288 
Rotation and Fission of Stars 
[CH. X 
vibration : forces of restitution at once come into play and equalise the angular 
velocities. Similar forces of restitution will be in operation right up to the 
instant of fission, so that in the final system the rotations may be expected to 
agree with the revolution, both in period and in phase. The two constituents 
will accordingly rotate about one another in a motion which must at least 
approximate to a rigid body motion, the periods of rotation and revolution 
being approximately identical and the eccentricity of the orbits being very 
small. 
These are precisely the conditions found in binary stars which have, 
according to all available criteria, just broken up by fission. The primary 
criterion for recent fission is the nearness of the two components, since we shall 
find that as a binary ages its two components move further apart. A class of 
stars exists, commonly described as stars of the /3 Lyrae type, in which the 
light curve varies continuously as the components undergo successive eclipses, 
this indicating that the two constituent masses must be either in actual contact 
or very close to actual contact. 
This class of star seems to fulfil all the conditions which our theoretical 
investigation has shewn ought to be expected in stars which have just broken 
up by fission. The periods of rotation and revolution appear to be identical; 
any difference in these periods would shew itself in non-periodicity of the light 
curve; of this there is no evidence whatever. The eccentricity of orbit is 
invariably small, being almost zero for X Carinae and RR Centauri, in which 
the separation, calculated from the light curve, is zero or negative (correspond 
ing to imperfect fission), and ranging from 0 to 0’02 for stars such as /3 Lyrae 
H.D. 1337 and W Crucis in which the separation is excessively small.