July 7, 1921] 



NATURE 



i89 



w ould actually happen we may conjecture that the 

 process we have been describing is that of the fis- 

 sion of a single star into a binary of the familiar 

 type, but the conjecture is beset by many diffi- 

 culties. To mention one only : if we have truly 

 described the history of a star before fission, the 

 star ought during a moderate part of its life to 

 possess an ellipsoidal figure, and as this rotated 

 the light received from the star ought to vary to 

 an extent which just before fission might amount 

 to 09 magnitude. Yet I believe there are only 

 three known stars whose variation of light is such 

 as could possibly be accounted for by an ellipsoidal 

 surface, and even in these cases the interpreta- 

 tion is doubtful. On the other hand, very con- 

 siderable reassurance is provided by the researches 

 of Russell on multiple stars. After a star has 

 broken into two parts by fission both parts will 

 continue to shrink, so that either or both may in 

 turn again break up, and a triple or quadruple 

 system be formed. Russell finds that in a multiple 

 system which has been formed in this way the 

 distance between the stars formed by subsequent 

 fissions cannot be more than a small fraction, at 

 most about one-fifth, of the distance between the 

 pair generated by the original fission. A mere 

 glance at a catalogue of multiple stars will show 

 that this condition is fulfilled by the majority of 

 observed systems. On account of foreshortening 

 the apparent separations will not always appear 

 to conform to the rule,, but Russell has shown, 

 as the result of a careful statistical discussion, that 

 the exceptions agree, both in kind and in 

 number, with what might be expected from 

 foreshortening. 



We have now traced out the life-historv of a 

 rotating and shrinking mass from beginning to 

 end, from its start as a gaseous mass of very low 

 density, through its assumption of a lenticular 

 shape and its first break-up as a spiral nebula, 

 through its subsequent condensation into separate 

 stars, to their final fissions into binary and multiple 

 systems. The picture has been distressingly in- 

 complete, and it cannot be denied that the story 

 is beset by many difficulties and uncertainties. 

 The mathematical investigation is far from per- 

 fect ; gaps in theory have frequently been bridged 

 by nothing more substantial than conjecture ; in 

 many cases there has been room for grave doubt 

 as to the identification of observed formations with 

 those predicted by theory ; in one instance at least 

 a formation predicted by theory, the ellipsoidal 

 star, is practically unknown to the observing 

 astronomer. But, after allowing for all imperfec- 

 tions, we have a tolerably complete knowledge, so 

 far as the main outlines arc concerned, of the 

 whole chain of configurations which will be 

 assumed in turn by the rotating shrinking mas? 

 of Laplace, and on this cham there does not appear 

 to be any room for the solar system. 



Apart from this, there are weighty reasons 

 for thinking that our system has not been 

 formed as the result of a rotational break- 

 up. The angular momentum of a svstem 



XO. 2697, VOL. 107] 



remains constant during a process of break- 

 ing up, and, as was pointed out by Babinet in 

 1 86 1, even if the whole angular momentum of the 

 solar system were now concentrated in the sun it 

 would still have less than a quarter of the angular 

 momentum requisite for breaking up at its present 

 density. Except in the improbable event of the 

 solar system, since fission^ having been robbed by 

 a passing star of by far the greater part of its 

 angular momentum, its rotation can never have 

 been sufficient to cause a break-up. Clearly there 

 is a case for examining whether some other agency 

 cannot produce a system such as ours. 



The sun and moon, as we know, raise tides on 

 our earth the height of which forms only an inap- 

 preciable fraction of the earth's radius. If our earth 

 were replaced by a mass of liquid or gas of low 

 density the fraction would be greater, varying in- 

 versely as the density of the mass. If the sun and 

 earth were placed much nearer to one another than 

 now the tides would be increased in the ratio of 

 the inverse cube of their distance apart. We can 

 easily imagine conditions under which the heights 

 of the tides would be comparable with the radius 

 of the earth, and here the simple formulae which 

 the mathematician uses to calculate the heights of 

 terrestrial tides become useless. The general 

 investigation of the succession of shapes which 

 will be assumed by a gaseous or plastic mass as 

 the tidal forces on it continually increase presents 

 a difficult but not altogether intractable problem 

 for the mathematician. 



It is found that the tides will be of the general 

 type with which we are familiar on the earth until 

 a certain critical height of tide is reached. This 

 critical height is comparable with half the radius 

 of the mass, being greater or smaller according 

 as the mass is of more or less uniform density. 

 After this critical height has been passed, there 

 is no longer a configuration of equilibrium under 

 the tidal forces. Dynamical motion ensues, and 

 the general nature of this motion will consist in 

 the ejection of two arms or jets of matter, one 

 towards the attracting mass and one, which may 

 be smaller, or may be absent altogether, in the 

 exactly opposite direction. If the tide-generating 

 forces should be suddenly removed at this stage 

 the jets would, of course, fall back into the mass 

 from which they emerged, and this would in time 

 resume its spherical form. But if the tidal forces 

 persist, the jets will continue to be thrown out, 

 and it can be shown that a continuous distribu- 

 tion of density in these jets would be unstable, 

 just in the same way, and for similar reasons, as 

 in the case we previously discussed of the jets 

 thrown out from a rotating mass of gas. Con- 

 densations would form in the jets, and ultimately 

 the jet would break up into separate detached 

 masses. 



According to the tidal hypothesis of the origin 

 of the solar system, the sun was at some past time 

 subjected to intense tidal forces from a passing 

 star, the sequence of processes we have just 

 described took place, and the emitted jet broke 



