Popular Science Monthly/Volume 22/February 1883/Methods in Modern Physical Astronomy
|METHODS IN MODERN PHYSICAL ASTRONOMY.|
IT has become an almost consecrated custom for the President of this Association, instead of relating the progress which has been made in all the sciences that are the objects of your investigations, to treat more particularly of a single one of them, and to present a summary of the progress it has made. This custom appeal's an excellent one to me. By it we gain in precision and authority what we lose in extensiveness; and we owe to it many masterly efforts, the impression of which has not yet been effaced from our minds, and which cause in me a just apprehension that I may fall far short of their standard.
I shall endeavor to present to you a picture, sketched in broad outline, of the progress and influence of a branch of research which has played a considerable part in contemporary scientific movements, and the discoveries of which have not only revolutionized our astronomical knowledge, but have also opened new and unexpected horizons to philosophy—I mean physical astronomy.
Physical astronomy is a wholly modern—yes, as to its best parts, contemporary—science; and it can be regarded as old only as concerning its object. From the earliest times, in fact, that men began to look toward the sky, and the first reflections on nature were born of these first observations, man has asked what is that sun whose immense and beneficent function caused it to be designated at that early period as the soul of nature. He has asked himself what is the cause which lends to the moon the sweet and mysterious light that gives to the nights so poetical a charm; and afterward questions arose concerning those brilliant points with which the celestial vault is strewed. All these problems appertain to our science, but how little was man then in a condition to deal with them! Long ages of observations and labor were necessary before even the corner of the veil could be raised.
This was because physical astronomy presupposes a very clear knowledge of the properties of light, both as to itself and as to its relations with bodies, and great perfection in the mechanical arts to permit the construction of the apparatus, at once extensive and delicate, which it employs.
Astronomy as a description of motions, on the contrary, only required the eyes and very simple instruments. Therefore the first astronomers began with that branch. At a later period, the science, ceasing to be purely descriptive, became geometric, and at last took a sublime flight; and by the application of the higher calculus we had the celestial mechanics.
During this long period, the physical branch of the science, to speak correctly, did not exist. Reduced to hypotheses that could not be verified, the theories of celestial physics had even fallen into discredit. It should be said that the beauty and importance of the discoveries with which the geometricians endowed the elder sister of our branch contributed no little to this result. Three great discoveries have, however, completely changed the situation, by giving to the physical branch arms which permit it at last to enter gloriously into the competition. I refer to the telescope, spectrum analysis, and photography.
The foundations of physical astronomy were laid in the invention of the telescope. Every one has heard of the emotion which filled Europe at the announcement of the discovery of an instrument which had the power of making distant objects appear as if they were near. It was at that time that Galileo, having only learned that such an instrument existed, discovered its arrangement, constructed one, turned it toward the sky, and, with this aid, fertilized by his genius, made a series of magisterial discoveries. These discoveries belong pre-eminently to physical astronomy, and form its first courses. If we except the sun and moon, which have a very sensible diameter, and admit of some observations without the aid of the telescope, all the stars appear to the eye only as brilliant points, and admit of no studies except of their motions. Therefore, an astronomy without the telescope would never have permitted us otherwise than as a matter of probability to consider the planets as like the earth in form, constitution, and office. But when it was seen that these brilliant and almost blazing points were resolved under the telescope into well-defined disks, showing indications of continents, clouds, and atmospheres; when satellites were perceived around those globes playing the same part to them as the moon plays to the earth then probabilities gave place to a clear certainty. Telescopes, then, are the instruments by means of which the constitution of the solar system has been definitively unveiled, and the earth has been assigned its part and its rank in the system of the planets. The discovery of the spots on the sun and of its rotation completed the conception of the solar system and prepared for the theory of its formation. Here is marked a well-determined phase in the history of human ideas respecting the universe, and it is characterized by the great name of Galileo.
Was it possible at once to go beyond this? Was it possible to question the stars in their turn, and inquire if, like the sun, they had a sensible disk, spots, a rotation, and planets revolving around them; was it possible, in short, to extend to the stellar universe the notions we had already acquired concerning the solar system? The methods in use did not yet permit this.
