The Story of Evolution | Page 7

space some 16,000,000,000 miles in diameter. That is a very small area in the new universe. Draw a circle, 100 billion miles in diameter, round the sun, and you will find that it contains only three stars besides the sun. In other words, a sphere of space measuring 300 billion miles in circumference--we will not venture upon the number of cubic miles--contains only four stars (the sun, alpha Centauri, 21,185 Lalande, and 61 Cygni). However, this part of space seems to be below the average in point of population, and we must adopt a different way of estimating the magnitude of the universe from the number of its stellar citizens.
Beyond the vast sphere of comparatively empty space immediately surrounding our sun lies the stellar universe into which our great telescopes are steadily penetrating. Recent astronomers give various calculations, ranging from 200,000,000 to 2,000,000,000, of the number of stars that have yet come within our faintest knowledge. Let us accept the modest provisional estimate of 500,000,000. Now, if we had reason to think that these stars were of much the same size and brilliance as our sun, we should be able roughly to calculate their distance from their faintness. We cannot do this, as they differ considerably in size and intrinsic brilliance. Sirius is more than twice the size of our sun and gives out twenty times as much light. Canopus emits 20,000 times as much light as the sun, but we cannot say, in this case, how much larger it is than the sun. Arcturus, however, belongs to the same class of stars as our sun, and astronomers conclude that it must be thousands of times larger than the sun. A few stars are known to be smaller than the sun. Some are, intrinsically, far more brilliant; some far less brilliant.
Another method has been adopted, though this also must be regarded with great reserve. The distance of the nearer stars can be positively measured, and this has been done in a large number of cases. The proportion of such cases to the whole is still very small, but, as far as the results go, we find that stars of the first magnitude are, on the average, nearly 200 billion miles away; stars of the second magnitude nearly 300 billion; and stars of the third magnitude 450 billion. If this fifty per cent increase of distance for each lower magnitude of stars were certain and constant, the stars of the eighth magnitude would be 3000 billion miles away, and stars of the sixteenth magnitude would be 100,000 billion miles away; and there are still two fainter classes of stars which are registered on long-exposure photographs. The mere vastness of these figures is immaterial to the astronomer, but he warns us that the method is uncertain. We may be content to conclude that the starry universe over which our great telescopes keep watch stretches for thousands, and probably tens of thousands, of billions of miles. There are myriads of stars so remote that, though each is a vast incandescent globe at a temperature of many thousand degrees, and though their light is concentrated on the mirrors or in the lenses of our largest telescopes and directed upon the photographic plate at the rate of more than 800 billion waves a second, they take several hours to register the faintest point of light on the plate.
When we reflect that the universe has grown with the growth of our telescopes and the application of photography we wonder whether we may as yet see only a fraction of the real universe, as small in comparison with the whole as the Babylonian system was in comparison with ours. We must be content to wonder. Some affirm that the universe is infinite; others that it is limited. We have no firm ground in science for either assertion. Those who claim that the system is limited point out that, as the stars decrease in brightness, they increase so enormously in number that the greater faintness is more than compensated, and therefore, if there were an infinite series of magnitudes, the midnight sky would be a blaze of light. But this theoretical reasoning does not allow for dense regions of space that may obstruct the light, or vast regions of vacancy between vast systems of stars. Even apart from the evidence that dark nebulae or other special light-absorbing regions do exist, the question is under discussion in science at the present moment whether light is not absorbed in the passage through ordinary space. There is reason to think that it is. Let us leave precarious speculations about finiteness and infinity to philosophers, and take the universe as we know it.
Picture, then, on the more moderate estimate, these 500,000,000 suns scattered over tens of thousands of billions of miles. Whether