558 The Genesis of Double Stars 



smooth dotted curve will be explained hereafter, but, by reference 

 to the scale of magnitudes on the margins of the figure, it may 

 be used to note that the dots might be brought into a perfectly 

 smooth curve by shifting some few of the dots by about a hundredth 

 of a magnitude. 



This light-curve presents those characteristics which are due 

 to successive eclipses, but the exact form of the curve must depend 

 on the nature of the two mutually eclipsing stars. If we are to inter- 

 pret the curve with all possible completeness, it is necessary to make 

 certain assumptions as to the stars. It is assumed then that the 

 stars are equally bright all over their disks, and secondly that they 

 are not surrounded by an extensive absorptive atmosphere. This last 

 appears to me to be the most dangerous assumption involved in the 

 whole theory. 



Making these assumptions, however, it is found that if each of the 

 eclipsing stars were spherical it would not be possible to generate 



Fig. 6. 

 The shape of the star ER Centauri. 



such a curve with the closest accuracy. The two stars are certainly 

 close together, and it is obvious that in such a case the tidal forces 

 exercised by each on the other must be such as to elongate the figure 

 of each towards the other. Accordingly it is reasonable to adopt the 

 hypothesis that the system consists of a pair of elongated ellipsoids, 

 with their longest axes pointed towards one another. No supposition 

 is adopted a priori as to the ratio of the two masses, or as to their 

 relative size or brightness, and the orbit may have any degree of 

 eccentricity. These last are all to be determined from the nature 

 of the light-curve. 



In the case of RR Centauri, however, Dr Roberts finds the 

 conditions are best satisfied by supposing the orbit to be circular, 

 and the sizes and masses of the components to be equal, while their 

 luminosities are to one another in the ratio of 4 to 3. As to their 

 shapes he finds them to be so much elongated that they overlap, 

 as exhibited in his figure now reproduced as Fig. 6. The dotted curve 





