Recent Astronomical Discoveries in the Southern Hemisphere
AN important part of Dr. Gould’s labors at Cordoba, to which I did not allude in the paper in the Atlantic for May, 1898, consisted in training the astronomers who were to be his intellectual successors, the scientific heirs to whom he bequeathed the legacy of the continued exploration of the southern heavens. In addition to cataloguing the Stars he accomplished this educational work, and, fortunately for astronomy, Dr. John M. Thome, who had served under Dr. Gould for many years in the execution of the great Argentine catalogues, was destined to be the second director of the National Observatory at Cordoba, and to add new lustre to an observatory already famous beyond the dreams of its early promoters. Dr. Thome and Mr. Tucker, now of the Lick Observatory, continued Gould’s work in a manner analogous to the extension of Bessel’s zones by Argelander, and of the latter’s more extensive star census by Schönfeldt. Argelander at Bonn, on the Rhine, had catalogued the principal stars between the north pole and two degrees south declination ; and when this work was concluded, his students and successors executed a survey from the zone where their master left off to twenty-three degrees south declination, including some stars as faint as the tenth magnitude. In this way the Bonn census of stars assigns the positions and magnitudes of 325,000 objects. From 1885 to the present time the work of the analogous Cordoba census of the southern hemisphere has been steadily advanced, and is already completed over the whole of the zone from twenty-two to forty-two degrees south declination. This vast survey of Thome and Tucker is based upon the foundation laid by Gould, and the part already published includes the positions of 339,215 fixed stars. The two imposing volumes which have appeared are accompanied by accurate charts of that region of the heavens. An examination of these duplicate pictures of the sky must impress every beholder with the infinitude of the stellar points diffused in space, and the comparative insignificance of everything upon the terrestrial globe.
When this survey of Thome is carried to the south pole, the southern heavens will be better known than our own skies which have occupied the attention of observers from the earliest ages of astronomy. Nearly all this immense enterprise on the more inaccessible of the two celestial hemispheres has been executed in the last quarter of the present century, and entirely by American astronomers. The work of Gould and Thome must be credited to American genius and to the enterprise and liberality of Argentina, and it is needless to add that the achievement is sufficiently imposing to do honor to any age. Yet it happens that, during the same period, Dr. David Gill, her Majesty’s astronomer at the Cape of Good Hope, has been most active, and has already published a photographic star census of a large zone of the southern heavens. This far-reaching undertaking, carried out under Dr. Gill’s direction, consisted in taking photographs of areas of the heavens so apportioned that each gelatine plate showed some stars whose position is given by the earlier observations at the Cape or at Cordoba; so that when the plates are developed it is possible to measure, with a fine machine, the place of each luminous point with respect to known stars. In this way the places of all the stars photographed are determined absolutely, and with extreme rapidity and accuracy. The plates were taken at the Cape of Good Hope, but the work of discussing the results and reducing the catalogue was done chiefly at Groningen, Holland, under the direction of Professor Kapetyn. While this photographic survey is of very high importance as a supplement to the Cordoba census, it cannot be said to supplant it.
In recent years, American readers have become so much accustomed to reports of large telescopes that the impression seems to prevail widely that such instruments are the only conditions necessary for great discoveries. Need I point out to any thoughtful person that this strange impression is not justified ? Is it not equally important that the telescope should be located in an atmosphere which is quiescent and steady as well as free from clouds and fog ? In addition to good instruments and favorable climate, there must of course be an astronomer at the little end of the telescope capable of obtaining the best results which his instruments and conditions afford.
Unfortunately, it is only very recently that astronomers have realized the value of a good atmosphere, and though this achievement seems anything but striking, it has led to results of the most far-reaching character. Optical instruments have reached practical perfection in the last thirty years, but no atmosphere yet found is even approximately perfect: hence it is clear that the way to increase telescopic power is to improve the atmospheres through which our observing is done. In the modern search for good atmosphere, Professor W. H. Pickering, of the Harvard Observatory, made the first important step, and the work has since been especially prosecuted by the Lowell Observatory. The result of this search for climates which afford good seeing has been a rich harvest of discoveries which no one unfamiliar with the problems to be solved could have anticipated.
