Popular Science Monthly/Volume 3/September 1873/The Great Nebula in Orion
|THE GREAT NEBULA IN ORION.|
DURING the first four months of the year, the constellation Orion is very favorably situated for observation in the evening. This magnificent asterism is more easily recognized than the Great Bear, Cassiopeia's Chair, or the fine festoon of stars which adorns the constellation Perseus. There is, indeed, a peculiarity about Orion which tends considerably to facilitate recognition. The other constellations named above gyrate round the pole in a manner which presents them to us in continually-varying positions. It is not so with Orion. Divided centrally by the equator, the mighty hunter continues twelve hours above and twelve hours below the horizon. His shoulders are visible somewhat more, his feet somewhat less, than twelve hours. When he is in the south, he is seen as a giant with upraised arms, erect, and having one knee bent, as if he were ascending a height. Before him, as if raised on his left arm, is a curve of small stars, forming the shield, or target of lion's skin, which he is represented as uprearing in the face of Taurus. When Orion is in the east, his figure is inclined backward; when he is setting, he seems to be bent forward, as if rushing down a height; but he is never seen in an inverted position, like the northern constellations.
And we may note, in passing, that the figure of Orion, as he sets, does not exactly correspond with the image presented in that fine passage in "Maud:"
Listening now to the tide, in its broad-flung shipwrecking roar,
Now to the scream of a maddened beach dragged down by the wave,
"Walked in a wintry wind, by a ghastly glimmer, and found
The shining daffodil dead, and Orion low in his grave;"
and again, toward the end of the poem:
"When the face of night is fair on the dewy downs,
And the shining daffodil dies, and the charioteer
And starry Gemini hang like glorious crowns
Over Orion's grave low down in the west."
I would not, however, for one moment, be understood as finding fault with these passages of Tennyson's finest poem. Detached from the context, the image is undoubtedly faulty ; but there is a correctness in the very incorrectness of the image, placed as it is in the mouth of one
brooding evermore on his father's self-murder:
Mangled, and flattened, and crushed."
Let us pass on, however, to the subject of our paper.
Beneath the three bright stars which form the belt of Orion are several small stars, ranged, when Orion is in the south, in a vertical direction. These form the sword of the giant. On a clear night it is easy to see that the middle star of the sword presents a peculiarity of appearance: it shines as through a diffused haze. In an opera-glass this phenomenon is yet more easily recognizable. A very small telescope exhibits the cause of the peculiarity, for it is at once seen that what seemed a star is, in reality, a mass of small stars intermixed with a diffused nebulosity.
It is very remarkable circumstance that Galileo, whose small telescope, directed to the clear skies of Italy, revealed so many interesting phenomena, failed to detect
It would not, indeed, have been very remarkable if he had simply failed to notice this object. But he would seem to have directed his attention for some time especially to the region in the midst of which Orion's nebula is found. He says: "At first I meant to delineate the whole of this constellation; but, on account of the immense multitude of stars—being also hampered through want of leisure—I left the completion of this design till I should have another opportunity." He therefore directed his attention wholly to a space of about ten square degrees, between the belt and sword, in which space he counted no less than 400 stars. What is yet more remarkable, he mentions the fact that there are many small spots on the heavens shining with a light resembling that of the Milky-Way (complures similis coloris areolœ sparsim per cethera subfulgeant); and he even speaks of nebulae of this sort in the head, and belt, and sword of Orion. He asserts, however, that, by means of his telescope, these nebulae were distinctly resolved into stars—a circumstance which, as we shall see presently, renders his description wholly inapplicable to the great nebula. Yet the very star around which (in the naked-eye view) this nebula appears to cling, is figured in Galileo's drawing of the belt and sword of Orion!
