont, the accomplished director of the Observatory of Munich, and Mason, an undergraduate of Yale College, commenced observations at about the same time: one being supplied with all the appliances which were known to astronomers, and devoting all his energies to his chosen science in a city which was then the most famous in the world for its astronomical instruments; and the other, a mere hoy, oppressed by narrow circumstances, working in the intervals of his college duties with a telescope which he had himself constructed, with a fellow-student (Mr. Hamilton L. Smith) as his only assistant.

The work of both astronomers (for it is impossible to deny to Mason that title) is of great excellence, but it will not be claiming too much to assert that Mason's was by far the most valuable monographic study of a nebula which had appeared, and indeed, in its thorough appreciation of the problems to be solved and in its most skillful adaptation of the existing means toward that end, it deserves to rank with the greatest works of this class, with Bond's, Lassell's, Rosse's and Struve's. It is not only in the observations themselves nor in the exquisite and accurate drawings which accompany the memoir that we feel this excellence, but in the philosophical grasp of the whole subject and the masterly appreciation of the fundamental ideas of the problem. His memoir contains so much that bears on this general aspect, that we quote from it largely, as it is too little known among those not professional astronomers:

"Although a period of nearly fifty years has now elapsed since the researches of the elder Herschel exposed to us the wide distribution of nebulous matter through the universe, we are still almost as ignorant as ever of its nature and intention. The same lapse of time that, among his extensive lists of double stars, has revealed to us the revolution of sun around sun, and given us a partial insight into the internal economy of those remote sidereal systems, has been apparently insufficient to discover any changes of a definite character in the nebulæ, and thereby to inform us at all of their past history, the form of their original creation, or their future destiny. At the same time, the detection of such changes is in the highest degree desirable, since no other sources of evidence can be safely relied upon in these inquiries. That the efforts of astronomers have thus far ended, at least, in vague and contradictory conjectures, is principally attributable to the great difficulty of originally observing, and of describing to future observers, bodies so shapeless and indeterminate in their forms, with the requisite precision. For we cannot doubt, authorized as we are to extend the laws of gravitation far into the recesses of space, that these masses of diffused matter are actually undergoing vast revolutions in form and constitution. The main object of this paper is to inquire how far that minute accuracy which has achieved such signal discoveries in the allied department of the double stars may be introduced into the observation of nebulæ, by modes of examination and description more peculiarly adapted to this end than such as can be employed in general reviews of the heavens. . . . It will conduce to a clearer understanding of our object to point out, generally and rapidly, the distinctions between our own theory of observation and that commonly adopted. It consists not in an extensive review, but in confining the attention to a few individuals; upon these exercising a long and minute scrutiny, during a succession of evenings; rendering even the slightest particulars of each nebula as precise as repeated observation and comparison with varied precautions can make them, and confirming each more doubtful and less legible of its features by a repetition of suspicions, which are of weight in proportion as they accumulate; and, lastly, when practicable, correcting by comparison of the judgments of different persons at the same time.

"The assistance which is rendered to the faithful description of these remarkable objects by thus laying a groundwork of stars, may be well illustrated by the familiar expedient of artists, who divide any complicated engraving which they would copy, into a great number of squares, their intended sketch occupying a similar number. The stars, which are apparently interwoven throughout the whole extent of the nebula, furnish a set of thickly-distributed natural points of reference, which, truly transferred to the paper, are as available as the cross-lines of the artist in limiting and fixing the appearance of the future drawing.

"In nebulæ of great extent, however correctly estimated may be the stars immediately around the standard of reference, those in the distant parts of the nebula are liable to suffer from an accumulation of errors of nearly the same kind as that arising in an extended trigonometrical survey. But if the places of the larger stars are well settled by fixed instruments, there will be far less room for error in estimations which spread, as from so many centres, over the remaining intervals.

Fig. 3.—Mason, 1839.

