In the High Heavens/Chapter 7

by Robert Stawell Ball

CHAPTER VII.

POINTS IN SPECTROSCOPIC ASTRONOMY.

 

HE annual meeting of the British Association in 1891 had a peculiar interest for astronomers, inasmuch as the assembly at Cardiff was gathered together to hear an address on the subject of Modern Astronomy from the lips of one who is admittedly the founder of a great branch of astronomical physics.

There is no Englishman, there is no man of any other nation, who could speak with the same authority as Sir William Huggins on the achievements of the spectroscope in the exploration of the heavens. To realise fully what he has done we must contrast our present knowledge with the knowledge that was possessed thirty years ago. Up to the middle of the present century the progress of astronomy along the older lines had no doubt been marvellous. The discovery of Neptune had illustrated in a forcible manner the completeness of mathematical astronomy. The movements of the planets had become so thoroughly understood that, though here and there small discrepancies were recognised, yet it seemed that the difficulties remaining to be vanquished were only akin to those which had been already overcome. More comets no doubt could be found, more minor planets were being constantly discovered, but the older methods did not supply much fresh intellectual pabulum. They provided, it is true, additional material for the application of well-known formulae; they required the computation of tables similar in scope to scores of other tables that were already in hand. But it certainly seemed that if astronomy was to sustain the high interest that it had always possessed, some new departure was necessary in order that the science might exhibit that growth which seems to be an essential requisite of vitality. It was about thirty years ago that the much needed advance was made which opened up to research a vast department of science of a totally unexpected character.

Comte was one of those who, in alluding to the probable exhaustion of attainable astronomical science, indicated some problems which were apparently beyond the reach of our powers. We might, he surmised, find out much with regard to the movements of the heavenly bodies, we might survey their distances, measure their dimensions, and appraise their weight; but, said Comte, to find out their material composition or to learn the actual chemical elements of which they are composed, this problem, though it would be pregnant with interest for us, we could not but despair of solving. It was not many years before this rash assertion was disproved by the splendid discoveries which, to the astonishment of the world, explained the meaning of the dark lines in the solar spectrum, and demonstrated the existence of iron and other well-known metals in our great luminary.

It is essential to the right understanding of the subject to comprehend adequately the enormous accession to our knowledge which this indicated. Chemists had studied the structure of our globe for centuries; they had ascertained that it was composed of some sixty or seventy elements; but they knew nothing as to the composition of the heavenly bodies. The sun, moon, and stars might, for anything we knew at that time, be composed of elements quite as unknown to us as lithium or any other rare metal was to Aristotle. The only indication of the chemical composition of bodies external to the earth was obtained from meteorites. It was, indeed, noted with interest that meteorites contained no elements except those which were already known to exist on the earth. The origin of meteorites was, however, at that time too obscure to enable any sound inference to be drawn about the composition of the celestial bodies generally. Indeed it might have been urged with much force that as the meteorites had been falling on the earth for countless ages an appreciable proportion of the materials on the earth's surface may have been accumulated from this source, so that the meteoric elements must be already discoverable in the list of terrestrial substances. In fact, we knew absolutely nothing about the composition of the globes external to the earth, and any information that was forthcoming on this subject was thus presented in the light of a revelation.

I do not here attempt to give any historical account of the discoveries. My only object is to indicate the position which Sir W. Huggins occupies, so as to comment on the address which he so fitly delivered at Cardiff. It is natural in this connection to refer to the lecture which Huggins delivered at Nottingham before the British Association in 1866, On that occasion, as some of those who listened to him at Cardiff may perhaps remember, he described the memorable discoveries by which he extended the methods of spectrum analysis to several of the heavenly bodies. He showed the spectra which he and the late Professor Miller had already succeeded in obtaining of some of the brightest stars, notably of Aldebaran and Betelguese. He had measured the dark lines with which the spectra of these stars were crowded, and it was shown by their positions that certain well-known terrestrial substances must be present in those distant luminaries. In reference to many of these elements the coincidence is based not on one line but on several lines, so that it is impossible to shake the testimony which the spectroscope affords as to the identity, in part at all events, of the constituents of the stars with the materials in the solar system.

