STRUCTUEE OF THE GALAXY PLASKETT 



197 



plane of the disk where it is impossible to see more than the vertical 

 projection of part of the disk or, in other words, where we cannot 

 (listinirnish the wood for the trees? As it is impossible for us either 

 to determine the structure of the galaxy from our position within 

 it or to get outside the galaxy to examine it from without, the only 

 recourse is to see if we can find any prototypes outside the galaxy, 

 any outside systems whose structure may legitimately be assumed as 

 similar to our own. 



Figure 5. — The galactic system. 



It is now quite generally believed that the spiral nebulae, of 

 which many thousands or perhaps even millions can be detected by 

 modern telescopes, are external galaxies, are stellar systems, consti- 

 tuted very similarly to our own galax3^ Thanks to the work of 

 Hubble at the Mount Wilson Observatory, who Avas able, by examin- 

 ing the Cepheid variables in some of the nearer of these nebulae, to 

 determine their distances in the same way that Shapley obtained 

 the distances of the globular clusters in our own galaxy or nebula, 

 we now know the distances and the dimensions of some of the nearer 

 of these external galaxies, these spiral nebulae. 



The characteristic form of these external star systems is familiar 

 to all, and we see readily the reason for the name spiral, assigned 



