GALAXIES — SHAPLEY 143 



along with the available information on Cepheid variables and ordi- 

 nary novae, to get the distances of galaxies that are not more than a 

 few million light-years away. If a galaxy is as much as 20 million 

 light-years distant, even these supergiants cannot be individually 

 l^hotographed with jiresent telescopic and photographic equipment, 

 and resort must be made to other photometric means of estimating dis- 

 tances. Ordinary giant stars, like Vega and Arcturus, are not yet 

 photographed in any but the very nearest galaxies, and stars of avei-- 

 age mass and luminosity, like the sun, have so far not been photo- 

 graphed outside our own galactic system. 



The future of research on galaxies probably depends not so much 

 on the size of telescopes as on the speed and resolution of photographic 

 plates and on other radiation-registering devices. But even without 

 better facilities than those at present available, astronomers have 

 plenty to do in galactic research, for within reach are 1,000 million in- 

 dividual stars in our own galaxy, and at least 10 million other galaxies. 



