1919] on Giant Suns 419 



form the tail. But this force is not merely in existence on the 

 •outside of the sun, it permeates its whole body. A particle inside 

 the sun is of course receiving radiation-pressure from all its sur- 

 roundings, but the pressure will naturally be greater on the hotter 

 side, i.e. on the side of the sun's centre. AVorking out the problem 

 afresh with the addition of this new factor. Professor Eddington has 

 obtained results which agree satisfactorily with the observed effects, 

 and indeed the closeness of the agreement is startling. He is able to 

 utihse the fact noticed earlier in the lecture, that the masses of the 

 stars are not very different, so that it is easy to take three repre- 

 sentative cases — let us say one in which the mass is equal to that on 

 our sun, one in which it is 5 times greater, and one in which it is 

 5 times less — and by following these three cases in detail he can 

 show the distinctive features of different stars. Briefly, the step- 

 ladder is highest for the star of greatest mass, which may get hotter 

 and hotter "until it reaches type ; a star of intermediate mass like 

 our sun is arrested at a lower height and may not reach higher than 

 type F, or at best A, before it begins to fall down the inclined leg ; 

 while a star of small mass may reach no higher than type K at any 

 time. The golfers in the audience may be reminded of their handi- 

 caps. Those who are destined to be scratch players (probably, 

 however, not because of their great mass) improve very rapidly until 

 they reach the highest pitch of excellence, and it may even be only 

 in old age that they begin to travel downwards ; but then there are 

 others of long handicap, who although they may improve a little at 

 first never get beyond the fatal 1<S at their best, and on whom 

 •decHning years soon begin to leave their mark. 



One of the most remarkable suggestions of Professor Eddington's 

 work gives a reason for the close resemblance in mass of the stars. 

 There is a certain mass for which the radiation-pressure pressing 

 outwards nearly balances the force of gravitation pulling inwards, 

 and it is clear that for stars as large or larger than this a break-up 

 sooner or later is to be expected. This assigns very obviously the 

 upper limit to the masses — we can easily see why there are no stars 

 larger than a certain limit. But how about the lower limit ? Are 

 there no stars very much smaller than this ? Certainly there are : 

 we are living on one of them. Our earth is smaller by some 

 thousands of times ; but then it is not a star in the full sense, for it 

 is not shining with its own light. If it did ever so shine the light 

 must have been feeble at best and have only lasted for a very short 

 time. There may in fact be many small stars, but we do not see 

 them, and accordingly have not reckoned them in saying that the 

 masses of the stars are closely similar." 



* On reading this again I realise that it does not do full justice to 

 Eddington's suggestion for the lower limit. He shows a definite difficulty 

 in the formation of small stars. 