"I will here speak of a method that I hit upon for the exact representation of nebulæ, which has essentially contributed to the accuracy of the accompanying delineations. It was first suggested by the method usually adopted for the representation of heights above the sea-level on geographical maps, by drawing curves which represent horizontal sections of hill and valley at successive elevations above the level of the sea—that is, by lines of equal height—and it is the same in its principle. It is obvious that, if lines be imagined in the field of view, winding around through all those portions of a nebula which have exactly equal brightness, these lines, transferred to our chart of stars, will give a faithful representation of the nebula and of its minutiæ, and of the suddenness as well as of the amount of transition from one degree of shade to another.

"By far the greatest obstacles to the successful comparison of modern observations on nebulæ with those which own, at least, a brief antiquity, exist in the want of precision with which the labors of former observers have been conducted, and hence all attempts to trace the slow progress of their changes end in uncertain conjectures and conflicting probabilities. I shall not, therefore, incur the charge of unnecessary minuteness in endeavoring to render, by every means, our knowledge of the present form and state of at least these few nebulæ, as far as possible, standard; and, although laden with the necessary imperfections of original observations, yet free from adventitious and unnecessary vagueness in the communication of them. In order to supply, to any future observer, those slight particulars which a chart cannot easily urge upon the notice of any but the original compiler, and further, to indicate the degree of certainty with which different features of the nebulæ were recognized, it is thought proper to bring under this head the enumeration of various facts not expressed in the journal of observations. These are divided into 'things certain,' 'nearly certain,' 'strongly suspected,' and 'slightly suspected.' Thus much for observation—for rendering the idea of the object as perfect as may be in the mind of the observer. For the most unimpaired communication of this idea or perception, the theory of the process adopted is briefly—1. To form an accurate chart of all stars capable of micrometrical measurement in and around the nebula. 2. From these, as the greater landmarks, to fill in with all the lesser stars, down to the minimum visible by estimation, which, with care, need not fall far short of ordinary measurement. 3. On this, as a foundation, to lay down the nebula.

"The first intention was to intrust entirely to careful estimation the copying of the stars which were to form the groundwork of the nebula, since no means of measurement were then at hand. The following is a sketch of the course of procedure adopted in pursuance of this plan: The limits of the nebula were traced as far as long and close examination could discern them, and a rough chart was made of the principal stars within it. This preparation was indispensable, because, in the consequent mapping down of all the visible stars in the nebula, it was necessary to use a light out-of-doors, and the object, of course, became invisible. The distance between any two conspicuous stars favorably situated in the nebula was then chosen as a standard of reference; and, from this as a base, a kind of triangulation was carried out by the eye to all the stars in the neighborhood, and these were successively marked on a sheet of paper at the time; their magnitudes were also affixed to each according to a fictitious scale, for which a few stars, conveniently situated, furnished standards of reference as to size. A lamp was close at hand, whose light could be cut off at pleasure; and almost direct comparison was thus instituted between the stars in the field of view and those on the paper, and corrections made where any distortions in the latter were observable. As the work advanced from night to night, the reference to the lamp was necessarily less and less direct, since a longer exclusion of light was necessary to see the fainter stars. Finally, the nebula itself was drawn upon the map by the guidance of the stars already copied; and although only an occasional and unfrequent reference could be made to a lamp, the stars within it had become so familiar by their constant recurrence, that the memory could as easily as before retain its estimates of distance and direction, until mutual comparison could be made between the map and the heavens."

It will be seen what a great advance had been made in the conception of the application of the topographical method of contour lines to the delineation of degrees of brightness, although this method has practical limitations not spoken of by Mason, and we must consider the careful separation of the various results into classes ranged according to their degrees of certainty, as scarcely less important. In all former memoirs the chart included all the results reached, and there was no searching division of these in such a way as to give absolute data to the future investigator.

Throughout the entire memoir (which relates also to other nebulae than the one now in question) the whole endeavor is to reach a perfect definiteness of conception; and Mason evidently held the idea that, in the existing state of astronomy, it was eminently "better to do one thing well than many things indifferently."

Fig. 4.—Lamont, 1837.