On referring to this memorable lecture of 1866, it is indeed surprising to find how discoveries seemed to crowd together at the commencement of Sir William Huggins' career. He had at that time noticed the characteristic spectrum presented by white stars, of which Sirius is one type, and had demonstrated the existence of hydrogen in stars of this class. He had also examined coloured stars, like Alpha Herculis, and had found them to exhibit a spectrum, in which portions of the coloured bands are subdued by strong groups of lines in such a way as to afford an explanation of the hues which these stars display. He had demonstrated in the case of Beta Cygni that sufficient lines are found in the blue and violet parts of the spectrum of the large star to make the red and yellow rays predominate, thus giving to the lustre of the larger star of this celebrated pair a hue that is often known as
Fig. 23.—The Region of the Milky Way about β Cygni. Showing twelve bright stars.

topaz colour. On the other hand, the small and delicate blue companion shows a spectrum in which the strongest groups of lines occur in the orange, yellow, and part of the red.

There is no more pleasing phenomenon in sidereal astronomy than that presented by the contrasted hues often exhibited by double stars. It was, however, always in some degree a matter of uncertainty as to how far these varied hues were to be regarded as actually indigenous to the stars, for it seemed not at all impossible that there might be some optical explanation of colours so vividly contrasted emanating from points so contiguous. It was also remembered that blue stars were generally only present as one member of an associated pair, and it was thought, not it must be confessed without plausibility, that the blue hue which was exhibited might have arisen from some subjective cause, or at all events that it did not necessarily imply that the star actually possessed a bluish colour. When, Sir William Huggins showed that the actual spectrum of the object demonstrated that the cause of the colour in each star arose from absorption by its peculiar atmosphere, it became impossible to doubt the reality of the phenomena. Since then it has been for physicists to explain why two closely neighbouring stars should differ so widely in their atmospheric constituents, for it can be no longer contended that their beautiful hues arise from an optical illusion.

Another achievement in the early part of Sir William Huggins' career is connected with the celebrated new star that burst forth in the Crown in 1866. It seemed a fortunate coincidence that just at the moment when the spectroscope was beginning to be applied to the sidereal heavens, a star of such marvellous character should have presented itself. I well remember going with Lord Rosse in 1856 to pay my first visit to Sir William Huggins in Tulse Hill. One of the objects he showed us was the spectrum of this star, which on the 12th of May in that year suddenly burst forth with a lustre of the second magnitude in the constellation of the Northern Crown. At the time of my visit the star had considerably declined from its original radiance. The feature which made the
Fig. 24.—The Region of the Milky Way about β Cygni. Showing sixty stars.

spectrum of the new star essentially distinct from that of any other star that had been previously observed was the presence of certain bright lines superposed on a spectrum with dark lines of one of the ordinary types. The position of certain of these lines showed that one of the luminous gases must be hydrogen. It is impossible to dissociate the spectroscopic evidence from the circumstances known in connection with this star. The spectroscope showed that there must have been something which we may describe as a conflagration of hydrogen on a stupendous scale, and this outburst would account for the sudden increase in luminosity of the star, and also to some extent explain how so stupendous an illumination once kindled could dwindle away in so short a time as a few days. Viewed in the light of much later work, these early discoveries assume an increased significance.

If we were to choose that one of Sir William Huggins' achievements which gave the widest extension to our knowledge, I think we can hardly hesitate to select what Romney Robinson long ago called the "palmary discovery" of the spectrum of a nebula. It was here that in the most emphatic sense Sir W. Huggins broke new ground. The stars were known to be bodies more or less congenerous with our sun; and up to the time of which I am speaking, about a quarter of a century ago, nebulæ were often looked upon as clusters of stars too distant for us to perceive the rays from each individual point. In fact, with the erection of each great telescope the test of its performance was generally sought in its power to "resolve" nebulæ, as the process used to be called. It is true that many nebulæ wholly refused to disintegrate, but it was generally, though not universally, thought that, with increased power, even the most refractory nebula would exhibit itself as a mere cloud of stars. Remembering this fact, and remembering also the faintness of these mere stains of light, it may be readily believed that when Huggins first allowed one of these objects to throw its gleam on the slit of his spectroscope, he did not entertain much hope that this instrument, though so potent elsewhere, would avail to interpret such a dim object. If
Fig. 25.—The Region of the Milky Way about β Cygni. Showing thousands of telescopic stars.

