Astronomy of To-day Part 22

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 pa.s.sage through the "ether," that imponderable medium which is believed to transmit the luminous radiations through s.p.a.ce. 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 pa.s.sage 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 circ.u.mstances there could not remain the tiniest s.p.a.ce 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 s.p.a.ce. 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 ma.s.ses 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 "cl.u.s.tering power," _i.e._ a tendency on the part of stars to acc.u.mulate in certain places, thus leaving others vacant; and the fact that globular and other cl.u.s.ters 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 a.s.sumption that the stars extend indefinitely through s.p.a.ce.

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 s.p.a.ce 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 a.s.sume 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 s.p.a.ces, 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 s.p.a.ce 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 a.s.sumption 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 cl.u.s.ter. 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 cl.u.s.ters 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 s.p.a.ce itself. Whether what is called s.p.a.ce 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 phosph.o.r.escent glow.

CHAPTER XXVI

THE STELLAR UNIVERSE--_continued_

It is very interesting to consider the proper motions of stars with reference to such an isolated stellar system as has been pictured in the previous chapter. These proper motions are so minute as a rule, that we are quite unable to determine whether the stars which show them are moving along in straight lines, or in orbits of immense extent. It would, in fact, take thousands of years of careful observation to determine whether the paths in question showed any degree of curving. In the case of the more distant stars, the accurate observations which have been conducted during the last hundred years have not so far revealed any proper motions with regard to them; but one cannot escape the conclusion that these stars move as the others do.

If s.p.a.ce outside our stellar system is infinite in extent, and if all the stars within that system are moving unchecked in every conceivable direction, the result must happen that after immense ages these stars will have drawn apart to such a distance from each other, that the system will have entirely disintegrated, and will cease to exist as a connected whole. Eventually, indeed, as Professor Newcomb points out, the stars will have separated so far from each other that each will be left by itself in the midst of a black and starless sky. If, however, a certain proportion of stars have a speed sufficiently slow, they will tend under mutual attraction to be brought to rest by collisions, or forced to move in orbits around each other. But those stars which move at excessive speeds, such, for instance, as 1830 Groombridge, or the star in the southern constellation of Pictor, seem utterly incapable of being held back in their courses by even the entire gravitative force of our stellar system acting as a whole. These stars must, therefore, move eventually right through the system and pa.s.s out again into the empty s.p.a.ces beyond. Add to this; certain investigations, made into the speed of 1830 Groombridge, furnish a remarkable result. It is calculated, indeed, that had this star been _falling through infinite s.p.a.ce for ever_, pulled towards us by the combined gravitative force of our entire system of stars, it could not have gathered up anything like the speed with which it is at present moving. No force, therefore, which we can conjure out of our visible universe, seems powerful enough either to have impressed upon this runaway star the motion which it now has, or to stay it in its wild course. What an astounding condition of things!

Speculations like this call up a suspicion that there may yet exist other universes, other centres of force, notwithstanding the apparent solitude of our stellar system in s.p.a.ce. It will be recollected that the idea of this isolation is founded upon such facts as, that the heavens do not blaze with light, and that the stars gradually appear to thin out as we penetrate the system with increasing telescopic power. But perchance there is something which hinders us from seeing out into s.p.a.ce beyond our cl.u.s.ter of stars; which prevents light, in fact, from reaching us from other possible systems scattered through the depths beyond. It has, indeed, been suggested by Mr. Gore[38] that the light-transmitting ether may be after all merely a kind of "atmosphere"

of the stars; and that it may, therefore, thin off and cease a little beyond the confines of our stellar system, just as the air thins off and practically ceases at a comparatively short distance from the earth. A clashing together of solid bodies outside our atmosphere could plainly send us no sound, for there is no air extending the whole way to bear to our ears the vibrations thus set up; so light emitted from any body lying beyond our system of stars, would not be able to come to us if the ether, whose function it is to convey the rays of light, ceased at or near the confines of that system.

Perchance we have in this suggestion the key to the mystery of how our sun and the other stellar bodies maintain their functions of temperature and illumination. The radiations of heat and light arriving at the limits of this ether, and unable to pa.s.s any further, may be thrown back again into the system in some altered form of energy.

But these, at best, are mere airy and fascinating speculations. We have, indeed, no evidence whatever that the luminiferous ether ceases at the boundary of the stellar system. If, therefore, it extends outwards infinitely in every direction, and if it has no absorbing or weakening effect on the vibrations which it transmits, we cannot escape from the conclusion that practically all the rays of light ever emitted by all the stars must chase one another eternally through the never-ending abysses of s.p.a.ce.

