168 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 



The answer is simple if you have a large enough telescoj)e. With the 

 large telescopes in existence today, it is possible to photograph the 

 separate stars and clusters in the largest (and closest) of the spirals, 

 the great Andromeda nebula. Using the brightness method, and tak- 

 ing account of the nearby smokiness of our own Milky Way system, but 

 assuming that space outside is transparent, Hubble has shown that the 

 Cepheid variables and clusters in the Andromeda nebula must be about 

 a million light-years away. Once the distances of another half dozen 

 extragalactic nebulae were found by these methods, Hubble determined 

 their average intrinsic luminosity (about 85 million times the sun's), 

 and can now determine the distances to much more distant nebulae with 

 the aid of this information. Using longer and longer exposures on 

 faster and faster photographic plates with larger and larger tele- 

 scopes, Hubble has pushed the confines of observable space out to over 

 a billion light-years. In this vast volume he estimates there are some 

 hundreds of millions of extragalactic nebulae, basing this estimate, 

 of course, on a limited number of "sample" plates. Surveys now 

 under way with the 20-inch refractor at the Lick Observatory and 

 the 48-inch Schmidt camera at Palomar will cover somewhat smaller 

 volumes more completely. 



What can we find out about these distant objects so far beyond the 

 Milky Way ? It seems to me that the studies which have already been 

 undertaken, and which are now under way, can be grouped in this man- 

 ner, although there is some overlapping : First, we can find out more 

 about individual extragalactic nebulae, their distances, sizes, masses, 

 forms, and contents; second, as with any large class of objects, we 

 can try to group them into further meaningful subclasses; third, we 

 can study their motions^ so far as they can be measured, and finally, 

 their numbers and distribution in space. This last problem, literally 

 the biggest in modern science, turns out to be linked with our funda- 

 mental notions of space and time. 



Although it may seem simple, in principle, to determine the linear 

 size of an object from its angular size and its distance, there are serious 

 practical difficulties in the case of the nebulae. They have no clearly 

 defined edges. Photographs of longer exposure show greater exten- 

 sions, and there are reasons to believe that the Andromeda nebula, for 

 example, is well over five times as large as what we can see in a tele- 

 scope. The best photographs of this spiral show an angular extent 

 corresponding to a diameter of about 40,000 light-years, at its distance 

 of over 700,000 light-years. This is roughly the same size as the Milky 

 Way system, which seems reasonable enough. (In fact, our picture of 

 the Milky Way system has been developed partly by analogy to the 

 form of spiral nebulae ; in other words, it has long been in the back of 

 astronomers' minds that the galaxy and the spirals are the same class 