the nebula was of the same order as stars which had been observed, then its light would be expanded by the prism, and weak as the light was at the beginning it would become much weaker still when spread out in the act of dispersion. When, however, Huggins found that he could see light in the spectroscope he so little realised the nature of his important discovery that he thought for the moment what he saw must have had its origin in some maladjustment of his apparatus. But it was not so. He discovered that the nebula he was looking at, as well as many other objects of the same class, was not a mere distant cluster of stars, but that they were masses of glowing gas.

The action of the prism on light from a star is utterly different from its action on the light emitted from glowing gas. In the former case the light is spread out into the long band displaying the rainbow hues if bright enough; in the latter case the light is condensed into one or more luminous lines. The light from the gaseous nebula is exhibited by the spectroscope in a number of bright lines instead of being spread out over the entire length of the spectrum. That nothing should be wanting to complete this splendid contribution to our knowledge of the universe Sir William Huggins discovered the nature of the gases which glow in these faint bluish nebulæ. Even at this early period he succeeded in establishing the existence of hydrogen in these remote regions of space.

The important discoveries we have named may be said to have initiated the application of spectroscopic research to the sidereal heavens. The address that Huggins delivered at Cardiff presents a splendid picture of the harvest of discoveries by this time accumulated. It is natural that so attractive a field of research should have engaged the co-operation of many zealous explorers. To their labours the address rendered ample justice.

At the present moment the attention of the astronomical world is especially directed towards the development of the resources of photography in the various applications which it has to their art. Already the camera has become an indispensable adjunct in the observatory, and we are every day learning more and more of what it can do for us. The chemical eye is often more sensitive than the human eye; it is always more patient. It will display for us a magnificent nebula like that which surrounds the Pleiades, and which is wholly invisible to the unaided eye, and only to be seen with the telescope under very special conditions not often realised. Naturally, Huggins has discussed at length the applications of photography. It would be impossible for him not to have mentioned that photograph obtained by the late Dr. Roberts of the great nebula in Andromeda, which was produced by exposing a highly sensitive plate for four hours in the focus of a powerful reflector. The result has been to produce a picture which has been said, and I believe with truth, to be the most suggestive representation of any celestial object that has ever been obtained. Features which had been dimly traced in the nebula when visually examined in powerful telescopes are now seen to be parts of an organic whole, visible on the photograph, though not otherwise discernible by the keenest sense.

Such a study of this great nebula was all the more acceptable because it is one of the most baffling of these objects. It is bright enough to be perceived by the unaided eye, and it might have been expected that so striking a celestial structure ought by this time to have disclosed its character either as a distant cluster of stars, or as a truly gaseous object. Herschel long ago called it one of the least resolvable of the nebulæ, but yet it does not appear to possess a spectrum similar to that of the gaseous nebulæ of which we have been speaking. The character of this object, both as to its actual physical nature and as to the materials present in it, is at present undetermined. This consideration lends a certain amount of mystery to Dr. Roberts' great photograph, a mystery we do not feel to a corresponding extent when we look at the photograph of Andromeda's only rival, the great nebula in Orion. The pictures of the latter exhibit a glorious object which is certainly known to be gaseous, and we have also the assurance that hydrogen is among the materials of which it is composed.