The most delicate measurement of parallaxes has proved that the star nearest to us is two hundred thousand times as far off as we are from the sun. We should, then, need a telescope magnifying more than two hundred thousand times to show us, under the most favorable circumstances, a star's diameter equal to that which the sun presents to the naked eye. Such a power is a hundred times greater than the strongest powers that can be used. We are, therefore, obliged to stay within the limits of our system, and proceed by analogy when we desire to go out of it. The analogies, it is true, were very powerful means even with Copernicus and Galileo, but with Kirchhoff and Huggins they were destined to acquire very shortly an irresistible force. Nature habitually reserves for the assiduous and sagacious observer surprises that exceed his hopes. So, while the study of the stars, considered as individual worlds, still remained beyond our reach, a great observer discovered facts of a very general bearing.
This leads us to a second phase of the period of telescopes—a phase that was characterized by the observations of the great Herschel. Herschel changed the form of the instrument, and adopted one that was more adaptable to the realization of the great powers he sought to obtain. Then, by his magnificent studies of the nebulæ, and by his discovery of multiple stars revolving around one another, he laid the foundations of the theory of worlds with multiple centers. This was a new conception, which did not proceed from that of the solar system, but was much more general. The problem was thus resolved in its extreme terms; but between these extremes yawned an immense gap.
The gap has not yet been filled. We can not directly study those worlds which each star forms with the planets of its suite, but a new method of investigation has come forward to shed unexpected light on the unanswered questions.
The first period of physical astronomy was opened with the modest telescope of Galileo, and closed with the large telescopes of Herschel. As early as the beginning of this century, when the astronomer of Slough had finished his review of the sky, it was felt that the telescopic harvest was nearly gathered, and another instrument of progress was looked for. Arago thought he had found it in the discovery of Malus, to which he made brilliant additions, and earnestly endeavored to found a new branch of physical astronomy on polarization. He was not successful. His discoveries ceased after a few beautiful applications of his method. At this time the polariscope process is only employed to determine whether light is reflected or emitted.
Quite different was it with a method the origin of which, we believe, goes back to the very birth of optics. This method is likewise founded on the actions of bodies on light, but, so rich and profound are the modifications with which it has to deal, that it has been able to pass over that in the matter which concerns only its general properties, and look into its peculiar individuality, its specific chemical quality. The principle which is the basis of the new method of spectrum analysis is as simple as general, and may be stated by saying that the elementary rays emitted by every kind of radiating gasiform matter depend upon and characterize the chemical species to which that matter belongs. Hence it follows that the spectral image resulting from the analysis of the beam of rays emitted by any body will vary according to the chemical nature of that body. Spectum analysis is founded upon the examination of spectra.
It must be added that the chemical nature of a body is not the exclusive element in the constitution of its spectrum, but that that may vary with the physical circumstances of the phenomenon, the temperature, the cause generating the radiation, etc.; but these are subordinate effects, which only add to the richness of the method, without detracting from its certainty and its value.
We have been able to leap over the enormous distance which separates the conception of the body, viewed as to its general properties, from the notion of it as individualized in such a manner as to constitute a chemical species by regarding light not only as a whole, but also in its elementary parts; by not only studying the whole beam and the general modifications that affect it, but by extending the examination to the elementary rays of which it is composed. The little mass of matter forming the chemical molecule, when it can vibrate freely, as in the gaseous state, emits a peculiar system of waves, a system which varies principally with the chemical species of the molecule, but which varies also, though rather secondarily, with the distance apart of the molecules and the nature and intensity of the forces that induce a vibratory movement in it. We might illustrate the nature of the system of luminous rays emitted by such a molecule by comparing it to the system of sounds given off by a vibrating cord, which is dependent for the principal characteristic on the length of the cord, and for the secondary phenomena of volume, tone, etc., on other circumstances accompanying the vibration.
It is proper to remark at this point that, when we analyze light in this way to examine it in its elements, we perform an operation entirely parallel to that of the chemist who separates the simple elements of a compound body. The elementary ray is a chemical species in light. It has all the characteristics of a species. It is incapable of decomposition, it has an individuality of its own, characterized by its wave-length, by the physiological effects it induces, whether acting alone or in association with other rays, and by the phenomena which it exhibits in its relations with bodies. We then bring the two sciences together by performing on light an operation parallel to the one that has been made on matter. Chemical analysis by light was performed in posse on the day when its rays were regarded in the light of chemical species.
