1919] Giant Suns 411 



WEEKLY EVENING MEETING, 

 Friday, January 31, 1919. 



The Hon. Richard Clere Parsons, M.A. M.Inst.C.E., 



Vice-President, in the Chair. 



Professor H. H. Turner, D.Sc. D.C.L. F.R.S. ■ F.R.A.S. 



Giant Suns. 



We have all been fascinated by Giants, from the times we read of 

 Jack the Giant Killer in our childhood to those more recent when 

 we read of the exploits of Lient. Warneford and his successors in 

 their fights with the Giant Zeppelins. 



I make no apology for shortening my title a little from what 

 astronomers might expect. I might have chosen " Giant and Dwarf 

 Stars,'' but stars are suns, as we shall see presently, and though I 

 shall include " dwarf " suns, the real dwarfs of science are the tiny 

 atoms at the other end of the scale of investigation — or rather, the 

 electrons into which they have been broken up. 



How shall we gauge the size of a star to see whether it is a 

 giant ? We must know two things : first of all, the apparent size of 

 its disc, and secondly, its distance. In the old days it was thought 

 that the size of the sun could be estimated from one of these only — 

 from the size of the disc. Lucretius,""' following Epicurus, believed 

 that the sun was a small body. He arrived at this conclusion by 

 neglecting entirely the consideration of the distance and judging by 

 the appearance to our senses. 



Now, without attempting to decide whether the sun is the size of 

 a soup-plate, or of a threepenny-bit, or what is the size that it seems 

 to be, we may remark that it seems to be about the same size as the 

 moon, and that by Lucretius' principle the sun and moon ought to 

 be of the same actual size. 



However, we now know that the sun is 400 times bigger than 

 the moon, because its distance is 400 times greater. We have 

 measured the distance of the sun and found it to be nearly 100 

 million miles, and we have measured that of the moon and found it 

 to be nearly a \ million ; and since the discs appear of about the same 



* " Nee nimio solis major rota nee minor ardor 



Esse potest, nostris quam sensibus esse videtur." 



Lucret., De Nat. Bev., v. 564-5. 