No part of Sir William Huggins' address was better than that which treated of the exquisite application of the spectroscope to the discovery of the movement of approach or movement of recession in the object from which the light emanates. In fact there is no passage in the address which seems to me more pregnant in significance than that in which he remarks that: "In the future a higher value may indeed be placed upon this indirect use of the spectroscope than upon its chemical revelations/' As to the accuracy of this method, it enables us, under favourable circumstances, to measure speed of recession or approach "to within a mile per second, or even less." What this means is that such a speed as that of the revolution of the earth in its orbit around the sun could be determined to within five or six per cent, of its amount. It is to Sir W. Huggins himself that we are indebted for the first application of this principle to astronomical measurement. The earliest observations were made by him in 1868, but for many years the application of this method was retarded by a want of perfection in the instruments necessary for so delicate a branch of research. However, such improvements have been made within the last two years, by means of photography, at Potsdam, and by eye observations at the Lick Observatory, that the method has been elevated to a precision that entitles its measurements to the respect which has always been accorded to those made by the appliances of the older astronomy.

Professor Vogel at Potsdam photographs a small part of the spectrum of the star in the vicinity of the line G, and for the purpose of comparison introduces with all needful precaution the hydrogen line in that neighbourhood. For certain stars he has recently used some of the lines of iron. The result we must give in Sir W. Huggins' own words. "The perfection of these spectra is shown by the large number of the lines, no fewer than 250 in the case of Capella, within the small region of the spectrum on the plate. Already the motions of about fifty stars have been measured with an accuracy, in the case of the larger number of them, of about an English mile per second."

In a method of such delicacy, involving results of so great interest, it is obviously desirable to have confirmatory measures made under circumstances as widely different as possible. These have been forthcoming from the Lick Observatory in California, thanks to the late Professor Keeler, at that great institution. He has succeeded in obtaining determinations, by direct eye observation with superb instruments, and he has found it possible to execute measurements of a spectrum with an accuracy as great as that obtained by Professor Vogel. The result is so significant that we must again give it in the words of Huggins:

"The marvellous accuracy attainable in Professor Keeler's hands on a suitable star is shown by observations on three nights of the star Arcturus, the largest divergence of Keeler's measures being not greater than six-tenths of a mile per second, while the mean of three nights' work agreed with the mean of five photographic determinations of the same star at Potsdam to within one-tenth of an English mile. These are determinations of the motions of a sun so stupendously remote that even the method of parallax practically fails to fathom the depth of intervening space, and by means of light waves, which have been, according to Elkin's nominal parallax, nearly 200 years upon their journey."

It is impossible for any lover of astronomy to read of these achievements without some emotion. The alliance between photography and spectroscopy is here rendered available for extending our knowledge of the movements of the heavenly bodies in a direction wholly inaccessible to every other appliance of the astronomer. I may mention one of the points in which the importance of the new method can hardly be overrated. In the older process of ascertaining the proper motions of stars, the lapse of long periods of time was indispensable. A star would have to possess a movement more rapid than that of any of the stars, except a very few, if it could be determined by our meridian instruments in less than a twelvemonth. In the majority of cases an interval of many years would be necessary before the movement of the star could be certainly concluded from such measurements. With such small movements as those possessed by most of the stars, various causes combine to render the measurements highly uncertain; and yet for astronomers who desire to learn the constitution of the heavens, there would be no information more valuable than copious and accurate knowledge of the proper motions of the stars. It seems from these discoveries at Potsdam and at Lick that we may now entertain a hope that abundant and accurate information of the character that I have indicated will be promptly forthcoming.

The researches of Professor Keeler at Lick have already afforded us some information with regard to the proper motions of the nebulæ in the line of sight. Here, indeed, an entirely new departure has been made. Most of these objects are so ill-defined that their position cannot be measured, or cannot by ordinary methods be even specified with the accuracy necessary for the determination of their proper motions. The vagueness of nebulæ is not, however, a bar to the application of the spectroscope in the measurement of its movements in the line of sight. We still know nothing as to the movements of nebulæ athwart that line. But it is something for us to have obtained information as to the progress of these bodies in one direction at all events. An attempt was made to solve this problem a good many years ago by Sir W. Huggins himself; but the apparatus that was then available did not possess the refinement necessary for measurements so delicate. The resources of the splendid equipment at Lick have provided what is required, and Prof. Keeler has ascertained the movements of some nebulæ. As an illustration of his results, we may take the famous nebula in Orion. He finds that this object is retreating from our system at the rate of about ten miles a second. The most rapid movement he has yet discovered in one of these nebulous objects is a pace of forty miles a second.

