EXTENT OF THE SIDEREAL HEAVENS.—Sir Robert S. Ball
Of all the discoveries that have ever been made in science there are two which especially baffle our powers of comprehension. They lie at the opposite extremes of nature. One relates to objects which are infinitely small, the other relates to objects which are almost infinitely great. The microscope teaches us that there are animals so minute that if a thousand of them were ranged abreast they would easily swim without being thrown out of line through the eye of the finest cambric needle. Each of those minute creatures is a highly organized number of particles, capable of moving about, of finding and devouring its food, and of behaving in all other respects as becomes an animal as distinguished from an unorganized piece of matter. The mind is capable of realizing the structure of these little creatures, and of fully appreciating their marvelous adaptation to the life they are destined to lead. If these animals excite our astonishment by reason of their extreme minuteness, there is an appeal made to conceptions of an entirely different character when we learn the lessons which the telescope teaches. As the microscope reveals the excessively minute, so does the telescope disclose the sublimely great. In each case myriads of objects are submitted to our astonished view, but while the microscope brings before us creatures of which countless millions could swim about freely in a thimbleful of water, the telescope conducts our vision to uncounted legions of stars, many of them millions of times larger than the earth.
The grandest truth in the whole of nature is conveyed in that first lesson in astronomy which answers the question: What are the stars? This is a question that a child will ask, and I have heard of a child’s pretty idea that the stars were little holes in the sky to let the glory of heaven shine through. The philosopher will replace this explanation by another hardly less poetical, which will enable us to form some more adequate notion of the real magnificence of the universe. Each star that we see is, it is true, only a glittering little point of light, but that is merely because we are a long way from it. An electric light which will dazzle your eye when quite close will be reduced to an agreeable illumination if it is at a little distance, will become a faint light a mile away, and at no great distance will become altogether invisible. We must remember that out in space there is plenty of room—there are no bounds; and therefore when we see light glistening in the far distant depths we can not at once conclude that the light is a faint one because it appears to us to be faint. It may be that the light is only faint because it comes from such a tremendous distance. In fact, the brightest light conceivable could be reduced to the insignificance of a small star if only it were removed sufficiently far.
The most intense light we know of comes, of course, from the light which rules by day, from our sun himself. The sun pours his unrivaled beams around us in all directions with prodigal abundance, notwithstanding his enormous distance of ninety-three millions of miles. Let me describe an experiment with respect to our sun, an experiment, it is needless to say, which could never be performed, but the results to which it leads us are none the less certain. Astronomers have demonstrated them in many other ways.
Suppose that the sun were gradually to be moved away further and further into space; suppose that by this time to-morrow the great luminary should be twice as far as it is now, and the next day should be three times as far, and the day after that four times, and so on until in a year’s time we should find that the sun was 365 times the distance from us that it is at present. Let us now trace the changes which we should see in the brilliancy of our orb of day. When he had reached double his distance from us, we should find that the light had decreased to a quarter of its present amount, and the heat which we derived from his beams would have decreased in the same proportion. In ten days we should find that the light had become so feeble as to be only one-hundredth part of that which we enjoy now. The apparent size of the sun would also be steadily decreasing, for as the distance of a body increases its apparent dimensions diminish. Sometimes the diminution of apparent size with distance is well illustrated on a clock tower. You would hardly believe that the hands and face of a clock like that at Westminster were so large until you happen to see a man cleaning or repairing it, when he appears a mere pigmy in comparison with the mighty dial which points out the hours. In a similar way with every increase of distance, the apparent size of the sun would decline, and in the lapse of a year the sunlight would be reduced to a feeble twilight. The sun itself would remain visible for many years, even if it were steadily moving away, though its lustre would continually decline, and its size would continually diminish, until at last it would have shrunk to the insignificance of a small point of light, still visible as a glittering object, but too minute to enable any definite form to be perceived.
Further still, the sun might recede until it passed beyond the reach of vision of the unaided eye; the telescope would, however, be able to pursue the retreating luminary until at last it sank into the depths of space beyond the reach of any instrument whatever.