In 1887, the fund left to the Harvard Observatory by Mr. Boyden, of Boston, “ for the prosecution of astronomical research in a mountainous region as free as possible from the impediments due to the atmosphere,” became available, and an expedition was sent to Colorado to test the seeing on Pike’s Peak and other high mountains of that region. The observers afterward experimented on Mount Wilson, in California, and the outcome was the conclusion that other conditions besides mere elevation must be taken into account, and that dryness of the atmosphere, above all, is of the highest importance. As the ultimate aim of the movement was to explore the southern stars, an expedition was dispatched beyond the equator to test the atmospheric conditions in the Andes of Peru and Chile.
Experiments were eventually made at a number of points along the Chilean and Peruvian coasts, and at various elevations in the desert of Atacama, as well as in the high mountains between Lima and Arequipa, Peru, and Lake Titicaca, Bolivia. The conclusion arrived at was that the best seeing is afforded in a dry region from six to ten thousand feet above the level of the sea, where the movement of the atmosphere is reduced to a minimum. Though very good conditions were found at Mount Harvard, near Chosica, Peru, and at Copiapo, in the Atacama desert, it was finally decided that the city of Arequipa offered the most favorable conditions, when all the needs of the observatory were considered. Situated in an excessively dry region, where the sky is seldom obscured by clouds, the site selected stands 8060 feet above the level of the sea, and overlooks an immense gorge which drains the great mountains of El Misti and Chichani above and some fifteen miles away. This site has proved a happy one, and already the observatory has become celebrated by discoveries made there in the last nine years. As this station was selected for the clear sky and good seeing it would afford, it was particularly well adapted to the investigation of the brightness of the southern stars ; and accordingly, the earliest opportunity was utilized for making a photometry of the southern heavens. A part of this work had already been done at Mount Harvard. Altogether this included the critical study of 7922 stars, and led to the detection of a number of variables. Being a continuation of a similar system of work extending over forty thousand stars of the northern heavens, and based upon hundreds of thousands of observations made at Cambridge, the high importance of the southern photometry is at once apparent.
In the programme of the new southern station the developments of photography were given a prominent place, and it was not long before impressions were made of the whole region invisible in Cambridge. Besides general photographic reproductions of the whole southern sky, a detailed investigation was made of particular portions. Thus long exposure of plates on the Magellanic Clouds revealed the amazing variety of phenomena in those luminous patches; and photographs of great clusters, such as Omega Centauri and 47 Toucanæ, showed in durable form their infinite complexity, previously discernible only with great telescopes. Omega Centauri was found, by the plates taken at Arequipa, to contain over seven thousand stars, all packed within a space smaller than the moon. To the naked eye it is a luminous patch resembling a faint cloud or nebula. Continued examination of these cluster photographs led Professor Baily to detect in some of the masses of stars a large number of objects which are variable. In Omega Centauri alone he found one hundred and twenty, and in the cluster Messier Five about eighty - five fluctuating points of light formerly assumed to be of constant brilliancy. This discovery is of very high importance, because previously only a few cases of variables in clusters were certainly known ; and this rich find is likely to throw light upon the cause of the light changes, if the observations are continued with system and regularity.
It may not seem strange that a star should increase or decrease in brilliancy ; but when we remember that a variation of five magnitudes, which occasionally occurs, means an increase and then a decrease in brightness of a hundredfold, vve may indeed wonder at the causes which could produce such amazing pulsations in brightness. In some instances the causes of these changes are known, as in the case of Algol and other stars of that type, where the bright star is eclipsed by a dark satellite moving in an orbit situated in the plane of vision, so that at regular intervals the lucid star fades or diminishes in brightness when the dark body intercepts its light, and then as regularly shows forth in full splendor. But in the great majority of cases, though many temporary hypotheses have been put forward, no acceptable explanation has yet been made.
