236 



SCIENCE 



[N. S. Vol. LII. No. 1341 



with their heat and radiate at least a hundred 

 times as fast as the sun. The supply of energy 

 which suffices to maintain the sun for 10,000,- 



000 years would be squandered by a giant star 

 in less than 100,000 years. The whole evolu- 

 tion in the giant stage would have to be very 

 rapid. In 18,000 years at the most a typical 

 star must pass from the initial M stage to 

 type G. In 80,000 years it has reached type 

 A, near the top of the scale, and is about to 

 start on the downward path. Even these fig- 

 ures are probably very much overestimated. 

 Jtfost of the naked-eye stars are still in the 

 giant stage. Dare we believe that they were 

 all formed within the last 80,000 years? The 

 telescope reveals to us objects not only remote 

 in distance but remote in time. We can turn 

 it on a globular cluster and behold what was 

 passing 20,000, 50,000, even 200,000 year? ago 

 —unfortunately not all in the same cluster, 

 but different clusters representing different 

 epochs of the past. As Shapley has pointed 

 out, the verdict appears to be " no change." 

 This is perhaps not conclusive, because it does 

 jnot follow that individual stars have suffered 

 no change in the interval; but it is difficult to 

 resist the impression that the evolution of the 

 stellar universe proceeds at a slow, majestic 

 pace, with respect to which these periods of 

 time are insignificant. 



1 There is another line of astronomical evi- 

 dence which appears to show more definitely 

 ithat the evolution of the stars proceeds far 

 more slowly than the contraction hypothesis 

 allows; and perhaps it may ultimately enable 

 us to measure the true rate of progress. There 

 are certain stars, known as Cepheid variables, 

 which undergo a regular fluctuation of light 

 of a characteristic kind, generally with a 

 period of a few days. This light change is not 

 4ue to eclipse. Moreover, the color quality of 

 the light changes between maximum and mini- 

 mimi, evidently pointing to a periodic change 

 in the physical condition of the star. Al- 

 though these objects were formerly thought to 

 be double stars, it now seems clear that this 

 was a misinterpretation of the spectroscopic 

 evidence. There is in fact no room for the hy- 

 pothetical companion star; the orbit is so small 



that we should have to place it inside the prin- 

 :cipal star. Everything points to the period of 

 the light pulsation being something intrinsic_ 

 in the star; and the hypothesis advocated by 

 iShapley, that it represents a mechanical pul- 

 ;sation of the star, seems to be the most plaus- 

 ible. I have already mentioned that the ob- 

 served period does in fact agree with the cal- 

 iCulated period of mechanical pulsation, so that 

 /the pulsation explanation survives one fairly 

 iStringent test. But whatever the cause of the 

 ivariability, whether pulsation or rotation, pro- 

 tvided only that it is intrinsic in the star, and 

 pot forced from outside, the density must be 

 ;the leading factor in determining the period. 

 ilf the star is contracting so that its density 

 ;changes appreciably, the period can not re- 

 jmain constant. Now, on the contraction hy- 

 pothesis the change of density must amount to 

 ]at least 1 per cent, in 40 years- (I give the 

 figures for 8 Cepliei, the best -known variable of 

 this class.) The corresponding change of 

 period should be very easily detectable. For 

 B Cephei the period ought to decrease 40 sec- 

 ;Onds annually. 



Now S Cephei has been under careful obser- 

 vation since 1785, and it is known that the 

 change of period, if any, must be very small. 

 S. Chandler found a decrease of period of %o 

 second per annum, and in a recent investiga- 

 tion E. Hertzsprung has found a decrease of 

 Vio second per annum. The evidence that 

 there is any decrease at all rests almost en- 

 tirely on the earliest observations made be- 

 fore 1800, so that it is not very certain; but 

 in any case the evolution is proceeding at not 

 more than Moo of the rate required by the 

 contraction hypothesis. There must at this 

 stage of the evolution of the star be some 

 other source of energy which prolongs the life 

 of the star 400-fold. The time-scale so en- 

 larged would suffice for practically all reason- 

 able demands. 



I hope the dilemma is plain. Either we 

 must admit that whilst the density changes 1 

 per cent, a certain period intrinsic in the star 

 can change no more than V^oo of 1 per cent., 

 or we must give up the contraction hy- 

 pothesis. 



