ORIGIN OF THE SOLAR SYSTEM JEANS 156 



very different from what they are now. We come to a time, which 

 we have ah-eady considered, when our sun had not yet assumed its 

 present stellar characteristics. It was a condensation in the arm of 

 a spiral nebula moving with thousands of similar condensations 

 towards a free career in space. Its density was enormously lower 

 than it now is, and its size correspondingly greater. It was also 

 nnich nearer to its neighbors than, in all probability, it has ever 

 been since. In this early stage of its existence, the tidal ellects of 

 its neighbors may well have been enormous; we shall pass to exact 

 figures in a moment. 



In general, the passage of one star past another merely raises a 

 tide which subsides as the tide-raising body recedes. Even when the 

 approach is so close that the height of the tide raised is greater than 

 the original radius of the star, the recession of the disturbing star 

 may result in the disturbed star relapsing merely to its original 

 spherical form. But there is a limit which must not be passed, and 

 if the disturbing body passes this limit, all hope of the star resuming 

 its original shape is lost. The distance of the limit depends pri- 

 marily on the mass of the disturber ; to a lesser degree it depends on 

 the rotation, shape, and density-distribution of the primary star; 

 and to some extent it depends on the velocity of the two stars rela- 

 tive to one another. We shall get a tolerable idea of the march of 

 events if we suppose the primary star to be surrounded by an imag- 

 inary sphere the radius of which depends solely on the mass of the 

 disturbing star. If this mass is equal to the mass of the primary, the 

 radius of this imaginary sphere will be about 2I/4 times the radius of 

 the primary; if the disturbing star has 8 times the mass of the pri- 

 mary, the radius of the imaginary sphere will be 4i^ times that of 

 the primary, and so on. So long as the center of the visiting star 

 remains outside the sphere, a tide is raised which recedes as the visit- 

 ing star disappears, but the moment the visiting star invades this 

 sphere an entirely new phenomenon appears. 



As the approach of the disturber raises the tide to higher and 

 Iiigher levels, the highest points of the tide move ever farther away 

 from the star's center into regions where the gravitational attraction 

 of the star gets weaker and weaker. At the same time, of course, the 

 gravitational pull of the visiting star gets stronger and stronger. 

 Finally, just as the visiting star crosses the critical sphere, its gravi- 

 tational pull just balances that of the primary — it is this condition 

 that defines the critical sphere. If the visiting star further invades 

 this critical sphere, the particles at high tide are shot away from the 

 primary star, the resultant gravitational force on them now being 

 definitely toward the visiting star; they are, of course, immediately 

 replaced by others, which are shot off in turn, and so on. The total 

 effect is that a filament or jet of gas is shot out from the point of 