Besides the cluster variables found by Baily, some three hundred individual and tested variables, often bright stars, are given in well-known recent catalogues. In many cases the law of the light variation is known accurately, though in general the cause is wholly obscure. For different cases the curves which represent the brightness are of very different character ; some exhibiting one sharp or round maximum or minimum, others a double maximum and minimum, as in the case of the celebrated northern variable Beta Lyræ. If the light variations of the Algol stars arise from the occultation of dark bodies, it is natural to suppose that other variations are in some way connected with attendant bodies, either by way of occultations of dark or partially dark bodies, or by tidal action due to masses wholly invisible in our telescopes. In view of this probable dependence of variables on other bodies, Baily’s discovery of so many variables in clusters, where all necessarily are connected in one immense system, opens up far-reaching suggestions, though such complicated phenomena will be difficult to unravel. It is to be hoped that the Harvard Observatory may be able to continue to watch the objects it has discovered ; and in due time, no doubt, we shall have the law of the light fluctuation for each of the handsome group of new variables it has announced.
One other object which has long engaged the attention of the Harvard Observatory is the extensive photography of stellar spectra, and this has recently been extended to the southern hemisphere. Many years ago Professor E. C. Pickering revived the plan, originally used by Fraunhofer, of putting a large glass prism in front of the objective of a telescope, so that the light of a star entering the lens is no longer a bundle of white rays, but a spectrum in which all colors are spread out; and the result is that, instead of an image, the eye perceives a spectrum. Replacing the eye by a sensitive photographic plate, these spectra may be photographed in large numbers, as many of them appearing on a plate as there are stars in the field of the telescope. By designing an instrument which has a large angular aperture, or short focus, so that the field of view is extensive, it is possible to take on a single piece of glass the spectra of a great many stars. By this means the spectra of more than ten thousand stars have been photographed in the northern heavens, the results composing the celebrated Draper Catalogue of stellar spectra. In the course of this work, each plate was carefully examined to find the type to which the spectrum belongs ; and it was soon ascertained that a few peculiar objects do not belong to any of the spectral types recognized by Huggins, Vogel, Rutherford, Secchi, Lockyer, or Pickering. Some of the spectra are found to be crossed by bright lines, like a few stars in Cygnus recognized by the French astronomers Wolf and Rayet in 1867. Professor Pickering, who took up this work in memory of the lamented Henry Draper, has now noted in the northern heavens more than sixty such objects, where only a few were known before. In the more recent study of the southern heavens other bright-line stars have been encountered, and the Harvard Observatory has the honor of finding the only ones known in that extensive region. This considerable list of bright-line stars, besides two new or temporary stars detected in the constellations Norma and Carina, constitutes a unique and somewhat unexpected contribution made in the course of regular work on stellar spectra provided for by the Henry Draper Memorial. The full import of these new bright-line stars cannot yet be made out, but it is assumed that they are closely related to nebulæ, which have in their spectra bright lines of a different type, and are known to be self-luminous masses of gas of which hydrogen is the only element heretofore recognized. It turns out that all these new bright-line stars are situated in or near the plane of the Milky Way or in the Magellanic Clouds, which thus disclose more directly their connection with the Galaxy.
Some of the most important discoveries made at the Arequipa station of the Harvard Observatory relate to what are known as spectroscopic binaries, or binary stars so close together that they cannot be resolved in any existing telescope, and must be inferred to exist from certain phenomena of their spectra. The spectra of most stars usually consist of certain dark lines projected on a luminous background; the positions of these lines are determined by the wave lengths of the light emitted ; and as characteristic wave lengths are emitted by particular chemical elements, these lines indicate the presence of certain elements in the atmospheres of the stars. Thus one series of lines will arise from the presence of iron, another from that of cadmium, still another from that of sodium. Hydrogen and carbon are very abundantly diffused throughout nature, and of course each gives a characteristic series of lines, though it is not yet settled that we are familiar with these lines under all conditions.
There is another principle of great interest in connection with stellar spectra. It was pointed out from theoretical grounds by Christian Doeppler, of Prague, in 1842, that a star moving toward the eye would transmit more, and conversely a star receding from us send fewer, light waves per second than an object at rest. By one of those singular oversights which not infrequently occur in the history of thought, this natural inference from the undulatory theory of light remained more or less barren of results till 1868, when Sir William Huggins applied it to the motion of stars in the line of sight, by means of the spectroscope, in which the chemical elements known upon the earth were made to supply the light corresponding to the ideal body at rest. The outcome of this fruitful line of inquiry has been an entirely new development of astronomy, now generally called astro-physics. By the most modern appliances, motion of stars toward or from the earth, amounting to one mile per second, may be accurately measured.
