CHAPTER XXV

THE STELLAR UNIVERSE

The stars appear fairly evenly distributed all around us, except in one portion of the sky where they seem very crowded, and so give one an impression of being very distant. This portion, known as the Milky Way, stretches, as we have already said, in the form of a broad band right round the entire heavens. In those regions of the sky most distant from the Milky Way the stars appear to be thinly sown, but become more and more closely massed together as the Milky Way is approached.

This apparent distribution of the stars in space has given rise to a theory which was much favoured by Sir William Herschel, and which is usually credited to him, although it was really suggested by one Thomas Wright of Durham in 1750; that is to say, some thirty years or more before Herschel propounded it. According to this, which is known as the "Disc" or "Grindstone" Theory, the stars are considered as arranged in space somewhat in the form of a thick disc, or grindstone, close to the _central_ parts of which our solar system is situated.[35] Thus we should see a greater number of stars when we looked out through the _length_ of such a disc in any direction, than when we looked out through its _breadth_. This theory was, for a time, supposed to account quite reasonably for the Milky Way, and for the gradual increase in the number of stars in its vicinity.

It is quite impossible to verify directly such a theory, for we know the actual distance of only about forty-three stars. We are unable, therefore, definitely to assure ourselves whether, as the grindstone theory presupposes, the stellar universe actually reaches out very much further from us in the direction of the Milky Way than in the other parts of the sky. The theory is clearly founded upon the supposition that the stars are more or less equal in size, and are scattered through space at fairly regular distances from each other.

Brightness, therefore, had been taken as implying nearness to us, and faintness great distance. But we know to-day that this is not the case, and that the stars around us are, on the other hand, of various degrees of brightness and of all orders of size. Some of the faint stars--for instance, the galloping star in Pictor--are indeed nearer to us than many of the brighter ones. Sirius, on the other hand, is twice as far off from us as [a] Centauri, and yet it is very much brighter; while Canopus, which in brightness is second only to Sirius out of the whole sky, is too far off for its distance to be ascertained! It must be remembered that no parallax had yet been found for any star in the days of Herschel, and so his estimations of stellar distances were necessarily of a very circumstantial kind. He did not, however, continue always to build upon such uncertain ground; but, after some further examination of the Milky Way, he gave up his idea that the stars were equally disposed in space, and eventually abandoned the grindstone theory.

Since we have no means of satisfactorily testing the matter, through finding out the various distances from us at which the stars are really placed, one might just as well go to the other extreme, and assume that the thickening of stars in the region of the Milky Way is not an effect of perspective at all, but that the stars in that part of the sky are actually more crowded together than elsewhere--a thing which astronomers now believe to be the case. Looked at in this way, the shape of the stellar universe might be that of a globe-shaped aggregation of stars, in which the individuals are set at fairly regular distances from each other; the whole being closely encircled by a belt of densely packed stars. It must, however, be allowed that the gradual increase in the number of stars towards the Milky Way appears a strong argument in favour of the grindstone theory; yet the belt theory, as above detailed, seems to meet with more acceptance.

There is, in fact, one marked circumstance which is remarkably difficult of explanation by means of the grindstone theory. This is the existence of vacant spaces--holes, so to speak, in the groundwork of the Milky Way. For instance, there is a cleft running for a good distance along its length, and there is also a starless gap in its southern portion. It seems rather improbable that such a great number of stars could have arranged themselves so conveniently, as to give us a clear view right out into empty space through such a system in its greatest thickness; as if, in fact, holes had been bored, and clefts made, from the boundary of the disc clean up to where our solar system lies. Sir John Herschel long ago drew attention to this point very forcibly. It is plain that such vacant spaces can, on the other hand, be more simply explained as mere holes in a belt; and the best authorities maintain that the appearance of the Milky Way confirms a view of this kind.

Whichever theory be indeed the correct one, it appears at any rate that the stars do not stretch out in every direction to an infinite distance; but that _the stellar system is of limited extent_, and has in fact a boundary.

