In this connection attention may be called to the great desirability of having a large telescope in the southern hemisphere, for the study of an extensive zone around the south pole which is still very largely unexplored. It is a misfortune, hitherto apparently unavoidable, that nearly all the principal instruments of the world are in the northern hemisphere, which includes the great civilized nations of the Earth, and the only peoples devoted to the cultivation of the sciences. The result is that a large space, beneath our horizon, round the southern celestial pole, including three eighths of the celestial sphere, and incomparably rich in objects of surpassing interest, is almost as little known as the antarctic continent. A few of the more obvious phenomena have been studied, either hurriedly or with inferior instruments, and enough attention has been given to the contents of that part of the universe to assure us of its exceeding richness; but there has been no general and exhaustive survey of that part of the sky, such as is demanded by the present state of our knowledge of the northern heavens. The largest telescope in the southern hemisphere is an eighteen-inch refractor at the Cape of Good Hope, where, unfortunately, the climate is so poor that little can be done in the way of discovery; while the northern hemisphere has at least twenty telescopes of greater power than any one in the southern hemisphere.
The dry climate and elevated plains of Peru offer atmospheric conditions probably unsurpassed on the face of the terrestrial globe; and this location above all others is to be recommended to the builders of our future great telescopes. Explorations in this region will be pioneer work; their value to the future progress of astronomical science will be priceless. The Harvard College Observatory, fully alive to the advantage of this southern location, already has a magnificent station at Arequipa, Peru, devoted to the photographic study of the southern stars and their spectra. Discoveries of the highest interest have recently been made at this site, which, it is interesting to note, was recommended by Alexander von Humboldt nearly a century ago. In his account of the exploration of the countries west of the Andes he points out that this is a dry and elevated plain, where the air is so steady that the stars scarcely twinkle when at any considerable elevation, but rather shine with a steady lustre, like the planets in our own climate. This steadiness of the atmosphere enables the telescope to perform to its full theoretical capacity, and would enable one powerful instrument in Peru to do more important work of discovery than a dozen great telescopes in the northern hemisphere.
So far as can now be estimated, it is safe to say that several thousand new stellar systems of great interest would be disclosed by an adequate exploration of the zone within sixty degrees of the south pole, which includes the constellations Scorpius, Centaurus, Lupus, Crux, Toucana, Grus, Eridanus, Corona Australis, Phoenix, and the great ship Argo, besides many less famous groups. The two wonderful Magellanic Clouds adorn this area, and the equally renowned voids known as the Coal Sacks. These latter are so named because they appeared to the early navigators as black holes in the densest portion of the Milky Way, near the Southern Cross. It is difficult to overestimate the high interest attaching to this part of the sidereal universe, which in point of variety of remarkable objects surpasses in importance every other portion of the celestial sphere. No area of the same extent in either hemisphere has so many promising objects for exploration, and no other portion of the sky is so truly a coelum incognitum. Under the circumstances, it cannot be considered singular that all astronomers hope for the early exploration of this interesting region by a powerful telescope, which will alone enable us to form a correct estimate of the extent and variety of bodies composing the material universe.
The observations since the time of Herschel show that the double stars obey the law of gravitation. This law, being established for many individual cases, is inferred to be true universally; and hence, in the few instances where certain anomalies appear, it is inferred that the regular motion is disturbed by unknown bodies, usually dark and wholly unseen. The discovery of double and multiple stars from the effects of the gravitational attraction on their luminous components is known as the "Astronomy of the Invisible." It was first suggested by the illustrious Bessel about 1840, to account for certain irregularities in the proper motions of the two dog stars, Sirius and Procyon; both of which have since been shown to be real binaries, the bright stars being in both cases attended by faint but massive satellites. More recently, Professor Seeliger, of Munich, Mr. Lewis, of Greenwich, the writer, and others have added to the Astronomy of the Invisible by showing that certain double stars are in reality triple, with one component yet to be disclosed. But the greatest extension of the Astronomy of the Invisible has been made by Professor Campbell, of the Lick Observatory. In the course of the regular work on the motion of stars in the line of sight, carried out with a powerful spectroscopic apparatus presented to the Observatory by Hon. D. O. Mills, of New York, he has investigated during the past five years the motion of several hundred of the brighter stars of the northern heavens. The velocities toward and from the Earth developed in different cases were, of course, very different; and with this splendid spectrograph, which Professor Campbell has used with decisive effect, the accuracy attainable is little short of marvelous. An error in the final result of one mile per second is quite impossible. With such unprecedented telescopic power and a degree of precision in the spectrograph which can be safely depended upon, it is not unnatural that some new and striking phenomena should be disclosed. These consisted of a large number of spectra with double lines, which undergo a periodic displacement, showing that the stars in question were in reality double, made up of two components, moving in opposite directions—one approaching, the other receding from, the Earth. There were thus disclosed spectroscopic binary stars, systems with components so close together that they could not be separated in any existing telescope, yet known to be real binary stars by the periodic behavior of the lines of the spectra so faithfully registered on different days by the powerful Mills spectrograph attached to the thirty-six-inch telescope at the Lick Observatory. Some of the more famous of these new stars are Capella, Polaris, Xi Ursae Majoris, Kappa Pegasi, Castor, Spica, Algol, Beta Lyre, and Eta Aquilae. In all, about fifty such stars are now known.