The investigation of these phenomena now occupies the attention of some of our foremost observatories, and the motions of a considerable number of stars have already been determined. In 1889 Professor Pickering detected at Cambridge two stars in the northern heavens, Zeta Ursæ Majoris and Beta Aurigæ, in which the spectral lines were not single, as is usually the case, but sometimes appeared as broad bands, and at other times as two closely adjacent lines distinctly separated. The natural interpretation of this broadening and doubling of the spectral lines, which were found to recur with moderate regularity, is that the objects are not single stars, but close binary systems, revolving so rapidly that the motion of the two components, one toward and the other from the earth, causes the separate spectra to be relatively displaced, and thus apparently doubled. These so-called spectroscopic binaries (no one of them has yet been seen double in any telescope) have been augmented recently by three similar discoveries in the southern heavens, — Zeta Scorpii, Gould 10534, and Beta Lupi.1
It ought to be said that a little doubt still attaches to the received interpretation of these phenomena. As no one of these stars appears double in the largest telescopes, our conclusion that they are double must be based wholly upon the evidence of the spectroscope. Now, unfortunately, our argument that these objects are double stars is not conclusive. We can show that a binary system such as we imagine would produce just the phenomena of spectral doubling observed, but we are not able to show that no other suitable explanation can be found. In fact, there is another explanation, lately developed, which is not improbable. Dr. Zeeman, a noted Dutch physicist, has found that when the radiating body is placed in a strong magnetic field, the lines of certain elements broaden and become double, not unlike the doublingobserved in the spectra of certain stars. This, however, does not account for the periodic character of the doubling; and should that phenomenon be clearly and fully established, as an inflexible law operating at a constant period, it would tend to exclude such an explanation as that suggested by Zeeman’s experiments. But should it turn out that the lines in question double with a periodicity which is not perfectly fixed, it might very well be that the spectroscopic binaries are in reality single stars, in which the atmospheres are periodically charged with strong electric or magnetic tension. This outcome, to be sure, does not seem very probable, but yet it is far from an impossibility, and its discovery is one of the notable scientific events of the past two years.
I cannot conclude this article without calling attention to another result in spectroscopic astronomy, of very farreaching consequence, recently obtained at the Johns Hopkins University by Professor W. J. Humphreys, now of the University of Virginia. This young American investigator finds that the absolute wave lengths of the elements are modified by pressure, and to some extent by temperature. Thus the positions and character of the spectral lines are not definitely fixed except under given conditions, and the question at once arises whether the shifting of the lines interpreted as motion in the line of sight is due wholly to that cause, or is to some extent influenced by the pressure and temperature of the star as well. It is too early to pronounce a definite opinion on this question, yet it seems certain that some small displacements of the lines of stellar spectra do arise from pressure. Further experiment alone can decide how far this new discovery will modify received results. But it appears highly probable that Mr. Humphreys’ achievement is so fundamental that it is easily the most important advance in the spectroscopic line since the early work of Kirchhoff, Bunsen, and Huggins, thirty years ago. The new result may modify the theory developed by Huggins, only in a quantitative way, so that the grand application which he gave Doeppler’s principle is likely to stand, at least in its essential features. Whatever be the outcome of disputed points, the immense strides made by spectroscopic astronomy, under the leadership of Sir William Huggins, must be very gratifying to that venerable and worthy successor of the great Newton, with whom astronomers not infrequently associate him. From a tiny but luminous speck in the sixties it has grown to fullorbed splendor within the lifetime of its aged but still active founder.
Among the planetary researches executed at Arequipa may be mentioned the discovery and delineation of new and striking features on the planet Mars, such as a part of the canals, in the light and dark regions, and the peculiar changes of color since investigated more in detail at the Lowell Observatory. In this work Professor Pickering showed meritorious originality, and put forth a number of suggestions of great promise. Though some of his views were at first contested by certain more conservative persons, who always look askance at anybody who brings forth new ideas, they have since been generally acquiesced in, and have been prolific of important developments. Under the steady atmosphere of Arequipa he obtained views of the markings of the planet Mercury, first seen by Schiaparelli, and since confirmed and extended by Lowell in so conclusive a manner as to place the rotation of the planet beyond doubt. Another investigation in which he displayed equal originality and freedom from prejudice was the study of the forms of Jupiter’s satellites, never before suspected to have other than perfectly globular figures. He found that the first satellite is egg-shaped, and that it rotates upon its shortest axis in about thirteen hours, — a discovery subsequently confirmed and extended at the Lowell Observatory.