It seems almost inconceivable that Galileo should have overlooked the nebula, assuming its appearance in his day to have resembled that which it has at present. And, as it appears to have been established that, if the nebula has changed at all during the past century, it has changed very slowly indeed, one can scarcely believe that in Galileo's time it should have presented a very different aspect. Is it possible that the view suggested by Humboldt is correct—that Galileo did not see the nebula because he did not wish to see it? "Galileo," says Humboldt, "was disinclined to admit or assume the existence of starless nebulae." Long after the discovery of the great nebula in Andromeda—known as the "transcendently-beautiful queen of the nebulæ"—Galileo omitted all mention in his works of any but starry nebulae. The last-named nebula was discovered in 1614 by Simon Marius, whose claims to the discovery of Jupiter's satellites had greatly angered Galileo, and had called forth a torrent of invective, in which the Protestant German was abused as a heretic by Galileo, little aware that he would himself, before long, incur the displeasure of the Church. If we could suppose that an unwillingness, either to confirm his rival's discovery of a starless nebula, or to acknowledge that lie had himself fallen into an error on the subject of nebulæ, prevented Galileo from speaking about the great nebula in Orion, we should be compelled to form but a low opinion of his honesty. It happens too frequently that—
An eye well-practised in Nature, a spirit bounded and poor."
That Hevelius, the "star-cataloguer," should have failed to detect the Orion nebula is far less remarkable; for Hevelius objected to the use of telescopes in the work of cataloguing stars. He determined the position of each star by looking at the star through minute holes or pinnules, at the ends of a long rod attached to an instrument resembling the quadrant.
The actual discoverer of the great nebula was Huyghens, in 1656. The description he gives of the discovery is so animated and interesting that we shall translate it at length. He says:
"While I was observing the variable belts of Jupiter, a dark band across the centre of Mars, and some indistinct phenomena on his disk, I detected among the fixed stars an appearance resembling nothing which had ever been seen before, so far as I am aware. This phenomenon can only be seen with large telescopes such as I myself make use of. Astronomers reckon that there are three stars in the sword of Orion, which lie very close to each other. But, as I was looking, in the year 1656, through my 23-feet telescope, at the middle of the sword, I saw, in place of one star, no less than twelve stars, which, indeed, is no unusual occurrence with powerful telescopes. Three of these stars seemed to be almost in contact, and with these were four others which shone as through a haze, so that the space around shone much more brightly than the rest of the sky. And, as the heavens were serene and appeared very dark, there seemed to be a gap in this part, through which a view was disclosed of brighter heavens beyond. All this I have continued to see up to the present time " (the work in which these remarks appear, the "Systema Saturnium," was published in 1659), " so that this singular object, whatever it is, may be inferred to remain constantly in that part of the sky. I certainly have never seen any thing resembling it in any other of the fixed stars. For other objects once thought to be nebulous, and the Milky-Way itself, show no mistiness when looked at through telescopes, nor are they any thing but congeries of stars thickly clustered together."
Huyghens does not seem to have noticed that the space between the three stars he described as close together is perfectly free from nebulous light, insomuch that, if the nebula itself is rightly compared to a gap in the darker heavens, this spot resembles a gap within the nebula. And, indeed, it is not uninteresting to notice how comparatively inefficient was Huyghens's telescope, though it was nearly eight yards in focal length. A good achromatic telescope two feet long would reveal more than Huyghens was able to detect with his unwieldy instrument.
Dominic Cassini soon after discovered a fourth star near the three seen by Huyghens. The four form the celebrated trapezium, an object interesting to the possessors of moderately good telescopes, and which has also been a subject of close investigation by professed astronomers. Besides the four stars seen by Cassini, there have been found five minute stars within and around the trapezium. These tiny objects seem to shine with variable brilliancy; for sometimes one will surpass the rest, while at others it will be almost invisible.
After Cassini's discovery, pictures were made of the great nebula by Picard, Le Gentil, and Messier. These present no features of special interest. It is as we approach the present time, and find the great nebula the centre of quite a little warfare among astronomers—now claimed as an ally by one party, now by their opponents—that we begin to attach an almost romantic interest to the investigation of this remarkable object.
In the year 1811, Sir W. Herschel announced that he had (as he supposed) detected changes in the Orion nebula. The announcement appeared in connection with a very remarkable theory respecting nebulæ generally—Herschel's celebrated hypothesis of the conversion of some nebulas into stars. The astronomical world now heard for the first time of that self-luminous nebulous matter, distributed in a highly-attenuated form throughout the celestial regions, which Herschel looked upon as the material from which the stars have been originally formed. There is an allusion to this theory in those words of the Princess Ida:
If that hypothesis of theirs he sound."
