GIANT SUNS TURNER. 181 



side, i. e., on the side of the sun's center. Working 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 

 utilize 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 of 

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

 five times less — and bjr 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 O ; 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, how- 

 ever, 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 downward; but then there are 

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

 first, never get beyond the fatal 18 at their best and on whom 

 declining 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 

 outward nearly balances the force of gravitation pulling inward, 

 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 thou- 

 sands 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. 3 



I can not give you a better idea of the value of Prof. Eddington's 

 work than by quoting a few words from a letter written to me by 



3 On reading this again I realize that it does not do full justice to Eddington's sug- 

 gestion for tie lower limit. He shows a definite difficulty in the formation of small stars. 