Among the problems which the spectroscope has yet failed to solve must be mentioned that of the Aurora Borealis. No doubt something has been learned; but still it must be confessed that the prism has been more successful up to the present in its application to objects which lie like the nebulæ on the very confines of the visible universe, than it has to the aurora, which is, comparatively speaking close at hand. Sir William Huggins gave a summary of our knowledge on this subject. It is certain that the glow of the aurora is in the main due to the effect of electric discharges in the upper regions of the atmosphere. Seeing that we are familiar with the spectra of the atmospheric gases, as produced in our laboratories, it might have been expected that the interpretation of the spectrum of the aurora would be a comparatively easy task. We are still ignorant of the source of the principal line in the green, which, as Huggins remarked, may have an origin independent of that of the other lines.

He also referred to the supposition that the aurora is produced by the dust of meteors; but with reference to this, he noted that experiment has shown that fine metallic dust suspended in gases conveying an electric discharge like that of an aurora will not cause the spectrum to exhibit the characteristic line of the metallic dust in question. There is much to be said for Professor Schuster's suggestion that the principal line in the aurora may be due to some extremely light gas which is present in too small a relative quantity in the lower strata of the atmosphere to permit of its existence being disclosed by spectroscopic or any other form of chemical analysis. In the upper regions where the auroral displays take place, the ordinary gases have assumed extreme tenuity, and the lighter gas becomes of more relative importance, and gives a character to the spectrum.

As it is instructive to learn, as far as may be, the boundary between the known and the unknown, it is interesting to read what Sir W. Huggins has to tell us about the solar corona. The nature of this marvellous appendage to the sun is still a matter of uncertainty. There can, however, be no doubt that the corona consists of highly attenuated matter driven outwards from the sun by some repulsive force, and it is also clear that if this force be not electric it must at least be something of a very kindred character. Dr. Schuster suggests that there may be an electric connection between the sun and the planets. In fact, with some limitations we might even assert there must be such a connection. It is well known that great outbreaks on the sun have been immediately followed, I might almost say accompanied, by remarkable magnetic disturbances on the earth. The instances that are recorded of this connection are altogether too remarkable to be set aside as coincidences. Sir William Huggins has not referred in this connection to Hertz's astonishing discoveries; but it seems quite possible that research along this line may throw much light on the subject, at present so obscure, of the electric relation between the sun and the earth. So far as the spectrum of the corona is concerned we may summarise what is known in the words of Huggins: "The green coronal line has no known representative in terrestrial substances, nor has Schuster been able to recognise any of our elements in the other lines of the corona."

Sir William Huggins regarded it as surprising that our first accurate knowledge of the spectrum of hydrogen should have been ascertained not from a course of refined laboratory experiments, but from photographs of the spectra of the white stars to which Sirius belongs. Hydrogen has a few visible lines in its spectrum, and the photograph shows that these belong to an organized system of lines which are wonderfully displayed in the spectra of the white stars, first fully obtained by Sir W. Huggins. The hydrogen spectrum possesses a special interest, inasmuch as Dr. Johnstone Stoney many years ago pointed out that the three principal visual lines were members of a harmonic series, and the interesting discovery has been since made by Professor Balmer that a more comprehensive law includes both these harmonic members and the rest of the series. Thus the hydrogen spectrum appears to present a simplicity not found in the spectrum of any other gas, and therefore it is with great interest that we examine the spectra of the white stars, in which the dark lines of hydrogen are usually strong and broad. In stars of this class we often look in vain for those dark metallic lines so characteristic of other stars which have a nature more nearly resembling our sun.

