138 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 193 8 



tually, if the sample is fair, we may be able to infer the nature of the 

 universe as a whole from the observed characteristics of the sample 

 available for inspection. This possibility is the ultimate goal of the 

 explorations of space. The sample must be fair, and the characteris- 

 tics must be determined with precision. How near we are to the 

 realization of these conditions, we do not know, but we like to believe 

 that the new telescope — the 200-inch reflector destined for Mount 

 Palomar — may furnish an answer. 



Meanwhile it is imperative to learn as much as possible with existing 

 instruments, concerning the nature of the inhabitants of space. The 

 nebulae are the landmarks that must necessarily be used in the general 

 study. The more intimately we know their characteristics — their 

 luminosities, dimensions, masses, structures and contents — the more 

 reliably can we interpret the reports of the final surveys. 



Our present information is still fragmentary, but by piecing bits 

 together we can construct a fairly coherent picture. The nebulae, it 

 is found, are all members of a single family. The forms vary widely 

 but they fall readily into a sequence which represents the progressive 

 variation of a single fundamental pattern. Thus it is possible to 

 reduce the nebulae to a standard type, and to study them as one 

 homogeneous group ; or, it is possible to select conspicuous, neighboring 

 nebulae, and to present them as typical examples of the family. In 

 the discussion which follows, the latter alternative will be adopted. 



THE LOCAL GROUP 



On the grand scale, the nebulae are scattered more or less at ran- 

 dom; one large volume of space is much like another. Nevertheless, 

 the small scale distribution is quite irregular, and presents many 

 analogies with the distribution of stars. Thus we find isolated 

 nebulae, multiples, groups, clusters, and, possibly, clouds. Our own 

 stellar system is the chief component of a triple nebula, the Magel- 

 lanic Clouds playing the role of satellites. The great neighboring 

 spiral in Andromeda is also favored with two satellites. These two 

 triple systems, together with four or five other nebulae, form a loose 

 group more or less isolated in the general field. The local group has 

 played an important role in the development of nebular research for 

 it furnished a small sample collection of nebulae so near that their 

 brighter stars could be studied with existing telescopes. These stars, 

 and, in particular, the Cepheid variables, furnished the criteria which 

 established the scale of distances, and thereby opened the realm of 

 the nebulae to actual exploration. 