This great idea of the specific character of luminous rays is due to Newton. It was introduced into science when that great genius gave his explanation of the action of the prism on white light. The foundations of spectrum analysis were laid at that time, and the study of it might have been begun then at once; but the human mind does not proceed with so penetrating and absolute a logic as that. The successive and often fortuitous acquisition of revealing facts had to be left to time. It must, however, be said that, when the facts were presented, their real significance would have been overlooked, notwithstanding the genius of the experimenters, had not the grand idea of Newton illuminated them with its brilliant light. The conception of the individuality of the rays made its way so slowly into our minds that it bore its fruits, as it were, unknown to us. But history, whose vision goes back to the beginnings, will have to assign their respective parts to the causes which have been influential toward the end. The allotment will in no degree diminish the admiration which is due to the creators of the marvelous instrument. They have given a body of life to what was slumbering in posse; they have thus shown themselves worthy continuers of the work of Newton.
You know that this spectrum analysis made its appearance very suddenly in science. You may recollect the emotion that affected us all when it was announced that the solar atmosphere had been chemically analyzed, and a list of the metals it contained was published. You are, however, too well acquainted with the history of science to suppose that a method as complete as the one that was thus announced had no antecedents. The antecedents in fact existed, and they were even numerous. With the labors which contributed to the constitution of the definitive method were associated the names of Sir John Herschel, Talbot, Miller, Wheatstone, Swan, Masson, Foucault, etc.; but Kirchhoff and Bunsen were able to make a synthesis of all these efforts, and they gave the method its general and practical form. When spectrum analysis presented itself to the scientific world, it held in one hand cresium and rubidium and in the other hand a list of the metals recognized in a star ninety-three million miles away. Why, then, should we be surprised at the enthusiastic reception that was given it?
At first it was believed that the incandescence of gases was one of the conditions of elective absorption by them. A French physicist, judging that the phenomenon related rather to the gaseous condition than to the temperature, was led to believe that the earth's atmosphere, as well as the atmosphere which was supposed to exist around the sun, might exercise such an action; and he showed that the solar spectrum contained a whole system of dark and fine lines, comparable to the lines of solar origin, but which were due to the action of our atmosphere. Brewster had already discovered that the solar spectrum was enriched with dark bands at sunrise and sunset, but that in his instrument they wholly disappeared during the day. Both Brewster and Gladstone, his eminent co-laborer, declared in their last memoir on this subject, in 1860, that they could not determine the cause of the phenomenon.
The absorbing action of our atmosphere was still more plainly demonstrated by an experiment on the Lake of Geneva, in which the absorption rays were obtained with the light of a fire passing over Lake Leman, from a distance of fourteen miles. It was also shown in an experiment made at Villette, with a tube filled with vapor at seven atmospheres of pressure and one hundred and twenty feet long, that the vapor of water has a complete absorption spectrum, and that the largest proportion of the absorption phenomena of our atmosphere should be attributed to it.
These observations and experiments doubled the field of investigation opened to spectrum analysis. Not only could the incandescent atmospheres of the sun and the stars now be made to reveal their nature and their composition to us, but our researches might also be extended to objects having a still greater interest for us. We could at once take our own atmosphere for an object, investigating high and inaccessible regions, and making analyses in them which could not be attempted by any other means. Then, going away from the earth, we could interrogate the planetary atmospheres, and seek in them the vapor of water, and with it one of the first conditions of the development of terrestrial life. We could also, comparing the composition of the planetary atmospheres with the astronomical facts which permit us to judge of the geological conditions of the surfaces of the planets, follow in them the atmospheric evolutions which on the earth belong to the domain of the past or of the future. Finally, the same study of the planetary atmospheres, when it shall have become more complete, will show us whether our atmosphere is a type reproduced everywhere, the composition of which appears from that fact indispensable to the existence of living beings, or whether, discovering atmospheres of varied compositions, we shall be led to suppose that life may appear and be developed in media essentially different. The planetary stars are not, however, the only ones that lend themselves to these applications. There are also fixed stars the spectra of which reveal the characteristics of the vapor of water. Now, we know that the atmosphere of a star must be considerably cooled to permit the gases of which water is constituted to combine and generate a vapor. Our sun is still very far from this critical condition. It is also remarkable that the stars presenting these characteristics are generally red or yellow ones. In this manner the spectroscope may help us to estimate in some degree the age of a sun, and measure the length of the career which it has already accomplished.
