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possible direction, but it has been recently discovered that considerable groups of stars often move in the same direction and at the same rate. The Pleiades exhibit this phenomenon, but much larger groups have the same kind of motion, and this has led to the theory that in certain parts of the heavens there is a star-drift in fixed directions. Our sun is now known to have its own "proper motion," the direction and rate of which has been determined approximately. This will, of course, produce an apparent movement in all the stars, except those situated exactly in the line of our motion, and the displacement thus caused has to be allowed for in determining the true motion of the stars in space. Should any of the stars be moving obliquely towards us, we shall only perceive that portion of the motion which is at right angles to the direction of the star from us, but the beautiful method of determining motion in the line of sight by means of the spectroscope has overcome this difficulty, and by its means we now know the real motion of many stars, both in direction and velocity, when we have been able to measure their distance from us.
This measurement of the distance of the stars is the most difficult of all the instrumental determinations of modern astronomy, both on account of the extreme remoteness of most of them, and because owing to the motions of the stars themselves, we have no fixed point from which to determine changes of position.
Most people know that by means of a measured base-line, the distances of very remote and inaccessible objects can be determined with considerable accuracy, depending upon the length of the base and its careful measurement, and equally upon the extremely accurate measurement of the angles taken at each extremity of the base. It is in this way that the position of moun
tain peaks is determined, as well as the distances across narrow seas, while all civilized countries have been trigonometrically surveyed in this manner.
In the case of the stars the base line used is the diameter of the earth's orbit, more than one hundred and eighty millions of miles. Every six months we are at opposite ends of this base, and if we had any absolutely fixed point in the heavens, in the right position, from which to take our angles, we could in this way determine the distance of some of the stars. But as almost all the stars are moving at various rates and in various directions; as our sun itself is moving; and as the proper motions of the stars can only be determined in relation to other stars, there is everywhere a complication of opposing motions, and nowhere the assured fixity we require for such delicate measurements. But notwithstanding all these difficulties astronomers have by various ingenious methods now measured the distances of a number of stars with considerable precision, notwithstanding the failures of their predecessors for nearly two centuries. The nearest of all the stars are so remote that the distance between the earth and the sun as seen from the star would subtend an angle of considerably less than one second of arc; while most of those measured are so excessively distant that this angle is often one-tenth of a second or even considerably less. To understand how small a quantity this is and what a distance it implies, it may be stated that, viewed at a mile distant, the small letter o in this page would subtend an angle of about one-tenth of a second. From a star of an average distance from us, therefore, the earth and sun, if they could be seen, would appear only as far apart as the opposite sides of the letter o when a mile away from us. But stars twice as far as these have been measured, it is be
lieved with some degree of certainty, and the distances of about sixty stars have now been satisfactorily ascertained.
It was long supposed that the brightest stars were the nearest to us, but it is now known that there is little or no relation between brightness or magnitude and distance. The nearest star yet measured is, indeed, a very bright one in the Southern Hemisphere, Alpha Centauri, but one almost as near, 61 Cygni, is of the fifth magnitude only, and another still nearer in the constellation, Piscis Australis, is of the seventh magnitude. Other stars of the first magnitude which have had their distances measured have a parallax of considerably less than one-tenth of a second, and are therefore among the remoter stars.
The true relation, as was long suspected theoretically, is between proper motion and distance, those which move fastest being nearest to us. It is as if, from a mountain-top, we observed ships at sea from two or three miles to forty or fifty miles distant, and kept a record of their angular movements. All might be really moving at not very different speeds-from five to perhaps fifteen or twenty miles an hour, yet while some would appear to move rapidly, others would seem to be almost stationary, and this would depend almost entirely on their distance from the observer. So with the stars. All may have, and probably have, real motions which do not differ very greatly in rapidity, but only in those which are comparatively near us can we detect any motion at all. This theoretical conclusion being confirmed by all the stars, whose distances have been measured, we have a most valuable and trustworthy
means of ascertaining their comparative distances from us, since those whose proper motions are either exceedingly small or cannot be detected at all, are certainly very much
farther from us than those which have well-marked and large, proper motions. It is by such indications that we are enabled to arrive at some definite conclusions as to the real form and struc ture of the stellar universe, as we will proceed to show.
THE GALAXY, OR "MILKY WAY." By far the most prominent feature in the starry heavens is that vast irregular nebulous ring which in all ages has attracted the attention and excited the admiration of observers. This great ring divides the whole heavens into two hemispheres, making an angle of about 63° with the equinoctial, so that portions of it pass not far from the North and South Poles. Its nebulosity is now believed to be almost wholly due to the massing together of myriads of minute stars, since each increase in the power of the telescope shows more and more of these stars, while the best photographic plates show them everywhere closely packed but still with a luminous haze between them indicating yet more stars beyond.