It appears from the investigations so far made that the brilliant star Capella is made up of two nearly equal components, which revolve in a period of one hundred and four days. The period of Polaris is about four days. In other cases the periods vary according to the objects: some being very short indeed, say only two days; others amounting to a considerable portion of a year, or even as much as three years in the case of Beta Capricorni.
It should be pointed out that these are not indeed the first spectroscopic binaries ever discovered. Professors Pickering and Vogel led in the initial search for these remarkable objects; yet with the means at their disposal they found only a few isolated examples, such as Beta Aurigae, Alpha Virginis, and Zeta Ursae Majoris. Campbell's work at the Lick Observatory derives increased importance from its systematic character, which enables us to draw some general conclusions of the greatest interest. He has thus far made known the results of his study of the spectra of two hundred and eighty of the brighter stars of the northern heavens. Out of this number he finds thirty-one spectroscopic binaries, or one ninth of the whole number of objects studied. Professor Campbell also points out that as some of the stars are multiple in character, composed of three or more components, with periods ranging from a few days to a year, or even several years, it cannot be assumed that all the spectroscopic binaries have been found in the first study of his photographic plates. In fact, it seems certain that a more thorough study will materially increase the number of spectroscopic binaries; and Professor Campbell thinks one sixth, or even one fifth, of all the objects studied may eventually prove to be binary or multiple systems. Such an extraordinary generalization opens up to our contemplation an entirely new view of the sidereal universe. If there be five or six thousand stars in both hemispheres which are sufficiently bright for study with the powerful apparatus now in use at the Lick Observatory, it will indicate that there are at least one thousand spectroscopic binary stars awaiting exploration—a number of stellar systems decidedly inferior, to be sure, to those of the visual class, yet undeniably impressive, and ample for furnishing us the general laws for all such objects, seen and unseen, throughout the immensity of space. If the labors of the next twenty years should give us accurate knowledge of even forty spectroscopic binaries, these would enable us to obtain a good estimate of the probable character of all such systems whatsoever. So far as they have been studied, it appears that the double stars observed visually in our telescopes are remarkable for two chief characteristics : (1) the high eccentricities of their orbits, which average about 0.5, or are twelve times larger than the eccentricities prevailing in the solar system; (2) the masses composing the systems, which are equal or comparable, not enormously disproportionate, like those of the planets relative to the Sun, or those of the satellites relative to the planets about which they revolve. Thus the stellar systems heretofore discovered are of a very different type from what we find in our own solar system, where the satellites are insignificant compared to the planets, and the planets insignificant compared to the Sun, and all the orbits nearly circular. And the number of such stellar systems, both visual and spectroscopic, appears to be truly enormous. Campbell finds that the general characteristics of high eccentricities and comparable masses, first attributed to double stars by the writer of these lines, some years ago, are true also of the spectroscopic binaries, which therefore are likewise of a different type from anything found in the solar system.
Since our telescopes do not enable us to recognize bodies anything like as faint as the planets attending the fixed stars, it is obviously impossible to affirm that no other systems similar to the solar system exist in the immensity of space; yet it is very clear that a vast number of systems of a radically different type are widely diffused. Some of these systems are self-luminous, like ordinary double stars; others probably are burnt out and already comparatively dark, so that they are correctly classed with the Astronomy of the Invisible; while yet others are spectroscopic in character, composed of one, two, or more associated bright and dark bodies revolving under the action of their mutual gravitation.
If we accept the conclusion that with our finest telescopes, in the best climates, on the average one star in twenty-five is visually double, it will follow from Campbell's work on some three hundred stars that five times that number are spectroscopically double. Thus, although over a million stars have been examined visually, and some five thousand interesting systems disclosed by powerful telescopes, the concluded ratio would give us, at last analysis, four million visual systems among the hundred million objects assumed to compose the stellar universe. On the other hand, the large ratio of spectroscopic binaries to the total number of stars examined by Campbell would lead us to conclude that in the celestial spaces there exist in reality no less than twenty million spectroscopic binary stars! Could anything be more impressive than the view thus opened to the human mind? Millions and millions of systems, of all sizes and representing all stages of cosmical evolution; with light, dark, and semi-obscure masses, all moving in orbits of considerable eccentricity, and by gravitational attraction generating in their fluid globes enormous bodily tides, which, working and reacting through the ages, modify the shape and size of the orbits and the stability of the systems! Since there are doubtless many millions of dark bodies, both large and small, as yet wholly unseen and even unsuspected, it seems not unreasonable to suppose that probably the great majority of the stars are in some way attended by satellites. The mass of matter composing the stupendous arch of the Milky Way is thus very much greater than has been supposed by those who have enumerated the stars disclosed by our telescopes, and computed the total amount of it on the assumption that all of the star dust is luminous.