These satellites of Jupiter have been regularly observed since their discovery by Simon Marius and Galileo, in 1610 ; and with the mass of observations available toward the end of the last century, Laplace discussed their motions and determined their mutual perturbations with a degree of care and rigor unexcelled in the whole range of celestial mechanics. In the course of this work he discovered a remarkable law connecting their motions, which has accurately represented their phenomena from the earliest times, and which nothing apparently can overthrow or disturb. This law is the result of the mutual action of the satellites, under which the motions are of such a character that the satellites tend inevitably to follow it as the path of least resistance, just as a resisted pendulum tends to come to rest at the lowest point of the arc of its oscillation. It is needless to say that the analysis by which Laplace reached this result is one of the most recondite inquiries in the whole domain of physical science ; and consequently, such a law, established by the greatest master of mechanics since Newton, is not easily set aside. When Professor Pickering announced that the satellites of Jupiter are not perfectly round, it led some to believe that the result of his observations violated the firmly established law of Laplace, and hence they were at first received with hesitation. It is now rendered highly probable by more recent investigations that the figures of these bodies are not round, but slightly ellipsoidal; and in the case of the first satellite the ellipticity is so marked as to be a matter of wonder that it was not detected before. The work at the Lowell Observatory indicates that this satellite is of the form of an egg, flattened on the sides, and thus an ellipsoid of three unequal axes, — a possible gravitational figure of equilibrium, as was shown many years ago by the celebrated geometer Jacobi. The rotation about this shortest axis gives the body a maximum moment of momentum, and the rotation is perfectly stable even if the mass be perturbed by the other satellites. Thus, after all, the law of Laplace is not invalidated, and yet the figures of the satellites are not such as that great mathematician imagined. These satellites appear to be covered with streaks, which in a good atmosphere are distinctly visible, and enable the astronomer to find the periods of rotation about their axes with great precision. The work begun at Arequipa by Professor Pickering has thus been productive of unexpected results ; and we may attribute his good fortune in opening up this new field of discovery to the exceptional steadiness of the atmosphere at the Harvard station in Peru, which enabled him to use high magnifying powers, and at the same time preserve well-defined telescopic images.
The latest discovery in the southern hemisphere also relates to the satellites, and like the foregoing was made by Professor Pickering. It is the new satellite of Saturn, discovered at Harvard from the examination of photographic plates taken at Arequipa, and made known to astronomers only a few months since. On three photographs of Saturn taken in August, 1898, Professor Pickering detected a faint point of the fifteenth magnitude, which had relative motion among the neighboring stars. Further examination showed that this tiny point, which no mortal eye has ever yet beheld, must be a satellite of Saturn ; and a study of all the photographs now available shows that the body revolves about Saturn in about seventeen months, at a distance of seven million miles. The period and distance of this satellite are by far the greatest yet disclosed for any similar body in the solar system. This object will prove to be of very high interest to astronomers on account of the great perturbations it suffers from the action of the sun and of Jupiter, which will assume greater importance for this satellite than for any known member of the solar system. It turns out that the solar perturbations will become three times as great as they are in the case of the moon, where these forces have such magnitude that it has taken geometers two hundred years to explain their full effects. Accordingly, the problems in mathematics presented by the new satellite, which Professor Pickering has named Phœbe, will probably occupy the attention of astronomers for a number of years. This new and obscure attendant of Saturn promises to become the most famous of satellites, and it is a matter of great congratulation that, like other recently discovered satellites, it has been added to the solar system by an American. If the photographic method which the Harvard Observatory has so splendidly developed is applied to other planets in the same way, it seems certain that additional satellite discoveries will be made, and none can foretell what treasures the future may bring forth.