Lamont tells us in Annalen der K. Sternwarte bei München, Band xvii. (1868), that his early researches on this and other nebulæ were prosecuted in the hope that something might be determined as to their real nature, and he expresses his opinion that all nebulæ consist essentially of clusters of stars, more or less remote. His original researches were published in 1837, accompanied by figures, and they are of high authority on this subject. We give Lamont's figure above. These two drawings having been executed by different observers with different telescopes (Lamont's refractor of nine inches aperture, and Mason's reflector of twelve inches) will afford in the cases in which they agree indubitable evidence as to the existence of any feature shown in them. The non-existence of any feature not shown in either is probable, although not certain.

Sir John Herschel's "Results of Astronomical Observations at the Cape of Good Hope" was published in 1847, and his drawing (our Fig. 2), in the order of publication, belongs after Fig. 4,

In his first paper he describes Fig. 1 as follows:

"The figure of this nebula is nearly that of a Greek capital omega, Ω, somewhat distorted, and very unequally bright. It is remarkable that this is the form usually attributed to the great nebula in Orion, though in that nebula 1 confess I can discern no resemblance whatever to the Greek letter. Messier perceived only the bright eastern branch of the nebula now in question, without any of the attached convolutions which were first noticed by my father. The chief peculiarities which I have observed in it are—1. The resolvable knot in the eastern portion of the bright branch, which is, in a considerable degree, insulated from the surrounding nebula; strongly suggesting the idea of an absorption of the nebulous matter; and, 2. The much feebler and smaller knot at the northwestern end of the same branch, where the nebula makes a sudden bend at an acute angle. With a view to a more exact representation of this curious nebula, I have at different times taken micrometrical measures of the relative places of the stars in and near it, by which, when laid down as in a chart, its limits may be traced and identified, as I hope soon to have better opportunity to do than its low situation in this latitude will permit."

This opportunity was afforded him at his southern station, and his Fig. 2 is accordingly much more detailed. He says of it in the work last cited that his Fig. 1 is far from an accurate expression of its shape:

"In particular the large horseshoe-shaped arc . . . is there represented as too much elongated in a vertical direction and as bearing altogether too large a proportion to [the eastern] streak and to the total magnitude of the object. The nebulous diffusion, too, at the [western] end of that arc, forming the [western] angle and base-line of the capital Greek omega (Ω), to which the general figure of the nebula has been likened, is now so little conspicuous as to induce a suspicion that some real change may have taken place in the relative brightness of this portion compared with the rest of the nebula; seeing that a figure of it made on June 25, 1837, expresses no such diffusion, but represents the arc as breaking off before it even attains fully to the group of small stars at the [western] angle of the Omega. . . . Under these circumstances the arguments for a real change in the nebula might seem to have considerable weight. Nevertheless, they are weakened or destroyed by a contrary testimony entitled to much reliance. Mr. Mason, a young and ardent astronomer, . . . . whose premature death is the more to be regretted, as he was, so far as I am aware, the only other recent observer who has given himself with the assiduity which the subject requires to the exact delineation of nebulæ, and whose figures I find at all satisfactory, expressly states that both the nebulous knots were well seen by himself and his coadjutor Mr. Smith on August 1, 1839, i.e., two years subsequent to the date of my last drawing. Neither Mr. Mason, however, nor any other observer, appears to have had the least suspicion of the existence of the fainter horseshoe arc attached to the [eastern] extremity of Messier's streak. Dr. Lamont has given a figure of this nebula, accompanied by a description. In this figure [our Fig. 4], the nebulous diffusion at the [western] angle and along the [western] base-line of the Omega is represented as very conspicuous; indeed, much more so than I can persuade myself it was his intention it should appear."

When Lassell mounted his great four-foot reflector at Malta, lie devoted much of his time to a systematic review of those nebula; which had previously been figured either by himself or by Rosse and

Fig. 5.—Lassell, 1862.

others, and, as was expected from the excellence of the climate, the superiority of the great telescope and the skill of the observer, this series of drawings at once took its place among the acknowledged classics on this subject. Too much praise can hardly be given to Lassell for confining his attention principally to objects previously figured, and for resisting the temptation to roam in those fields which his own telescope had opened with its list of six hundred new nebulæ. And it may be remarked in passing that it is just this intelligent devotion to a definite aim and object which, in this case as in all, has led to brilliant results. We give Lassell's figure above, remarking that it was constructed, as indeed all the preceding ones had been, by first measuring the relative position of the brighter stars, then inserting by careful eye estimates the fainter ones, and finally by drawing among these stars, guided by their configurations, the details of the nebula itself.

