IN STARRY SKIES

Evening Post, Volume CXII, Issue 91, 14 October 1926, Page 19

IN STARRY SKIES

THE NEBULAR HYPOTHESIS

AND RECENT RESEARCH

(By "Omega Centanri.")

The Nebular theory of Laplace \vi!l be by no means easy to replace. For a hundred years it reigned without any reeognj*td rival, and even after many unexplained difficulties with regard to it had been pointed out, astronomers clung to it tenaciously, because they were unable to suggest any more plausible hypothesis. Laplace's conception was such a magnificent one that it. could not fail to make a profound and lasting impression, on astronomical thought. Even now, when it is generally realised that it must either be. radically modified or else definitely, and completely, abandoned, its powerful influence is felt in. every discussion on cosmical evolution. Before considering any of the liv^l .theories which are. gradually . pushing ,-it, from its proud position, we must recall a few. of tho considerations which have told against its continual acceptance.

Our sun is a star. The stars arc suns. Far from being, as ivas at one time thought, unique in tho vast universe, the mighty ruler of our' solar system is found to be but one of a thousand million similar bodies. Stars and suns throw light upon one another. Much has already been learnt about the life history of the stars, and tho sum of this branch of knowledge is increasing every day. On the other hand, many great observatories are specialising on the' study of the suu. Each of these researches is helping the other. If we can find out definitely how the sun was born, a similar origin will be suggested for innumerable stars. At the same time all new light thrown by the telescope and spectroscope on the evolution of the stars will help to illuminate the problem of tho birth of the solar system.

It is now felt that in postulating as the initial state of our system a vast hot rotating nebulae, Laplace demanded a great deal.. Such a state is inherently unstable and transitory. . The nebula cannot always have been hot, for a hot body incessantly loses heat by radiation. How,, then, did it originally acquire its extremely high temperature? In looking back the mind cannot rest contentedly at such a point. The journey is as inconrplete as that up a scale of music which rises to the seventh and does not pass on to the key not* We seem impelled to search further. What was the state of the system before the nebula became hot? Laplace gives \is no hint. Then, again, how did the nebula acquire its rotation? ■ If Laplace gives us the true history of the birth of tho sun, stars must be- born in the same way. The difficulty of imagining one hot rotating nebula is infinitesimal compared with that of imagining a thousand million similar ones. This.'difficulty is not only a thousand million times as great, but is more nearly equal to the original difficulty raised to the . power of a thousand million;

Then, the solar system is an astonishingly, flat structure. So are the spiral uobylae. So is the Milky Way. The flatness in all cases is probably due to rotation, but the rotation must have been extremely rapid to .produce such a striking effec v t. Before we can be satisfied we must find an explanation of the universal prevalence of such rotation. JJa^tej. we : shall attempt to show that this is one of the effects of encounter, which appears to bo the life principle of the universe, but at present we merely mention it as one of the difficulties which beset the nebular hypothesis.

One serious difficulty was recognised by Laplace, himself. If each ring of matter thrown from the equator of the rotating nebula consists of separate particles, those which are nearer to the nucleus must move faster than those further away. If these particles condense into a single body, it should therefore rotate in. a direction opposite to its revolution round the contracting nobula. Now, tlys is exactly the opposite of what is found to be the case. Every planet, except Uranus, as far aa is known, rotates from west to east in the sanie direction as it revolves. - A.similar rule holds amongst the satellites of the planets, but exceptions are. found amongst the outermost satellites.;of the major planets. Laplace suggested that this difficulty might be got over by supposing cohesion to be so great between the particles that each ring revolved much as a solid body would do. This does not seem probable or even possible. Another'difficulty was pointed out by Darwin. If a ring concentrates, owing to gravitational forces, about any point, that point should be its centre of gravity. A ring surrounding a nebula in course of contraction would therefore be -ab : sorbed by tho nucleus, and could not form a planet or satellite. ;