The last and not the least important subject taken up at Arequipa was the discovery of new double and multiple stars in the extensive unexplored field round the south pole. Some two hundred new stars were detected with the thirteen-inch Boyden telescope in the hands of Professors Pickering and Baily. The northern heavens were first roughly searched for double stars by Sir William Herschel, one hundred and twenty years ago. After he had accidentally discovered that these objects are genuine systems of double suns revolving under the law of gravitation, and thus subject to the same laws of motion as are observed in the solar system, the interest in the new branch of science was greatly increased, and it has ever since remained one of the most dignified and important branches of astronomical research. Sir William Herschel discovered in all about five hundred double stars, including a number of the brightest objects in the northern heavens. From 1827 to 1838, William Struve, of Dorpat, Russia, executed a systematic survey of the northern heavens, in the course of which he examined more than one hundred and twenty thousand stars within one hundred and five degrees of the north pole. The result of this immense survey, made with the first large equatorial telescope ever mounted with clockwork, was a catalogue of 3112 double and multiple stars, which to this day has remained the fundamental work on double stars for the northern heavens. The exploration of our sky has since been continued by Sir John Herschel and Otto Struve, but above all by the American astronomer Sherburne W. Burnham, who has discovered within the last thirty years some thirteen hundred systems of surpassing interest. I mentioned in my article in the Atlantic for May, 1898, that Sir John Herschel, in the course of his survey of the southern skies, made at the Cape of Good Hope from 1884 to 1838, discovered about two thousand new double stars. After this early work the subject of southern double stars lay in abeyance for thirtyfive years, till 1870, when Russell of Sidney undertook a hurried remeasurement of Herschel’s stars, and in so doing came upon about four hundred new systems, some of which promise to be of high importance. Aside from this work and the exploration made by the Harvard observers at Arequipa, no work on the southern double stars worthy of mention had been done in fifty-eight years following the memorable expedition of Sir John Herschel.
The part of the heavens within seventy-five degrees of the south pole, rich in double stars of high interest, was practically neglected for half a century, at a period when all lines of science were advancing rapidly, and in which the great cataloguing plans of Gould and Thome and the photographic survey of Gill for the same region had been executed with a degree of exhaustiveness and care which would astonish Herschel himself could he now behold what has taken place. This region of the heavens includes three eighths of the celestial sphere, and comprises incomparably the most impressive portion of the visible universe ; and yet it was still unexplored by a great modern telescope. That it would reveal to the investigator some of the finest objects to be found anywhere was highly probable, and in this conviction its exploration was entered upon with the great telescope of the Lowell Observatory.
On beginning this survey for southern double stars, my first concern was to develop a plan of work which would enable me to sweep over an extensive region, and to study a large number of stars within the available time. A new method was soon devised, by which, under the best conditions, I could examine carefully, in a full night of six or eight hours’ work, as many as a thousand stars ; and in this way we sometimes swept upon forty stellar systems in a night. Thus it has been possible in a single year to examine something like a hundred thousand stars brighter than the tenth magnitude. The region swept over includes the zones of the sky visible near our southern horizon, which are rich in clusters and full of stellar objects of high interest.
It seems fairly certain that there is no object in that region, visible to the naked eye or through an opera glass, but has been repeatedly examined, and many of the brightest objects have been found to have companions hitherto unknown. Some of these stars have components which are very close together, while others are wide apart. A good many of the newly found systems are composed of equal or nearly equal members ; the rest show increasing disparity in brightness. When sidereal systems are made up of components of equal brightness, they generally present to us two stars of the same color ; in the more general case of unequal stars, the pair frequently exhibits all the contrasts of combinations of garnets and sapphires, topazes and rubies. Still more rarely we find a bright object attended by a dull or obscure satellite resembling rusty iron. Thus the variety of colors presented is almost infinite, and the same may be said of the lustre of associated stars. From August 1, 1896, to July, 1898, we studied nearly two hundred thousand stars, and in this immense survey found some two thousand double stars worthy of measurement. Of this number, about fourteen hundred had been seen (though not always measured) by Sir John Herschel and other early observers. The six hundred new pairs, never suspected to exist until resolved by the great Lowell telescope, have been discovered at Flagstaff, Arizona, and at the City of Mexico.