In the first place, Science has no grounds for supposing that light is in any way absorbed or destroyed merely by its passage through the "ether," that imponderable medium which is believed to transmit the luminous radiations through space. This of course is tantamount to saying that all the direct light from all the stars should reach us, excepting that little which is absorbed in its passage through our own atmosphere. If stars, and stars, and stars existed in every direction outwards without end, it can be proved mathematically that in such circumstances there could not remain the tiniest space in the sky without a star to fill it, and that therefore the heavens would always blaze with light, and the night would be as bright as the noonday.[36] How very far indeed this is from being the case, may be gathered from an estimate which has been made of the general amount of light which we receive from the stars. According to this estimate the sky is considered as more or less dark, the combined illumination sent to us by all the stars being only about the one-hundreth part of what we get from the full moon.[37]

Secondly, it has been suggested that although light may not suffer any extinction or diminution from the ether itself, still a great deal of illumination may be prevented from reaching us through myriads of extinguished suns, or dark meteoric matter lying about in space. The idea of such extinguished suns, dark stars in fact, seems however to be merely founded upon the sole instance of the invisible companion of Algol; but, as we have seen, there is no proof whatever that it is a dark body. Again, some astronomers have thought that the dark holes in the Milky Way, "Coal Sacks," as they are called, are due to masses of cool, or partially cooled matter, which cuts off the light of the stars beyond. The most remarkable of these holes is one in the neighbourhood of the Southern Cross, known as the "Coal Sack in Crux." But Mr. Gore thinks that the cause of the holes is to be sought for rather in what Sir William Herschel termed "clustering power," _i.e._ a tendency on the part of stars to accumulate in certain places, thus leaving others vacant; and the fact that globular and other clusters are to be found very near to such holes certainly seems corroborative of this theory. In summing up the whole question, Professor Newcomb maintains that there does not appear any evidence of the light from the Milky Way stars, which are apparently the furthest bodies we see, being intercepted by dark bodies or dark matter. As far as our telescopes can penetrate, he holds that we see the stars _just as they are_.

Also, if there did exist an infinite number of stars, one would expect to find evidence in some direction of an overpoweringly great force,--the centre of gravity of all these bodies.

It is noticed, too, that although the stars increase in number with decrease in magnitude, so that as we descend in the scale we find three times as many stars in each magnitude as in the one immediately above it, yet this progression does not go on after a while. There is, in fact, a rapid falling off in numbers below the twelfth magnitude; which looks as if, at a certain distance from us, the stellar universe were beginning to _thin out_.

Again, it is estimated, by Mr. Gore and others, that only about 100 millions of stars are to be seen in the whole of the sky with the best optical aids. This shows well the limited extent of the stellar system, for the number is not really great. For instance, there are from fifteen to sixteen times as many persons alive upon the earth at this moment!

Last of all, there appears to be strong photographic evidence that our sidereal system is limited in extent. Two photographs taken by the late Dr. Isaac Roberts of a region rich in stellar objects in the constellation of Cygnus, clearly show what has been so eloquently called the "darkness behind the stars." One of these photographs was taken in 1895, and the other in 1898. On both occasions the state of the atmosphere was practically the same, and the sensitiveness of the films was of the same degree. The exposure in the first case was only one hour; in the second it was about two hours and a half. And yet both photographs show _exactly the same stars, even down to the faintest_. From this one would gather that the region in question, which is one of the most thickly star-strewn in the Milky Way, is _penetrable right through_ with the means at our command. Dr. Roberts himself in commenting upon the matter drew attention to the fact, that many astronomers seemed to have tacitly adopted the assumption that the stars extend indefinitely through space.

From considerations such as these the foremost astronomical authorities of our time consider themselves justified in believing that the collection of stars around us is _finite_; and that although our best telescopes may not yet be powerful enough to penetrate to the final stars, still the rapid decrease in numbers as space is sounded with increasing telescopic power, points strongly to the conclusion that the boundaries of the stellar system may not lie very far beyond the uttermost to which we can at present see.

Is it possible then to make an estimate of the extent of this stellar system?

