GALAXIES- — SHAPLEY 137 



on the best of our lono-exposiire plates the hirgest individual galaxies 

 only to a distance of 10 or 15 thousand light-years from their centers. 

 Either those systems are very much smaller than ours, or we are unable 

 to explore the faint regions that are as remote from the nuclei as we 

 are from our nucleus in Sagittarius. 



It might turn out, therefore, that the bird's-eye observer from the 

 Andromeda nebula would report that our galactic system is no larger 

 than the Andromeda nebula ; or, if the research were rather casual, the 

 view might include only tlie nuclear portions of our galactic system, 

 which might even be cataloged as a spheroidal galaxy. When we, in 

 our turn, take a quick bird's-eye view of the Andromeda galaxy, and 

 measure its distance and dimensions, we immediately conclude that it 

 is much smaller than we are, even though it is a giant compared with 

 the average galaxy of our catalogs. But when the over-all extent of 

 the Andromeda galaxy is studied with precise measuring apparatus, 

 we double the dimensions as first seen and conclude that it is not very 

 much smaller than the Milky Way system. 



5. Why is it that we seem to be so baffled about the structure and 

 dimensions of our own system, although we bravely go out to dis- 

 tances of 100 million light-years in our explorations of other galaxies? 

 What is so troublesome about measuring something that completely 

 surrounds us and is near at hand ? 



That question finally brings out one feature of Milky Way structure 

 which must be clearly seen at first glance by the observer in Andro- 

 meda, but which has taken us many years and much labor to discover 

 and partially evaluate. This basic feature (and difficulty) is the pres- 

 ence throughout the Milky Way, especially near the Milky Way plane, 

 of interstellar absorbing niaterial — dust and gas, scattered and in 

 clouds, around the stars and in the spaces between them. Our vision 

 is not clear ; simple geometric relations between light and distance are 

 incorrect because our observing station is in a fog that unevenly dims 

 the light of the surrounding stars. 



Gradually we are learning through studies of colors, and otherwise, 

 how to make corrections for the interstellar absorption. It would not 

 be difficult at all if the absorbing material were uniform. But the 

 clouds of absorption are irregular. It is supposed that some of the 

 greatest irregularities would be apparent to the Andromeclan observer. 

 At any rate, our own bird's-eye views of hundreds of external galaxies 

 show immediately the dark lanes between spiral arms, or across them, 

 which indicate the interstellar absorption clouds that irregularly dim 

 the star fields of those distant stellar systems. 



In summary, our imaginary V)ird's-eye view has revealed our system 

 as discoidal in its main body of stars, probably surrounded by a thinly 

 populated spheroidal shell and dominated by a massive globular nu- 