A much more serious difficulty was discovered by BaMnet in 1861. There is a yery well known principle which is called.by a rather formidable name —the principle, of the conservation of the moment of momentum. This principle ' affirms that no body or system can, of itself withont the action of some other body, either start to rotate, stop rotating, or change in any way its total amount of rotation. Tha moment of momentum of any particle is the product of its mass, its distance from the axis of rotation passing through the centre of gravity of the body or system, and its velocity at right angles to the line joining it to the axis. The moment of momentum of the.whole systom is the sum of all sucli products. Using this principle Laplace saw that, as the nebula shrank, the speed of its rotation must continually increase, but he did not calculate the value of this moment in the solar system at present, and test whether it ia sufficient to account for the effects he postulated. This has been done by others, and the existing moment of momentum turns out to be utterly inadequate. It must have had csactly the same value in the far distant past aa it has to-day, unless a change has been wrought by some body external to the system. Moulton has calculated that for a ring to be tfrrown off, in the manner suggested by Laplace, to form the 'planet Neptune, the nebula would have required a moment of momentum more than 213 times as great, as that which.the solar system possesses. Where has the rest gone to? No method is known by which any fraction of it whatever can have been lost without disturbance from without. Looiing at it in another way Babinet showed that if Noptuno had been thrown off as a ring from a nebula which had the . exact moment of momentum which the solar system has at present, the ring, and consequently Neptune, would have taken more than 2,700,000 years to .make one revolution, whereas Keptune really takes only 165 years. Similarly, if the earth had been formed from such a ring, the year should haye1 been 3181 times as long as it is.

Then again, tho action pictured by Laplace was essentially intermittent. Successive periods of instability occurred. Aftor one ring was thrown off, a

long period ensued before tha next was ejected. In 1869 Kirkwood pointed out that, after tho process of escape onco started, it should have been continuous. When the nebula extended to the orbit of Neptune its density must have been millions of times less than that of the atmosphere. In this state of extreme tenuity, it could not: have had any power of resisting drain. The condensing nebula would thus uecome surrounded liy a. vast cloud of cosmic dust, from which it seems impossible to picture thd development of an orderly system of planets. Now the orderly character of the arrangements and movements of its members is the most impressive fact with regard to thu solar system, which is clearly the outcome of law, not a chance product of the fortuitous concourse of atoms.

Bui amongst all its marvellous congruencies there are curious exceptions, which are extremely difficult to explain. For example, although nearly all the bodies of the system revolve mid rotate in one direction, which is usually called from west to east, the eighth and ni!if}i satellites of Jupiter, the ninth satellite of Saturn, the four satellites of Uranus, and the single satellite of Xeptuue revolve in the opposito direction!. Whilst the. axis of rotation of the planets are mostly inclined at a high angle to the plane of their orbits, the axis of Uranus is only seven degrees from the orbital plane. Again while, according to the nebular hypothesis, every planet should rotate faster than any of its satellites,. JPhobbs/ the inner satellite of Mars, completes more than three revolutions round the planet whilst the latter is turning once upon its axis. The inner ring .at. Saturn is also anomalous. The meteorites of which: it is composed have jjeeu found to complete a revolution in-about half the time that the planet takes to rotate. If the original rotating nebula was perfectly,symmetrical in form, the planets developed from it in accordance with La.placo's hypothesis should all have moved exactly in the plane of the sun's equator, and thpy should have rotated about axis perpendicular to this plane, which should also have contained the orbits of their satellites.

It is clear, then,' that the evolution of the system cannot have had the beautiful simplicity postulated by Laplace. Tho theory, however, survived many rudo shocks. Occasionally objections that for a time appeared forcible, turned out afterwards to be fallacious. Thus when Lord Rosso's great reflector proved one apparent nebular after another to consist in reality of stars, the belief became stronger and stronger that gaseous nebulae did not exist. It became probable that c cry object which appeared nebulous was a distant stellar system. The heavens then presented no longer any example of a body known to be in the gaseous state predicted by Laplace. Nebulae were believed to be clusters so remote that their discrote character would not be detected. The spectrocopic discoveries of Huggins, which proved once for all the existence of gaseous nebulatc, turned the tables again, and holped to revive tho discredited nebular hypothesis. Subsequent research, , howover, disclosed more and more objections to it. As a result of a masterly series of investigations, Jeans has recently concluded that in maasss of stellar dimensions, the tendency is to separate into two portions not very unequal in mass, and never to form a system like that in which we live, with a great central sun and a family of planets comparatively insignificant in size.