Whatever estimates we may attempt to form cannot however be regarded as at all exact, for we know the actual distances of such a very few only of the nearest of the stars. But our knowledge of the distances even of these few, permits us to assume that the stars close around us may be situated, on an average, at about eight light-years from each other; and that this holds good of the stellar spaces, with the exception of the encircling girdle of the Milky Way, where the stars seem actually to be more closely packed together. This girdle further appears to contain the greater number of the stars. Arguing along these lines, Professor Newcomb reaches the conclusion that the farthest stellar bodies which we see are situated at about between 3000 and 4000 light-years from us.

Starting our inquiry from another direction, we can try to form an estimate by considering the question of proper motions.

It will be noticed that such motions do not depend entirely upon the actual speed of the stars themselves, but that some of the apparent movement arises indirectly from the speed of our own sun. The part in a proper motion which can be ascribed to the movement of our solar system through space is clearly a displacement in the nature of a parallax--Sir William Herschel called it "_Systematic_ Parallax"; so that knowing the distance which we move over in a certain lapse of time, we are able to hazard a guess at the distances of a good many of the stars. An inquiry upon such lines must needs be very rough, and is plainly based upon the assumption that the stars whose distances we attempt to estimate are moving at an average speed much like that of our own sun, and that they are not "runaway stars" of the 1830 Groombridge order. Be that as it may, the results arrived at by Professor Newcomb from this method of reasoning are curiously enough very much on a par with those founded on the few parallaxes which we are really certain about; with the exception that they point to somewhat closer intervals between the individual stars, and so tend to narrow down our previous estimate of the extent of the stellar system.

Thus far we get, and no farther. Our solar system appears to lie somewhere near the centre of a great collection of stars, separated each one from the other, on an average, by some 40 billions of miles; the whole being arranged in the form of a mighty globular cluster. Light from the nearest of these stars takes some four years to come to us. It takes about 1000 times as long to reach us from the confines of the system. This globe of stars is wrapt around closely by a stellar girdle, the individual stars in which are set together more densely than those in the globe itself. The entire arrangement appears to be constructed upon a very regular plan. Here and there, as Professor Newcomb points out, the aspect of the heavens differs in small detail; but generally it may be laid down that the opposite portions of the sky, whether in the Milky Way itself, or in those regions distant from it, show a marked degree of symmetry. The proper motions of stars in corresponding portions of the sky reveal the same kind of harmony, a harmony which may even be extended to the various colours of the stars. The stellar system, which we see disposed all around us, appears in fine to bear all the marks of an _organised whole_.

The older astronomers, to take Sir William Herschel as an example, supposed some of the nebulae to be distant "universes." Sir William was led to this conclusion by the idea he had formed that, when his telescopes failed to show the separate stars of which he imagined these objects to be composed, he must put down the failure to their stupendous distance from us. For instance, he thought the Orion Nebula, which is now known to be made up of glowing gas, to be an external stellar system. Later on, however, he changed his mind upon this point, and came to the conclusion that "shining fluid" would better account both for this nebula, and for others which his telescopes had failed to separate into component stars.

The old ideas with regard to external systems and distant universes have been shelved as a consequence of recent research. All known clusters and nebulae are now firmly believed to lie _within_ our stellar system.

This view of the universe of stars as a sort of island in the immensities, does not, however, give us the least idea about the actual extent of space itself. Whether what is called space is really infinite, that is to say, stretches out unendingly in every direction, or whether it has eventually a boundary somewhere, are alike questions which the human mind seems utterly unable to picture to itself.

[35] The Ptolemaic idea dies hard!

[36] Even the Milky Way itself is far from being a blaze of light, which shows that the stars composing it do not extend outwards indefinitely.

[37] Mr. Gore has recently made some remarkable deductions, with regard to the amount of light which we get from the stars. He considers that most of this light comes from stars below the sixth magnitude; and consequently, if all the stars visible to the naked eye were to be blotted out, the glow of the night sky would remain practically the same as it is at present. Going to the other end of the scale, he thinks also that the combined light which we get from all the stars below the seventeenth magnitude is so very small, that it may be neglected in such an estimation. He finds, indeed, that if there are stars so low as the twentieth magnitude, one hundred millions of them would only be equal in brightness to a single first-magnitude star like Vega. On the other hand, it is possible that the light of the sky at night is not entirely due to starlight, but that some of it may be caused by phosphorescent glow.