While studies of this kind were going on in France, spectrum analysis was receiving magnificent developments in England, more in the line which its authors had indicated. Messrs. Miller and Huggins entered upon the study of the stars, and found in all of them which they examined the solar elements in various combinations. This discovery had an immense philosophical bearing, for it proved that the matter forming the solar and the stellar world is obtained from the same elements. It was a demonstration of the material unity of the universe. The study was prosecuted still further. There are stars which we regard as situated on the confines of the visible universe, the light of which is so weakened by the immense journey it has to make to reach us that they appear only as feeble glows. Mr. Huggins succeeded in analyzing some of them, and showed that there exists a whole class of nebulæ which are really unresolvable into stars, and are formed of incandescent gases, among which hydrogen, which thus seems to be the principal element in the composition of the universe, is the most prominent.
So the whole visible universe—not only our central star and the planets of our family, but those stars, too, which are so far off that our most powerful telescopes can not give them a sensible diameter, and those nebulæ which only make a weak glow in our instruments—is reached by our chemistry, seized by our analysis, and made to furnish the proof that all matter is one, and that these stars are made of the same stuff as we. More, still, than this: at those great distances, and in the presence of the vague and indefinite forms of the nebulæ, it would not be possible to study precise movements and discover whether the great law of gravitation reigns in such remote regions. Chemistry here comes to the aid of mechanics, and we may say boldly that that matter, which is identical with ours, is subject, like it, to the laws of gravitation. Certainly, when Newton decomposed a beam of white light, and laid the first basis of the theory of the spectrum, he had not the slightest suspicion that his law of gravitation would, at a later period, find in it wings to carry it into regions where all measurement ceases and all calculation is powerless.
Spectrum analysis, after having in this manner, in a few years, gone through the universe and reaped the magnificent harvest I have just described, now returns to the sun, the point whence it departed, to take advantage of the opportunity afforded by eclipses. These phenomena, it is well known, exhibit a collection of magnificent spectacles of an extraordinary character, which had heretofore remained unexplained. Those rosy-colored protuberances of strange forms which surround the dark limb of the moon, that magnificent luminous corona, those radiances in the form of a glory and extending to enormous distances—all constituted so many riddles for astronomers till 1868. In that year one of the great eclipses of the sun took place. We might say that, at the very moment when the heavens had just suffered their most precious secrets to be revealed, the star of day had deigned to invite us to the study of his admirable structure.
The eclipse was observed, and the result surpassed even the general expectation. The nature of the protuberances was immediately recognized, and a method was discovered that permitted the study of these phenomena every day, without having to wait for the rare occasions of eclipses. This method led in a short time to the discovery of the chromospheric atmosphere, and this completed and explained the phenomena of the protuberances. The first results of the spectroscopic investigations may be stated thus:
The sun of Herschel and Arago, formed of a central nucleus and a luminous envelope, the photosphere, has an additional stratum formed chiefly of incandescent hydrogen. This stratum, in immediate contact with the photosphere, is very thin, being only from eight to ten seconds thick; it is the seat of small eruptions of metallic vapors rising from the photosphere, in which sodium, magnesium, and calcium predominate. Frequently, however, principally at the time when the sun-spots become abundant, there rise from the solar globe formidable eruptions of hydrogen, which pass through this same envelope and rise to a height of sixty thousand or ninety thousand miles. These eruptions are the protuberances of the total eclipses, the nature of which is thus revealed and the forms explained.
The corona and the phenomena exterior to it were the objects of study in the next eclipses. In 1874 French observations showed that the corona constituted a new solar atmosphere, a very rare one and enormously extended, in which hydrogen still dominated, while it presented spectral conditions as yet unexplained. This atmosphere seemed to borrow a part of the appearances of the protuberance-eruptions which penetrate it and are extinguished in it. It also seemed probable, and that opinion was expressed by the author of these observations, that the figure of the corona would vary with the condition of external activity of the sun. At the times of the maximum of spots, when the protuberance-eruptions were in the highest activity, the coronal atmosphere would be intersected by numerous and rich jets which would increase its extent and density, and change its aspect. This opinion was confirmed by one of the observers of the last eclipse in Egypt.