But beside these minute stars which give us the cloudy or milky appearance, it is found that stars of all degrees of brilliancy are more numerous in the Milky Way and in its vicinity, than elsewhere. The two poles of the Galaxy are the regions where stars are scantiest. Each 15° nearer to it, they increase in numbers, at first slowly, then more rapidly, till we reach its borders. The following series of numbers give the average number of stars in a square of 15 min. at each 15° from the pole of the Galaxy, as determined by Sir John Herschel 4-5-8-13 -24-53.
Later observations have fully confirmed this, while it has been shown by the late Mr. Proctor that all stars down to the tenth magnitude, more than 324,000 in number, when carefully mapped, mark out the Milky Way in
all its details by their greater density. Later still, the Italian astronomer, Schiaparelli, by using all the materials now available, arrives at the same result, and Professor S. Newcomb, of Washington, after a close examination of his maps, assures us that the Milky Way can be fairly traced out by the region of maximum agglomeration of stars.
These facts lead to the conclusion that the Galaxy is a vast annular agglomeration of stars, forming a great circle round the heavens, although in places very irregular, being split in two for about one-third of its circumference, and being, besides, full of irregular dark streaks and patches where the most powerful telescopes show very few stars, so that, as Sir John Herschel says, we are irresistibly led to the conclusion that, in those regions, "we see fairly through the starry stratum"; and this is further shown by the fact that in these parts "the ground of the heavens seen between the stars is for the most part perfectly dark, which would not be the case if multitudes of stars, too minute to be individually discernible, existed beyond." This great ring is, therefore, evidently not very much extended in the direction of its own plane-that is, the ring is not flat or greatly compressed (as is Saturn's ring, for example), or we should nowhere see through it.
But what is more important is, that we must be situated not in any part of it as was once supposed, but at or near the very central point in the plane of the ring, that is, nearly equally distant from every part of it. This must be the case, because from any other position the ring would not appear to us so symmetrical as it does. If we were much nearer to one side of it than to the other, the nearer side would appear broader, the more remote side narrower, and these two direc
tions would show a decided difference in the numbers of the visible stars. Sir John Herschel, indeed, thought the southern portion was nearer to us than the northern, because of its greater brightness, which, he says, is very striking, and conveys strongly the idea of greater proximity. But this may be deceptive, because the whole Milky Way shows great irregularities and variations in brightness, and it is a remarkable fact that the portions near the North and South Poles are both
equally narrow, while the parts 90° from them are both very broad, rather suggesting equality of distance in all directions. Nearness would be indicated by a widening out of stars of all magnitudes not necessarily by any general increase of brilliancy. The facts, therefore, seem to show that we are about equally distant from all parts of the Milky Way.
Very important, however, is Sir John Herschel's testimony to the close correspondence of the Galaxy as a whole to a great circle. He tells us that, following the line of its greatest brightness, it conforms, as nearly as may be, to that of a great circle inclined about 63° to the equinoctial, and cutting that circle in R.A. 6h. 47m., and 18h. 47m., while its poles are in R.A. 12h. 47m. N. Decl., 27°, and R.A. Oh. 47m., S. Decl., 27°. He therefore determines it, by the figures he gives, to lie in an exact great circle as seen from the earth, as nearly as so irregular an object can be defined. But neither he nor any other astronomer, so far as I am aware, makes any remark on the extraordinary nature of this fact, which proves that we are placed exactly in the plane of the medial line of the ring. The fact of the Galaxy forming a great circle as seen from the earth being so familiar, no one seems to have thought it worth while to ask why it is so. If we could look at such a fact from the outside, as it were, we should
certainly impute it to some causal connection between our system and the Galaxy. But before speculating what this relation may mean we must consider another point of equal importance in our relation to the system of stars.
OUR STAR CLUSTER.
It has long been observed that the brighter stars seem scattered over the whole heavens with no special abundance in or near the Milky Way, and this was thought to be due to their being much nearer to us. It is now known, however, that brightness is no indication of nearness, so that this fact has little significance. But, as we have seen, we do possess a real test of nearness in the amount of the proper motion of stars, and this leads us to a very definite and most suggestive conclusion. For the stars which are nearest to us, judged by this test, not only have no apparent relation to the Milky Way, but are spread over every part of the heavens with tolerable uniformity. The most recent examination of this class of stars is by Professor S. Newcomb, who states the result in the following words:-"If we should blot out from the sky all the stars having no proper motion large enough to be detected, we should find remaining stars of all magnitudes, but they would be scattered almost uniformly over the sky, and show no tendency towards the Milky Way."