This little argument will prepare us for an explanation of the stars. They merely appear to us to be points of light of varying degrees of brightness, but we have seen that our own sun might be reduced in lustre to that of the very dimmest of the stars if only it were removed sufficiently far. If, therefore, the stars are at a great enough distance from our system, it may indeed be that they also are suns, possibly equaling, or possibly even surpassing, our own sun in magnificence.
Here is indeed an imposing suggestion. Can it be that the host of stars which adorn our midnight sky are actually suns themselves of an importance comparable with that of our own? This is a great thought, and we desire to test it by every means in our power. You will see from the reasoning I have given that the whole question turns simply on one point, and that is: How far off are the stars?
The tiniest point of light that is just seen as a glimmer in the mightiest of telescopes may be indeed a sun as great, or indeed a million times greater, than our sun, if only that star be sufficiently far off. To find the distance of a star is a problem which taxes the utmost powers of the painstaking astronomer; every refinement of skill in making his measurements and of care in the calculation of his observations have to be lavished on the operation. Alas! it but too often happens that the astronomer’s labors prove to be futile. The surveying navigator often has to mark on his chart that no bottom could be found in the depths of the sea. His appliances would not work, or work reliably, in those ocean abysses; so, too, the astronomer, when he tries to sound the depths of space to the distances of the stars, has also to mark, generally speaking, “No bottom here,” as the result of most of his investigations. When this is the case we know for certain that the star on which his calculations have been made must be a gorgeous sun, because we are assured of the greatness of its distance, even though we have not been able to find out what that distance was. There are, however, some few places through the sky where the astronomer’s sounding line can, so to speak, touch bottom; there are a few stars of which we do know the distance, and the result is not a little significant. Were our sun to be withdrawn from us to a distance so great as that of the very nearest of the stars, our magnificent ruler and benefactor would certainly have lost all his splendor; he would, in fact, have shrunk to the similitude of a little star not nearly so bright as many of those which we see over our heads every night. Imagine the sun’s light subdivided into two hundred thousand parts, each of which would give us only a feeble illumination, and then imagine that each of these parts was again divided into two hundred thousand parts more, and it is one of these last fragments that would represent the miserable lustre which the sun would then display.
From these considerations we can enunciate the magnificent truth which astronomy discloses to us. I do not think that in the whole range of nature there is any thought so magnificent or so imposing as that which teaches us to regard every star of every constellation as a sun. We can not indeed assert that they are all so great as our sun, but we can affirm with certainty that many of them are far greater and far more splendid. Considering that our sun presides over a system of worlds of which the earth is one, that it gives light and heat to those worlds, and guides them in their movements, it would greatly enlarge our conceptions of the universe if we were assured that there was even one more sun as large and as splendidly attended as is our own. But now we find that not only is there one additional sun, but that they teem in uncounted thousands through space. Look, for example, on the next fine night at the Great Bear, the best known of all our northern constellations, and there you see seven stars forming the well-known feature. Figure in your mind’s eye each one of those stars in the likeness of a majestic sun, as big, warm, and bright as our sun, and look at other parts of the sky and repeat the process with the other constellations, and your conception of the magnificence of the starry system will begin to assume proper proportions. But this is only the first step, you must next look at the smaller stars, and reflect that they, too, are also suns, only much further off as a general rule than the brighter stars, though this is by no means invariably the case. Thus your estimate of the number of suns in the universe will rise to thousands, but you will not stop there, you will get a telescope to help you, and, to your extreme delight and wonder, you will find that there are hosts of stars—too faint to be visible to the eye, but which the telescope will immediately disclose. You will get a more powerful instrument, and then you will perceive that the stars are to be numbered by tens of thousands, and even by millions, and with every fresh accession of power in your telescope fresh troops and myriads of suns are revealed. Suns in clusters, suns strewn thickly here and sparsely there, so as to give us the notion that the only limit to the number we can see is the power of the telescopes we are using. Attempts at actual numeration are futile, for who can tell the number of the stars?
We can, however, form an estimate, and by taking samples, so to speak, of the sky here and other samples there, we have been enabled to learn the overwhelming fact that our universe does contain at the very least one hundred millions of suns.