[38] _Planetary and Stellar Studies_, by John Ellard Gore, F.R.A.S., M.R.I.A., London, 1888.

CHAPTER XXVII

THE BEGINNING OF THINGS

LAPLACE'S NEBULAR HYPOTHESIS

Dwelling upon the fact that all the motions of revolution and rotation in the solar system, as known in his day, took place in the same direction and nearly in the same plane, the great French astronomer, Laplace, about the year 1796, put forward a theory to account for the origin and evolution of that system. He conceived that it had come into being as a result of the gradual contraction, through cooling, of an intensely heated gaseous lens-shaped ma.s.s, which had originally occupied its place, and had extended outwards beyond the orbit of the furthest planet. He did not, however, attempt to explain how such a ma.s.s might have originated! He went on to suppose that this ma.s.s, _in some manner_, perhaps by mutual gravitation among its parts, had acquired a motion of rotation in the same direction as the planets now revolve. As this nebulous ma.s.s parted with its heat by radiation, it contracted towards the centre. Becoming smaller and smaller, it was obliged to rotate faster and faster in order to preserve its equilibrium. Meanwhile, in the course of contraction, rings of matter became separated from the nucleus of the ma.s.s, and were left behind at various intervals. These rings were swept up into subordinate ma.s.ses similar to the original nebula. These subordinate ma.s.ses also contracted in the same manner, leaving rings behind them which, in turn, were swept up to form satellites. Saturn's ring was considered, by Laplace, as the only portion of the system left which still showed traces of this evolutionary process. It is even probable that it may have suggested the whole of the idea to him.

Laplace was, however, not the first philosopher who had speculated along these lines concerning the origin of the world.

Nearly fifty years before, in 1750 to be exact, Thomas Wright, of Durham, had put forward a theory to account for the origin of the whole sidereal universe. In his theory, however, the birth of our solar system was treated merely as an incident. Shortly afterwards the subject was taken up by the famous German philosopher, Kant, who dealt with the question in a still more ambitious manner, and endeavoured to account in detail for the origin of the solar system as well as of the sidereal universe. Something of the trend of such theories may be gathered from the remarkable lines in Tennyson's _Princess_:--

"This world was once a fluid haze of light, Till toward the centre set the starry tides, And eddied into suns, that wheeling cast The planets."

The theory, as worked out by Kant, was, however, at the best merely a _tour de force_ of philosophy. Laplace's conception was much less ambitious, for it did not attempt to explain the origin of the entire universe, but only of the solar system. Being thus reasonably limited in its scope, it more easily obtained credence. The arguments of Laplace were further founded upon a mathematical basis. The great place which he occupied among the astronomers of that time caused his theory to exert a preponderating influence on scientific thought during the century which followed.

A modification of Laplace's theory is the Meteoritic Hypothesis of Sir Norman Lockyer. According to the views of that astronomer, the material of which the original nebula was composed is presumed to have been in the meteoric, rather than in the gaseous, state. Sir Norman Lockyer holds, indeed, that nebulae are, in reality, vast swarms of meteors, and the light they emit results from continual collisions between the const.i.tuent particles. The French astronomer, Faye, also proposed to modify Laplace's theory by a.s.suming that the nebula broke up into rings all at once, and not in detail, as Laplace had wished to suppose.

The hypothesis of Laplace fits in remarkably well with the theory put forward in later times by Helmholtz, that the heat of the sun is kept up by the continual contraction of its ma.s.s. It could thus have only contracted to its present size from one very much larger.

Plausible, however, as Laplace's great hypothesis appears on the surface, closer examination shows several vital objections, a few of those set forth by Professor Moulton being here enumerated--

Although Laplace held that the orbits of the planets were sufficiently near to being in the one plane to support his views, yet later investigators consider that their very deviations from this plane are a strong argument against the hypothesis.

Again, it is thought that if the theory were the correct explanation, the various...o...b..ts of the planets would be much more nearly circular than they are.

It is also thought that such interlaced paths, as those in which the asteroids and the little planet Eros move, are most unlikely to have been produced as a result of Laplace's nebula.

Further, while each of the rings was sweeping up its matter into a body of respectable dimensions, its gravitative power would have been for the time being so weak, through being thus spread out, that any lighter elements, as, for instance, those of the gaseous order, would have escaped into s.p.a.ce in accordance with the principles of the kinetic theory.

_The idea that rings would at all be left behind at certain intervals during the contraction of the nebula is, perhaps, one of the weakest points in Laplace's hypothesis._

Mathematical investigation does not go to show that the rings, presuming they could be left behind during the contraction of the ma.s.s, would have aggregated into planetary bodies. Indeed, it rather points to the reverse.