And in the teaching of "comely Psyche:"
Till toward the centre set the starry tides,
And eddied into suns that, wheeling, cast
Few theories have met with a stranger fate. Received respectfully at first on the authority of the great astronomer who propounded it—then in the zenith of his fame—the theory gradually found a place in nearly all astronomical works. But, in the words of a distinguished living astronomer," The bold hypothesis did not receive that confirmation from the labors of subsequent inquirers which is so remarkable in the case of many of Herschel's other speculations." It came to pass at length that the theory was looked upon by nearly all English astronomers as wholly untenable. In Germany it was never abandoned, however, and a great modern discovery has suddenly brought it into general favor, and has in this, as in so many other instances, vindicated Herschel's claim to be looked upon as the most clear-sighted, as well as the boldest and most original, of astronomical theorizers.
Herschel had pointed out various circumstances which, in his opinion, justified a belief in the existence of a nebulous substance—fire-mist or star-mist, as it has been termed—throughout interstellar space. He had discovered and observed several thousand nebulæ, and he considered that among these he could detect traces of progressive development. Some nebulae were, he supposed, comparatively young; they showed no signs of systematic aggregation or of central condensation. In some nebulae he traced the approach toward the formation of subordinate centres of attraction; while in others, again, a single centre began to be noticeable. He showed the various steps by which aggregation of the former kind might be supposed to result in the formation of star-clusters, and condensation of the latter kind into the formation of conspicuous single stars.
But it was felt that the strongest part of Herschel's case lay in his reference to the great nebula of Orion. He pointed out that, among all the nebulas which might be reasonably assumed to be star-systems, a certain proportionality had always been found to exist between the telescope which first detected a nebula and that which effected its resolution into stars. And this was what might be expected to happen with star-systems lying beyond our galactic system. But how different is this from what was seen in the case of the Orion nebula! Here is an object so brilliant as to be visible to the naked eye, and which is found, on examination, to cover a large region of the heavens. And yet the most powerful telescopes had failed to show the slightest symptom of resolution. Were we to believe that we saw here a system of suns so far off that no telescope could exhibit the separate existence of the component luminaries, and therefore (considering merely the observed extent of the nebula, which is undoubtedly but a faint indication of its real dimensions) so inconceivably enormous in extent that the star-system of which our sun is a member shrinks into nothingness in comparison? Surely it seemed far more reasonable to recognize in the Orion nebula but a portion of our galaxy—a portion very vast in extent, but far inferior to that "limitless ocean of universes" presented to us by the other view.
And when Sir W. Herschel was able, as he thought, to point to changes taking place within the Orion nebula, it seemed yet more improbable that the object was a star-system.
But now telescopes more powerful than those with which the elder Herschel had scanned the great nebula were directed to its examination. Sir John Herschel, following in his father's footsteps, applied himself to the diligent survey of the marvellous nebula with a reflecting telescope 18 inches in aperture. He presented the nebula to us as an object of which "the revelation of the 10-feet telescope was but the mere rudiment." Strange outlying wisps and streamers of light were seen, extending far out into space. Yet more strange seemed the internal constitution of the object. So strange, indeed, did the nebula appear, "so unlike what had hitherto been known of collections of stars," that Sir John Herschel considered the evidence afforded by its appearance as sufficient to warrant the conclusion of a non-stellar substance.
But this eminent astronomer obtained a yet better view of the great nebula when he transported to the Cape of Good Hope an instrument equal in power to that which he had applied to the northern heavens. In the clear skies of the Southern Hemisphere the nebula shines with a splendor far surpassing that which it has in northern climes. It is also seen far higher above the horizon. Thus the drawing which Sir J. Herschel was able to execute during his three years' residence at the Cape is among the best views of the great nebula that have ever been taken. But, even under these favorable circumstances, Sir John records that" the nebula, through his great reflector, showed not a symptom of resolution."