The importance of these objects over those previously known is due mainly to their unusual closeness and difficulty of measurement, and the resulting probability that such physical systems will have rapid orbital motion. For it is found by the observations since the time of Sir William Herschel that, in general, rapid motion can be expected only in the case of objects closely adjacent; those which are widely separated either showing no motion, or revolving, as a general rule, much more slowly. Indeed, wide angular separation of objects at a given distance from the earth implies an orbit of great dimensions, and a large orbit requires a huge central mass to produce rapid revolution : thus, if a star with a large apparent orbit revolves rapidly, we know at once either that it must be comparatively near us, so that the orbit looks large, or, if removed, it must have an enormous mass to generate such motion. Accordingly, when we find new double stars which can be just separated in a great telescope, the probabilities are that the objects will be found to revolve with a comparatively short period ; and should events disclose a slow motion, we naturally conclude that the system is at a very great distance where the orbit appears diminutive, or that the stars are of small mass.
In general, the brightness is only a very rough index to the mass of the system, and the rule that mass is proportional to brightness is so frequently violated that we must accept it with due reservation for individual cases. Thus the companion of Sirius gives only one ten thousandth part as much light as Sirius itself ; yet mathematical investigations of the motion of this system show that the dull and obscure attendant is one half as massive as the great luminous star which controls its motion. Indeed, the mass of this satellite is so great that it perturbed the principal star appreciably, and the famous German astronomer Bessel, more than fifty years ago, predicted that Sirius was attended by a dark companion. This object, whose existence was first indicated by the refined methods of analysis, was duly discerned in 1862 by Alvan G. Clark, and has since been shown to be the real perturbing body announced by Bessel in 1844. In like manner, Procyon, the smaller dog star, was supposed to have a perturbation in its proper motion ; that is, instead of tracing a great circle in its forward motion across the sky, it was seen to be moving in a tortuous snakelike curve, now bending this way, now that. Bessel also foresaw, in the case of this body, a dark attendant, which was not disclosed to telescopic vision till November 2, 1896, when Professor Schaeberle, of the Lick Observatory, detected the long-lost body, hitherto known only by the irregularity in the motion of the bright star. It has since been seen at several observatories, and is found to move essentially in accordance with Lhe theory suggested by Bessel more than half a century ago. This companion is even darker than that of Sirius, and, wonderful to say, is equally massive. The attendant is of a dull purplish color, and revolves in about forty years.
The Algol variables, in which dark bodies occult the light of the brighter stars about which they revolve, give the closest analogy to the systems of Sirius and Procyon heretofore recognized. But should some of our new systems in the southern heavens turn out, as they appear, practically devoid of inherent light, and shining only with a dull, obscure lustre, as if reflected and strongly absorbed by a dark surface, other interesting objects will be added to the list. Thus the stars of our first catalogue, Lambda 76, 88, 289, 311, 408, 428, 429, are probably the most remarkable objects of the class known, and at present appear to occupy a unique place in astronomical literature. In every one of these cases we seem to perceive a mere sparkle of light from a body which is not only faint, but apparently obscure and more massive than its light would indicate. The color may be described as deep brown, or dingy, closely approaching black. Iron rust or iron ore, such as meteoric iron or black hematite, recalls vividly the hues of these companions as they appear in the great telescope. It is of course impossible to see these dark bodies, or “ planets,” except under the most unusually favorable conditions.
An interesting question arises with regard to the cause of this singular color. Heretofore I have been able to reach only one explanation. The labors of the past hundred years have established for double stars a peculiar law of color, according to which the companion has a bluish, while the large star frequently has an orange or a reddish tint. It has occurred to me that these planets may be like other companions, except that they are extreme cases, shining by ultraviolet instead of bluish or purplish light. Should they radiate ultraviolet light, which affects the eye but feebly, they would be almost invisible, and the color would be just such as we observe. If these satellites were more widely separated from their central stars, photography could decide the question whether their feeble luminosity is due to the predominance of ultraviolet rays or to actual reflection from a dark surface.