I shall conclude this brief review of the methods of physical astronomy with a word upon an art which has recently brought a really wonderful aid to all our scientific studies—I mean photography. Considered in its old and primary object, the aim of photography is to fix the images of the camera-obscura. Its aim, however, and its means have been singularly extended. We have to consider here only the aid and the applications which physical astronomy can expect from it.
The first application which was made of photography to the study of the sky was in France, whatever may be said about it. The first image of a fixed star upon the daguerrean plate was that of the sun, and it was taken by the authors of the admirable processes for measuring upon the earth the velocity of light—MM. Fizeau and Foucault. Shortly afterward, images of the moon were obtained in the United States. These labors were followed by others, of which the sun and the moon were also the objects. Beautiful proofs of lunar photographs have been taken by Mr. Warren de La Rue and Mr. Rutherfurd. Photographs of the sun are taken regularly in many observatories, as aids in the study of the spots and faculae of that star. More recently, Mr. Rutherfurd and Mr. Gould have begun to make celestial maps, and photographs of the nebula in Orion have been obtained in New York (by Mr. Draper) and at Meudon.
These works are all very important; they bear upon the primary object of astronomical photography, that of obtaining durable and trustworthy images of the stars and the phenomena produced upon them, available for further studies and measurements. Hitherto, observers had only memory, a written description, or a drawing, to depend upon for the preservation of the recollection of a phenomenon. Photography substitutes for this the material image of the phenomenon itself. It is an admirable artifice, which in a certain manner prevents the extinction of the phenomenon and its passage to among the things that were, and keeps it always with us for examination or study. Important as these results may be, the latest labors of which photography has been the object, especially in what concerns the sun, have demonstrated that the method may be employed as a means of making discoveries in astronomy.
The large solar images which have been obtained in the latest years at Meudon have revealed phenomena of the surface of the sun which our largest observatory instruments could not have shown, and which open a new field of studies. By their aid we can at last distinguish the real form of those elements of the photosphere, respecting which so many different and contradictory assertions have been made. The elements in question are composed of a fluid substance readily obedient to the action of external forces. At points of relative calm, the matter of the photosphere assumes forms more or less approaching the spherical, and its aspect is that of a general granulation. Where-ever, on the other hand, currents and more violent movements of the matter prevail, the granular elements are more or less drawn out, and present aspects suggesting the forms of grains of rice, willow-leaves, or veritable threads. The regions, however, where the photosphere is more agitated, are limited spots, and the granular form is generally observed in the intervals between them. The result of this peculiar constitution is, that the surface of the sun presents the aspect of a network, the web of which is formed of strings of more or less regular grains, with here and there elongated bodies drawn out in all directions. An attentive study of these curious phenomena leads us to a very simple explanation of them.
The stratum of luminous matter to which the sun owes its power of radiation is, as we know, very thin. If this stratum was in a state of perfect equilibrium, the fluid matter of which it is constituted would form a continuous envelope around the nucleus of the sun; and the granular elements being confounded together, the solar surface would have everywhere a uniform brightness. But the ascending currents, of which the eruptions of metallic vapors and the hydrogen-protuberances are evidences, rupture the fluid stratum which is tending to form at a great number of points. It is then broken up and divided into more or less considerable fragments. Wherever the perturbing forces leave the elements of the photosphere in a state of relative repose, they take a more or less pronounced globular form. At those points, on the other hand, which are the seats of ascending currents, these elements give evidence in their aspect of the violence of the actions to which they are subjected. Hence the variable forms of the elements of the photosphere, concerning which there has been so much discussion. Hence, also, the explanation of that net-work-like structure of the solar surface which has been revealed by photography.
These images also show the enormous difference between the luminous power of the elements of the photosphere and that of the medium in which they float, which seems quite dark by the side of them. A result of this constitution is, that the radiating power of the sun will be affected according to the number and brightness of these elements. The spots, then, can no longer be regarded as the principal element in the variations of the solar radiation; a new factor, the action of which may be preponderant, must hereafter be added to them.
These photographs permit another study, which promises results of extreme importance—the study of the motions which the granular elements take on under the action of the forces that rumple the photosphere. For the study of these motions, successive images of the same point on the surface of the sun are taken at very brief intervals with the photographic revolver. A comparison of the images demonstrates that the matter of the photosphere is animated by movements of the violence of which our terrestrial phenomena can convey only a very feeble idea.