Then Lassell turned his powerful mirror, two feet in diameter, upon the unintelligible nebula. But, though he was able to execute a remarkable drawing of the object, he could discern no trace of stellar constitution.
In 1845 Lord Rosse interrogated the great nebula with his three-feet mirror. Marvellous were the complexity and splendor of the object revealed to him, but not the trace of a star could be seen.
The end was not yet, however. Encouraged by the success of the three-feet telescope, Lord Rosse commenced the construction of one four times as powerful. After long and persistent labors, and at a cost, it is said, of £30,000, the great Parsonstown reflector, with its wonderful sis-feet speculum, was directed to the survey of the heavens. At Christmas, 1845, while the instrument was yet incomplete, and in unfavorable weather, the giant tube was turned toward the Orion nebula. Prof. Nichol was the first who saw the mysterious object as pictured by the great mirror. Although the observation was not successful so far as the resolution of the nebula was concerned, yet Nichol's graphic account of the telescope's performance is well worth reading:
"Strongly attracted in youth by the lofty conceptions of Herschel," he writes, "I may be apt to surround the incident I have to narrate with feelings in so far of a personal origin and interest; but, unless I greatly deceive myself, there are few who would view it otherwise than I. With an anxiety natural and profound, the scientific world watched the examination of Orion by the six-feet mirror; for the result had either to confirm Herschel's hypothesis in so far as human insight ever could confirm it, or unfold among the stellar groups a variety of constitution not indicated by those in the neighborhood of our galaxy. Although Lord Rosse warned me that the circumstances of the moment would not permit me to regard the decision then given as absolutely final, I went in breathless interest to the inspection. Not yet the veriest trace of a star! Unintelligible as ever, there the nebula lay; but how gorgeous its brighter parts! How countless those streamers branching from it on every side! How strange, especially that large horn on the north, rising in relief from the black skies like a vast cumulous cloud! It was thus still possible that the nebula was irresolvable by human art; and so doubt remained. Why the concurrence of every favorable condition is requisite for success in such inquiries may be readily comprehended. The object in view is to discern, singly, sparkling points, small as the point of a needle, and close as the particles of a handful of sand ; so that it needs but the smallest unsteadiness in the air, or imperfection in the shape of the reflecting surface, to scatter the light of each point, to merge them into each other, and present them as one mass."
Before long, Lord Rosse, after regrinding the great mirror, obtained better views of the mysterious nebula. Even now, however, he could use but half the power of the telescope, yet at length the doubts of astronomers as to the resolvability of the nebula were removed. "We could plainly see," he wrote to Prof. Nichol, "that all about the trapezium was a mass of stars, the rest of the nebula also abounding with stars, and exhibiting the characteristics of resolvability strongly marked." These views were abundantly confirmed by subsequent observations with the great mirror.
It will surprise many to learn that even Lord Rosse's great reflector is surpassed in certain respects by some of the exquisite refractors now constructed by opticians. As a light-gatherer the mirror is, of course, unapproachable by refractors. Even if it were possible to construct an achromatic lens six feet in diameter, yet, to prevent flexure, a thickness would have to be given to the glass which would render it almost impervious to light, and therefore utterly useless. But refractors have a power of definition not possessed by large reflectors. With a refractor eight inches only in aperture, for instance, Mr. Dawes has obtained better views of the planets (and specially of Mars) than Lord Rosse's six-feet speculum would give under the most favorable circumstances. And, in like manner, the performance of Lord Rosse's telescope on the Orion nebula has been surpassed—so far as resolution into discrete stars is concerned—by the exquisite defining power of the noble refractor of Harvard College (U. S.). By means of this instrument, hundreds of stars have been counted within the confines of the once intractable nebula.
It seemed, therefore, that all doubt had vanished from the subject which had so long perplexed astronomers. "That was proved to be real" Nichol wrote, " which, with conceptions of space enlarged even as Herschel's, we deemed incomprehensible."
Yet, even at this stage of the inquiry, there were found minds bold enough to question whether a perfectly satisfactory solution of the great problem had really been attained. Nor is it difficult, I think, to point out strong reasons for such doubts. I shall content myself by naming one which has always appeared to me irresistible.