In discussing the extent of the visible universe, it must always be borne in mind that the further a source of light is from us the fainter is the illumination which we receive from it. Suppose that a star which just lies on the limits of naked-eye visibility were somehow to be transported to a distance which is twice as great, then the lustre of that star would be diminished to one-fourth of its original amount. It would, therefore, be of course invisible to the unaided eye, but could still be easily perceived by a telescope. Indeed, the very word telescope means an instrument for looking at objects a long way off, and the effect of the telescope is to reduce the apparent distance of the object.
The bulk of a grain of sand as compared with the bulk of a football may illustrate the space accessible to our eyes when compared with the space accessible to one of the great telescopes. The larger of these spaces has a thousand times the diameter of the others; therefore, the relative quantities of these spaces are to be obtained by multiplying 1,000 by 1,000 and by 1,000 again. Thus we finally learn that the amplitude of our vision is augmented to one thousand million times its original extent by the use of our greatest telescopes. It need, therefore, be no matter for surprise that the number of stars visible through our great telescopes or recorded on the sensitive films of photographic plates should number scores of millions. In fact, it would sometimes seem surprising that the number of telescopic stars is not even greater than it actually appears to be. If we are able to explore one thousand million times as much space, we might expect that the number of objects disclosed would be also increased about a thousand million-fold, but this is certainly not the case. The truth seems to be that our sun is but one star of a mighty cluster of stars; we happen to lie near the middle of the cluster, and the rest of the stars belonging to it form what we know as the Milky Way. There are, of course, other clusters scattered through the heavens, some of them, perhaps, as great as that body of stars which forms the Milky Way. Owing to our residence in this cluster we see the neighboring suns in multitudes, and thus we receive the impression that the solar system lies in an exceptionally rich part of the universe in as far as the distribution of stars is concerned.
On the outskirts of the universe lie those faintest and dimmest of objects which we can just perceive through our greatest telescopes. We know that many of the stars around us would still remain visible in great instruments, even though they were removed a thousand times as far off. Among the myriads of faint stars which we see from our observatories, there may be many, indeed there must be many, which are fully a thousand times as distant as the bright stars which twinkle in our comparative neighborhood. We thus obtain some conception of the stupendous distance at which the outskirts of the universe are situated.
There are different ways of illustrating this point, but I think the simplest, as well as the most striking, is that which is founded on the velocity of light. It is a remarkable fact that the beautiful star known as Vega[3] has a distance from us so tremendous that its light must have taken somewhere about eighteen years to travel hither from thence. Notwithstanding that the light dashes along with such inconceivable speed that it will cover 185,000 miles in every second, notwithstanding that a journey at this pace will complete the entire circuit of this globe seven or eight times between two successive ticks of the clock, the light will, nevertheless, take eighteen years to reach our eye from the time it leaves Vega. We do not, therefore, see the star as it is at present; we see it as it was eighteen years ago. For the light which this evening enters our eyes has been all that time on its journey. Indeed, if Vega were actually to be blotted out from existence it would still continue to shine out as vividly as ever for eighteen years before all the light on its way had reached us.
We have been led to the belief that among the more distant stars in the universe there must be many which are fully a thousand times as far from us as is Vega, hence we arrive at the startling conception that the light they emit has been on its journey for 18,000 years before it reached us. When we look at those lights to-night we are actually viewing them as they were 18,000 years ago. In fact, those stars might have totally vanished 17,000 years ago, though we and our descendants may still see them glittering for yet another thousand years.
We shall realize a little more fully what this reasoning involves if we suppose that astronomers dwelt on such a star, and that they had eyes and telescopes sufficiently keen not only to discern our little earth, but even to scrutinize its surface with attention. Let us suppose that the stellar astronomers looked at England: do you think they would see a network of railways joining mighty and populous cities, furnished with immense manufactories and with countless institutions? Such would be the England of to-day. But from the distance at which these astronomers are situated light takes 18,000 years for its journey, and, therefore, what they would see would be England as it was 18,000 years ago. To them England would even now appear as a country mainly covered with forests inhabited by bears and wolves, and totally void of any trace of civilization. This illustration will, at all events, serve to convey some conception of the distance at which the outskirts of our visible universe are plunged in the depths of space.