Another, and a very rapid method of drawing nebulæ, is the following. It yields to the first in the accuracy of the positions of the stars, but it is probably even superior to it in facilities for the correct representation of the nebula and stars considered as one mass. A piece of glass is ruled carefully into squares (see Figs. 6 and 7) and this is placed in the focus of the telescope so as to be plainly visible; the telescope is then directed upon the nebula, and a clock-work motion is applied to the telescope so that it follows the apparent motion of the nebula from east to west accurately. Some one of the brighter stars is chosen, and it is kept by means of the clock-work accurately in the corner of one of the squares. A piece of paper ruled into squares similar to those of the glass reticle is provided, and on it the observer dots down the various stars in and about the nebula. This may take two, three, or four nights according to circumstances, but in all cases it requires much less time than the micrometric measurements of the brighter stars and the troublesome allineatious required to fix the positions of the smaller stars, and it has the great advantage that the work can be done in a perfectly dark field of view, whereas the micrometric measures demand the use of illuminated wires at least. After the stars are inserted, the principal lines are put in, not only by the star-groups, but also by the squares themselves. For my own use I have had constructed two reticles: one ruled in squares like those seen in Figs. 6 and 7, and another in which the heavy-lined large squares (each containing nine small squares, see Fig. 6) are still present, but are subdivided into small squares by lines parallel to their own diagonals. After making all the use possible of the first reticle, the second is put in, and an entirely new set of reference-lines is obtained, making an angle of 45° with the old set. This, of course, could be equally obtained by revolving the first reticle through an angle of 45°, but it is not quite so convenient.

After the stars and the principal lines of the nebula are inserted a new and higher power eye-piece is used, and the drawing is concluded by means of this. Fig. 6 is an example of a drawing of the Horseshoe Nebula made in this way by M. Trouvelot, of Cambridge, Massachusetts, the artist to whom we owe the exquisite plates of astronomical engravings published by Harvard College Observatory, under the superintendence of its late director, Prof Winlock.

During the last summer M. Trouvelot was invited by the superintendent of the United States Naval Observatory to visit Washington for the purpose of making drawings of nebulæ, etc., by means of the twenty-six inch Clark refractor. By the courtesy of Admiral Davis

Fig. 6.—Trouvelot, 1875 (with a 6 1/2-inch Refractor).

I am able to give a drawing of the Horseshoe Nebula as delineated by M. Trouvelot from observations made jointly by him and by myself. Pretty much the same method was adopted in this drawing as in Fig. 6, but the vastly more complex structure of the nebula itself is what might have been expected from an increase of eighteen times in the light, over M. Trouvelot's six-inch telescope.

From careful comparisons of different kinds, it has been found that the power of the Washington telescope is about the same as that of Lassell's great four-foot reflector, and the two drawings. Figs. 5 and 7, are therefore nearly comparable, i.e., almost as if made with the

Fig. 7.—Trouvelot, 1875 (26-inch Telescope).

same telescope at different times. It may be said of the last drawing that nothing is there laid down about which the slightest doubt is entertained; and although, in some respects, it was made in greater haste than is desirable, yet it is sufficiently accurate to found an argument on, for or against variation in the shape of any of the brighter portions of the nebula.

It is hoped that enough has been said to show how much care, skill, and patience, have been spent upon these drawings, and to show, too, how important are the conclusions which may be drawn from them. Their careful discussion involves considerations which might be out of place here, but which are well worth general attention. A full explanation of different methods has been given in the hope that some of the large telescopes in various parts of the United States in the hands of private gentlemen may be devoted to work of this class, in which it is easy for an amateur, with but a trifling expenditure of time and labor, to produce valuable results. Provided only that the work be done conscientiously and faithfully, it will be a definite gain to astronomy; without such care and fidelity, it will only introduce new confusion.