The Orion nebula as seen in powerful telescopes covers a large extent of the celestial sphere. According to the Padre Secchi, who observed it with the great Merz refractor of the observatory at Rome, the nebulous region covers a triangular space, the width of whose base is some eight times, while its height is more than ten times as great as the moon's apparent diameter—a space more than fifty times greater than that covered by the moon. Now, I do not say that it is inconceivable that an outlying star-system, so far off as to be irresolvable by any but the most powerful telescopes, should cover so large a space on the heavens. On the contrary, I do not believe that a galaxy resembling our own would be resolvable at all, unless it were so near as to appear much larger than the Orion nebula. I believe astronomers have been wholly mistaken in considering any of the nebulas to be such systems as our own. There may be millions of such systems in space, but I am very certain no telescope we could make would suffice to resolve any of them. But, what I do consider inconceivable is, that a nebula extending so widely, and placed (as supposed) beyond our system, should yet appear to cling (as the Orion nebula undoubtedly does) around the fixed stars seen in the same field with it. So strongly marked is this characteristic, that Sir John Herschel (who failed, apparently, to see its meaning) mentions among others no less than four stars—one of which is the bright middle star of the belt as "involved in strong nebulosity," while the intermediate nebulosity is only just traceable. The probability that this arrangement is accidental is so small as to be almost evanescent. However, as I have said, English astronomers, almost without a dissentient voice, accepted the resolution of the nebula as a proof that it represents a distant star-system resembling our own galactic system, but far surpassing it in magnitude.
The time came, however, when a new instrument, more telling even than the telescope, was to be directed upon the Orion nebula, and with very startling results. The spectroscope had revealed much respecting the constitution of the fixed stars. We had learned that they are suns resembling our own. It remained only to show that the Orion nebula consists of similar suns, in order to establish beyond all possibility of doubt the theories which had been so complacently accepted. A very different result rewarded the attempt, however. When Dr. Huggins turned his spectroscope toward the great nebula, he saw, in place of a spectrum resembling the sun's, three bright lines only! A spectrum of this sort indicates that the source of light is a luminous gas, so that, whatever the Orion nebula may be, it is most certainly not a congeries of suns resembling our own.
It would be unwise to theorize at present on a result so remarkable. Nor can we assert that Herschel's speculations have been confirmed, though his general reasoning has been abundantly justified. Astronomers have yet to do much before they can interpret the mysterious entity which adorns Orion's sword. On every side, however, observations are being made which give promise of the solution of this and kindred difficulties. We have seen the light of comets analyzed by the same powerful instrument; and we learn that the light from the tail and coma is similar in quality (so far as observation has yet extended) to that emitted from the Orion nebula. We see, therefore, that in our own solar system we have analogues of what has been revealed in external space. I would not, indeed, go so far as to assert that the Orion nebula is a nest of cometic systems; but I may safely allege that there is now not a particle of evidence that the nebula lies beyond our galaxy.
Nor need we doubt the accuracy of Lord Rosse's observations. More than a year before his death, indeed, he mentioned to Dr. Huggins that "the matter of the great nebula in Orion had not been resolved by his telescope. In some parts of the nebula he observed a large number of exceedingly minute red stars. These red stars, however, though apparently connected with the irresolvable blue material of the nebula, yet seemed to be distinct from it."
The whole subject seems to be as perplexing as any that has ever been submitted to astronomers. Time, however, will doubtless unravel the thread of the mystery. We may safely leave the inquiry in the hands of the able observers and physicists whose attention has been for a long time directed toward it. And we need only note, in conclusion, that in the Southern Hemisphere there exists an object equally mysterious—the great nebula round η Argus — which has yielded similar results when tested with the spectroscope. The examination of this mysterious nebula, associated with the most remarkable variable in the heavens a star which at one time shines but as a fifth-magnitude star, and at another exceeds even the brilliant Canopus in splendor may, for aught that is known, throw a new light on the constitution of the great Orion nebula.—Fraser's Magazine.