THE STARS AND NEBULAE 305 



unique feat of surgery, when Nature attempts it, turns 

 out a success, the sundered ends are successfully sealed 

 and cauterized, and the severance of the celestial Siamese 

 twins becomes an accomplished fact. However, there 

 is at least one case where a failure in this sort of cosmic 

 chirurgery seems to have been scored and resulted in a 

 double-nebula instead. Harking back to Darwin's tidal 

 evolution theory, the reader may argue, as indeed Darwin 

 and his adherents do, that since the fission of stars can 

 and does take place, the earth might have flung off the 

 moon. The cases, however, as the sincere reader will 

 see on second thought, are not parallel at all. In this 

 connection compare this interesting passage from Doctor 

 Campbell's book, (p. 291) : 



The two stars [Beta Lyrse] are enormous in size, but of very 

 low density. They are so close together as to be almost in con- 

 tact. The two bodies are in form approximately prolate ellip- 

 soids, with their longer dimensions in the line joining the two 

 bodies. The immediate cause of the variable brightness is due, 

 in large part, to the eclipsing of one body by the other, but there 

 are probably other factors entering to a minor degree, such as 

 tidal ebb and flow, which must exist, as the orbit seems to be 

 slightly eccentric. 



VARIABLE STARS 



In a sense all stars may be said to be Variable, for 

 all alike have explosive graduated shells. In the very 

 large stars, however, the shells are much more homoge- 

 neous and much more fluid, so that the ebullient action is 

 practically uniform. I am speaking now, not of eclipsing 

 variables, whose changefulness is manifestly due to the 

 intervention of companion bodies, but of those whose 

 flickering is due to a change that goes on in the star itself. 

 In the previous chapter I have already given the reader 

 my version of the true nature of sun-spots and the rea- 

 sons for their periodicity, and he has only to apply the 

 same principles to the analysis of particular cases and 

 phases of star variation. In this engrossing study, how- 

 ever, two exceedingly important points must be ever 

 borne in mind, namely; (1) that a very high percentage 