From these considerations it appears that new fields of research are constantly being opened up to the student of the stars, and that a few of the gems of the heavens have fallen to our lot. The more ultimate problems which invite the attention of the astronomer relate to the forces which control the stars in their orbits, and the processes by which these giant systems have evolved from nebulæ. On both of these recondite topics great and indeed satisfactory progress has already been made, yet the field of the unexplored grows wider and wider with each decade. Removed by one hundred and twenty years from the earliest labors of Herschel, we have at last attained a fair knowledge of some fifty orbits with indications of promising motion in other stars which will especially interest the next generation. It is hoped that some of the stars recently discovered will revolve with sufficient rapidity to interest living astronomers; others, which move with a more leisurely pace, presumably will remain fixed in the sky for several centuries. Thus, of the ten thousand double stars catalogued by previous observers, only about five hundred show any evidence of orbital revolution. Some which are clearly moving require a period almost equal to that of all recorded history for a single circuit of their immense paths. For example, of the brilliant double-double or quadruple system Epsilon Lyræ, one Component revolves in about nine hundred years, the other in twice that period; while still other bright stars, among the earliest discovered, have given no certain evidence of motion since the invention of the telescope.
It is satisfactory to find that all these stars, whatever be the rate of their motion, and whether glowing at a white heat on account of an enormous temperature, or barely visible by a dull reflected light, obey the grand law of gravitation ; and we are thus able to trace their motion through past and future ages with mathematical precision. It is a singular property of gravity that it appears to be in no way influenced by temperature, and is thus altogether different from the other physical forces with which we are acquainted upon the earth. Magnetic and electric forces lose their efficacy when acting on bodies subjected to enormous temperatures, because masses in such conditions lose their power of magnetism, and are not affected by corresponding forces. In the case of gravity, however, there seem to be no exceptions; bodies at all temperatures come equally under its sway. The motions of a variety of double stars in different parts of the universe show that they obey a central force, like the bodies of our solar system, and all the evidence tends to prove that Newton’s law of attraction is really universal. We must remember, however, that it is not sufficient to show mathematically that a star describing an ellipse obeys the law of gravitation ; we must also demonstrate, by elaborate observations of the highest refinement, that the paths of the stars are really ellipses. Fortunately, this is now established with great accuracy in a number of individual cases, and is thus inferred to be true universally. The extension and verification of the Newtonion law of attraction in the remotest regions of the universe must be accounted one of the sublimest achievements of the human intellect, and the recent discoveries in the southern hemisphere will contribute largely to its complete establishment.
While this extension of the theory of gravity is very gratifying to the mind, it is a somewhat remarkable fact that since the time of Newton no certain advance has been made toward explaining the nature of gravity itself. In the closing scholium of the Principia Newton says : “ Hitherto I have not been able to discover the cause of those properties of gravity from phenomena, and I frame no hypotheses. . . . To us it is enough that gravity does really exist, and act according to the laws which we have explained, and abundantly serves to account for all the motions of celestial bodies and of our sea.” The nature of gravity was given profound meditation by Laplace, who instituted numerous researches to ascertain whether it is propagated with a finite velocity, like light and electricity. The outcome of his labors was the conclusion that if gravity has a finite velocity, it must be millions of times greater than the velocity of light, which travels at the rate of one hundred and eighty-six thousand miles per second. From Laplace’s investigations, and those made since his time, the indications are that gravity acts instantaneously throughout the universe, and is not propagated like other forces with a finite velocity. The nature of gravity thus remains an enigma, and it is not easy to see how any light can be thrown upon the cause from which it arises.
From this brief sketch of recent discoveries, it appears that although the objects of the southern celestial hemisphere long remained comparatively unknown, their exploration within the last thirty years has been undertaken with a degree of thoroughness commensurate with the inherent interest of the richest portion of the celestial sphere.
Looked at historically, the exploration of the northern heavens was favored by circumstances, and by the traditions of consecrated labors bestowed upon science by the more civilized nations from the earliest ages. The hemisphere unknown to the ancients had to await the tide of civilization, or attract its devotees by the greater abundance of wonders held out to the faithful explorer. Although the northern terrestrial hemisphere will probably always be the seat of the world’s highest civilization, the development already made in the exploration of the great constellations near the south pole insures an ultimate equalization of our knowledge of the two celestial hemispheres. And we may venture the opinion that when the balance of fate shall finally decide the merits of achievements dating from our time, the contributions to universal knowledge resulting from discoveries in the southern heavens, made by contemporary astronomers, will not appear among those of least importance.
T. J. J. See.
- Professor Campbell, of the Lick Observatory, is now rapidly extending this line of research, and has attained very striking results. He concludes that Capella and the Pole Star are spectroscopic doubles.↩