GIANT SUNS. 1 



By Prof. H. H. Turner, D. Sc, D. C. L., F. R. S., F. R. A. S. 



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 Lieutenant "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 disk, 

 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 disk. 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 appear- 

 ance 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 mny 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 

 100,000,000 miles, and we have measured that of the moon and found 

 it to be nearly one-fourth million ; and since the disks appear of about 



1 Reprinted by permission from Proceedings of the Weekly Evening Meeting of the Royal 

 Institution of Great Britain, Jan. 31, 1910. 



2 " Xec nimio solis major rota nee minor ardor 



Esse potest, nostris quam sensibus esse videtur." 



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



173 