Professor Kapteyn, of Groningen, appears to have been the first to draw the obvious conclusion from these facts that these nearer stars spread around us in every direction, constitute a globular mass, which he termed the "solar cluster," nearly concentric with the Milky Way, and that our Sun is "deeply immersed" in this cluster.
Other astronomers have adopted this view, which seems to be almost indisputable if the facts are as stated. For,
if the cluster were not globular, its component stars would not appear to be so uniformly spread over the whole heavens; and if our sun were not situated at or near its centre but much nearer to one side of it than to the other, then we should inevitably find the stars of this type (those with measurable proper motions) much more numerous in one direction than in a direction exactly opposite. But although there may be some irregularities in their distribution, it has not been pointed out that there is any such regular inequality as this, and if there is not, then we must be situated very near indeed to the centre of this "solar cluster."
The results so far reached by astronomers as the direct logical conclusion from the whole mass of facts accumulated by means of those powerful instruments of research which have given us the New Astronomy, is, that our Sun is one of the central orbs of a globular star-cluster, and that this star-cluster occupies a nearly central position in the exact plane of the Milky Way. But I am not aware that any writer has taken the next step, and combining these two conclusions, has stated definitely that our Sun is thus shown to occupy a position very near to, if not actually at, the centre of the whole visible universe, and therefore, in all probability, in the centre of the whole material universe.
This conclusion is no doubt a startling one, and all kinds of objections will be made against its being accepted as a proved fact. And yet I am not acquainted with any great inductive result of modern science that has been arrived at so gradually, so legitimately, by means of so vast a mass of precise measurement and observation, and by such wholly unprejudiced workers. It may not be proved with minute accuracy as regards the actual mathematical centre. That is not of the least
importance. But that it is substantially correct in the terms I have stated there seems no good reason to doubt, and I therefore hold it to be right and proper to have it so stated and provisionally accepted, until further accumulations of evidence may show to what extent it requires modification.
This completes the first part of our enquiry; but an equally important part remains to be considered-our position in the Solar System itself as regards adaptability for organic life. Here, too, I am not aware that the whole facts have been sufficiently considered, yet they are facts that indicate our position in this respect to be, in all probability, as central and unique as is that of our Sun in the stellar uni
THE EARTH AS ADAPTED FOR LIFE.
Among the many writers who have more or less seriously discussed the question of the adaptability of other planets for the development of organic life, and of the higher forms of intellectual beings, I have not met with any who have considered the problem in all its bearings. They have usually been content to show that certain planets may possibly be now in a condition to support life in forms not very dissimilar from those upon our earth; but they have never adequately considered the precedent question: Could such life have originated and have been developed, upon these planets? This is the real crux of the problem, and I believe that a full consideration of the required conditions will satisfy us that, so far as we can judge, no other planet can fulfil them. Let us therefore consider what these conditions are.
The earlier writers on this subject could give free play to their imaginations and overcome difficulties of temperature, moisture, etc., by supposing that in other worlds there might be other elements which had different
properties from any we possess, and which might render life possible under conditions very unlike those which are essential here. But the revelations of spectrum-analysis have shown us the unity of the constitution of matter throughout the whole material universe, so that not only are the planets of the solar system all composed of the same elements, but that the farthest stars and remotest nebulæ alike consist of the very same elements with which we are so familiar, while the same physical and chemical laws undoubtedly prevail. We may be confident, therefore, that wherever organized life may have developed, it must be built up out of the same fundamental elements as here on earth.
The essential features of the structure of organized beings are, continuous growth and repair of tissues, nutrition by the absorption of dead or living matter from without, and its transformation into the various unstable compounds of which their bodies are built up. For these purposes a double system of circulation, gaseous and liquid, has to be constantly in operation, and this is carried on by means of minute tubular or cellular vessels which permeate every part of the body. These wonderfully complex and exquisitely adjusted circulating systems are entirely dependent on the continuous maintenance of a very narrow range of temperatures somewhere between the extremes of the boiling and the freezing points of water, but really within much narrower limits, since if the whole of the water at any time became solidified, all the higher forms of life would be destroyed, while a temperature very much below the boiling point, if permanently maintained, would be almost equally detrimental.
When we consider that the temperature of space is about-273° C., while that of the outer surface of the sun is about 9,000° C., we realize what a