144 
SOLUTION OF THE EQUATIONS 
ionisation of some predominant element or group of elements. In that 
case y could temporarily fall below £. There would be a sudden collapse 
of the star which would be arrested as soon as the contraction had pro 
vided enough energy to accomplish this critical ionisation; after that the 
star would resume orderly progress with y >£, starting from the more 
condensed condition. Cepheid pulsations might possibly be started by a 
collapse of this kind, but I do not think the hypothesis has much to 
recommend it.