Following the example of spectrum analysis, photography is making a circuit of the heavens. The year 1881 witnessed the first taking of the photograph of a comet, with a considerable portion of its tail. This picture has revealed some curious particulars of structure and has permitted a number of photometric measurements, the most notable of which is one showing that the tail, notwithstanding the brightness with which it appears to shine, is, at only a few decrees from the nucleus, two or three hundred times less luminous than the moon. There will doubtless be room enough to perfect these first efforts, for it will be of the highest importance to obtain by photography incontestable documents for the history of these stars, the nature of which still presents so many enigmas.
Equally interesting efforts have been made with respect to the nebulæ. Mr. Draper, in America, and the observatory at Meudon, have obtained photographs of the nebulæ in Orion. The nebulæ are of great importance in their bearing on the theory of the formation of stellar systems and the genesis of worlds. It would be immensely interesting to establish clearly the existence and the nature of changes going on in their structure, and good photographs of them would be valuable for this. They are, however, difficult subjects, on account of the extreme weakness of their light, the uncertainty of their outlines, and the variations of brightness in their different parts. Consequently, we are liable to have images of the same nebulæ, in no way comparable with each other, but varying according to the length of the exposure, the clearness of the sky, and the sensitiveness of the plate; and it becomes imperiously necessary to define the conditions under which the images are obtained.
The images of any object impressed by light upon the eye are fugacious, and can be of only a limited intensity. The images fixed upon the photographic plate are permanent, and can be made of an intensity that becomes cumulative with the duration of the exposure. The photographic retina may be expected, when the art has been perfected in the highest degree, to give us images corresponding with an extremely expanded range in the duration of the exposure. We now obtain photographic impressions of the sun in the one hundred thousandth part of a second, and can not yet guess what the final limit will be in the direction of brevity. On the other hand, the images of the comet required an hour, and that of the nebulæ in Orion more than three hours of luminous action. Thus the luminous action was more than five hundred million times as long in the last case as in the first. What phenomena can have wide enough ranges in brightness or obscurity to escape so admirable an elasticity?
The photographic plates, moreover, which are prepared now, are not only sensitive to all the elementary rays which excite the retina, but the power also extends into those ultra-violet regions and the opposite regions of dark heat in which the eye has no power.
The priceless advantages which photography offers for the prosecution of our experiments are, in short, the preservation of the images, the extension of sensibility, and the faculty of seizing phenomena of the most different degrees of illumination, including the extremely strong and the extremely weak.
The above is a very incomplete picture of what has been accomplished by physical astronomy. Is it not enough, however, to show that our branch of the science has already attained the height of its elder sister? Are not the two worthy of each other, and will they not be able to march hereafter at an equal pace to the conquest of the heavens? On one side we behold the calculus—that marvelous intellectual lever, putting the data of observation to work, and drawing from them magnificent and unexpected consequences. On the other side, that wonderful apparatus which analyzes light as if it were matter, which forces it to give images of near and distant objects alike, and, seizing the fugitive images, makes them fixed and durable.
Behold on one side, again, the mathematical genius that has created the analysis of the infinite, a genius of exactness and thoroughness, which is able to enter into all the elements of a question and disengage from the complication of data the ultimate consequences signified by them. On the other side, the genius of observation, which now watches phenomena with the innate and superior sense that enables it to discover their intimate relations, now questions Nature and carries on its experiments as the geometrician carries on his analysis when he wishes to prove or discover something, and now, illuminated by a sudden inspiration, makes at a stroke one of those approaches that open immense horizons.
On one side behold, finally, the heavens measured, the solar world placed in the balance, and its movements so well linked together by the law that governs them that soon, perhaps, past, present, and future will no longer exist for astronomy. On the other side, wonders still more astonishing: stars revealing to us their forms and the most minute details of their structure, as if they had left the depths of space to offer themselves submissively to our study; worlds intrusting the secrets of the matter that engenders them to the rays which they send us; and the history of the sky written by the sky itself. Finally, by the united efforts of the two, the entire universe, in its majesty and its grandeur, become the intellectual domain of man.
- President's address at the French Association for the Advancement of Science, La Rochelle, August, 1882.