The question is also discussed as to whether the radiance characteristic of the white stars may be regarded as an indication of an extremely high temperature as compared with that shown by other stars. It seems hardly possible to doubt that such a star as Sirius owes its great lustre not merely to its size, but also to its intrinsic brilliancy, indicative of a high temperature. It may illustrate the attention that has been paid to the spectra of the white stars to refer to some interesting observations of Scheiner; he has found that the objects of this class which are in the constellation of Orion agree in possessing a certain dark line, which appears to coincide in position with one of the bright lines in the famous nebula in the same constellation. He remarks that, with the single exception of Algol, he has not observed this same line in any other white star. These observations naturally suggest the remark that the stars in the constellation of Orion possess a certain affinity beyond that implied by their proximity in the same constellation. They are apparently a group associated by community of composition. In considering this circumstance we are reminded how the Great Nebula, with every increase of optical power and every increase in the period of exposure of the photographs, seems to cover an ever-widening area, extending, as we now know, so as to include several of the bright stars.

Still one more application of the spectroscopic method of measuring movements in the line of sight is found in Dunér's beautiful observations of the limb of the sun. By comparison between the approaching edge and the retreating edge he has been able to ascertain the velocity of the sun's rotation. It is not only interesting to find that these results corroborate the determinations already familiar by observations of the sun-spots, but the spectroscopic method admits of being applied to zones in the sun from which spots are absent. We thus obtain a very complete knowledge of the laws of rotation of our luminary. Dunér's measurements confirm the extraordinary fact that the equatorial regions in the sun accomplish a revolution in a shorter time than zones which are nearer to his poles.

The address of course gave some account of the progress of the combined effort to produce a great photographic chart of the heavens. About 22,000 photographs will be necessary, each covering a space of four degrees. Each star is to appear on two plates, so as to avoid errors, and by giving an exposure of rather less than an hour it is expected that all stars down to the fourteenth magnitude will be represented. Astronomers well know how large a share of credit is due to Sir David Gill in connection with this great work. This vast surveying task is only one of the pieces of astronomical work in which photographing the stars is now employed. In the delicate movements of annual parallax it has been proved that measurements made on the photographs can compare favourably with the finest measurements made on the heavens. We are only just beginning to realise the benefits from these photographic processes.

Sir William Huggins referred to the constitution of comets and their connection with meteors. Nothing is better established than the fact that the periodic meteor shower is a swarm of minute bodies revolving around the sun in an elliptic orbit, and that in the case of some of the greater showers, at all events, the highway pursued by the meteoric shoal is also the highway in which a great comet moves. That there is a connection between comets and meteors of this periodic class seems therefore unquestionable, though it does not seem easy to say what the precise nature of the relation may be. It is, however, especially necessary to observe the distinction between the ordinary luminous meteors and the solid meteorites which occasionally tumble down on the earth. It does not seem to be at all clear that meteorites have any connection whatever with comets. The meteorites do not stand in any ascertained relation to the periodic shooting-star showers. In fact, the only common feature which they may be said to possess is that they both come into the atmosphere from the outside. While, therefore, we must admit that such meteor showers as the Leonids are unquestionably connected with comets, yet we must distinctly hesitate to affirm that meteorites have any known relation to these bodies. On this matter Huggins has expressed himself with characteristic caution, though he acknowledged that there is some spectroscopic evidence which might be cited in support of the contention that the nucleus of the comet is not wholly different from the matter which falls down here as meteorites. With reference to the more characteristic features of comets, such as the rapid transformations which they undergo, and the marvellous tails which they shoot forth, the idea seems gradually developing that the phenomena are in the main of an electric character. Sir W. Huggins suggests that the recent discoveries of the electric action of the ultra-violet part of solar light may possibly help to explain the highly electrified condition of comets.

It would not be possible in a résumé of the achievements in modern astronomy to omit an account of the researches on the constitution of the sun made by the late Professor Rowland. He has shown that thirty-six terrestrial elements are certainly indicated in the solar spectrum, while eight others are doubtful. Fifteen elements have not been found though sought for, and ten elements have not yet been compared with the sun's spectrum. Reasons are also given for showing that though fifteen elements had no lines corresponding to those in the solar spectrum, yet there is but little evidence to show that they are absent from the sun. Sir William Huggins epitomises these very interesting results in the striking remark: "It follows that if the whole earth were heated to the temperature of the sun, its spectrum would resemble very closely the solar spectrum."

The science of the last century seems destined to be famous throughout the ages. To biologists it will be the century of natural selection; to physicists it will be the century of the spectroscope.