It may indeed well be that the dark and unseen portion of the universe is even greater than that which is indicated by our most powerful telescopes. Half a century ago Bessel remarked: "There is no reason to suppose luminosity an essential quality of cosmical bodies. The visibility of countless stars is no argument against the invisibility of countless others."
If, therefore, certain stars are called "runaway " stars, because their velocities appear to be too great to be accounted for by the attraction of the luminous bodies composing the sidereal universe, we should perhaps ask whether the unknown mass of matter scattered throughout space as dark stars, comets, meteors, and nebulae might not, after all, account for the discrepancy. For my part, I am satisfied that it probably would, and that the universe is much more massive than has been generally supposed. In this fact will doubtless be found the explanation of the great velocities of the runaway stars.
These discoveries shed an interesting light upon the general theories of the material universe, and show that the ultimate exploration of the heavens has, in fact, only begun. Moreover, it is now recognized that the self-luminous stars are fluid masses, and therefore binaries are of necessity agitated by tidal oscillations. In considering some recent observations bearing on this subject, Campbell has found in certain subsidiary displacements of the spectral lines of a few binary stars evidence of the enormous tidal waves which sweep over their flaming globes.
It is well known that our original conception of tides arose from the oscillations in the waters covering the Earth, first noted by the early navigators of our seas. These periodic motions of the oceans were correctly explained by Newton in 1687. The theory of the tides has since been placed on an adequate mathematical basis by the labors of numerous geometers; and as the law of gravitation is shown to hold among the double stars, we assume that the rotations and orbital motions of such systems are disturbed by the gigantic tidal waves generated in their globes of flaming fluid. Some years ago I explained in this way the high eccentricities of the stellar orbits, and, following the younger Darwin, pointed out tidal friction as a physical cause operating with more or less effect throughout the heavens. Since the generation of bodily tides depends merely on the mutual attractions of two connected fluid globes, the resulting tidal effects are obviously as universal as gravitation itself.
For the natural philosopher to be enabled to ascend from the comparatively minute and unimportant oscillations of our terrestrial seas, generated by the attractions of the Sun and Moon, to the bodily tides in the stars composing the Milky Way, which are great pulsating globes of self-luminous fluid; and to trace in this manner the effects of tidal friction, which with the flight of ages has enlarged and elongated the orbits of double and multiple stars, is a generalization which at least need cause no feeling of humiliation! A chain of reasoning connecting such grand phenomena may justly impress a philosopher of any age or country as alike honorable and gratifying to the human mind. And since this achievement is of comparatively recent origin, it may be cited as a specific proof that all the great generalizations of nature are not yet accomplished. Far from it!
Though three hundred years have elapsed since the death of Tycho Brahe, and the scientific world has only recently joined in celebrating worthily his immortal memory, it appears that we are in many lines almost as far from the ultimate goal as when he began the great work of exploring the skies, before the days of Kepler, when all Europe was slumbering in intellectual darkness. The science of the stars, indeed, has been refined and perfected in an unparalleled degree, and infinitely extended in all directions; but with the bounds of darkness pushed back step by step, the goal is not, and never will be, in sight. An infinity of objects and causes and an endless variety of phenomena are yet to be explored, and the work of the mind is rather a process of development to the perfect understanding of the universe than the solution of a simple mathematical problem. We cannot therefore subscribe to the doctrine announced by Professor Haeckel. If we did so, we should come back to the mental position of the schoolmen of the Middle Ages and of the unproductive Arabians. With them, the most that an acute and daring mind could hope for was to comment on the writings of Plato and Aristotle, or perhaps remeasure the earth and catalogue the stars by the methods of Ptolemy. Such an attitude indicates a mental condition unaccustomed to, and without hope of, solid progress, ill fitted to cope with real philosophic problems, such as have been handled successfully by the great natural philosophers of the past three centuries. And for my part, also, I am unwilling to believe that the universe is so simple or so easily exhausted that even a great number of the acutest minds could unravel its principal mysteries in a few centuries, flattering as such an achievement would be to the age in which we live.
It may be said that in some lines of applied science we have indeed well-nigh reached the appointed goal. Within the memory of this generation the Earth has been girdled with iron and steel, and the electric telegraph and the cable have practically annihilated terrestrial space: these modes of communication have come to stay, and they are ultimate. Whatever be the future progress of the world, it seems certain that nothing more rapid or more general will ever be used by the children of men. The velocity of electricity is the same as that of light, and no swifter messenger is possible or even desirable. The same approach to ultimate standards of speed may be observed in other lines of activity, as railroading and navigation, where the limits are fixed by the nature of organic life and by the physical properties of matter. But such physical limits do not restrict the powers of the mind for researches in pure science, whether in the biological or in the physical world. And if we continue to make discoveries throwing light upon the phenomena and principles underlying the arrangement and growth of the universe, who can doubt that some of them will augment continually the mental and physical comforts of mankind?