44 SECTIONAL ABDRESSES. 



by experimental evidence which it is difficult to dispute. Lord Kelvin's 

 date of the creation of the Sun is treated with no mote respect than 

 Archbishop Ussher's. 



The serious consequences of this contraction hypothesis are particu- 

 larly prominent in the case of giant stars, for the giants are prodigal 

 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 evolution in the giant stage Avould have to 

 be very rapid. In 18,000 years at the most a typical st-ar 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 figures are probably very much over-estimated.' 

 Most of the naked-eye stars are still in the giant stage. Dare we 

 beheve 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 years ago — unfoi'tunately 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 not 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. 



There is another line of astronomical evidence which appears to 

 show more definitely that the evolution of the staa's proceeds far more 

 slowly than the contraction hypothesis allows ; and perhaps it may ulti- 

 mately 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 due to eclipse. Moreover, the 

 colour quality of the light changes between maximum and minimum, 

 evidently pointing to a periodic change in the physical condition of the 

 tetaa-. Although 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 hypothetical 

 companion star ; the orbit is so small that we should have to place it 

 inside the principal star. Everything points to the period of the light 

 pulsation being something intrinsic in the star ; and the hypothesis 

 advocated by Shapley, that it represents a mechanical pulsation of the 

 star, seems to be the most plausible. I have already mentioned that the 

 obsei'ved period does in fact agree with the calculated period of 

 mechanical pulsation, so that the pulsation explanation survives one 

 fairly stringent test. But whatever the cause of the variability, 

 whether pulsation or rotation, provided only that it is intrinsic in the 



' I have taken the ratio of specific heats at the extreme possible value, § ; 

 that is to say, no allowance has been made for the energy needed for ionisa- 

 tion and internal vibrations of the atoms, v/hich makes a further call on the 

 scanty supply available. 



