120 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 2 



out as far as telescopes can reach. They are so distant that, in gen- 

 eral, they appear as small faint clouds mingled among the stars, and 

 many of them have long been known by the name "nebulae." Their 

 identification as great stellar systems, the true inhabitants of the uni- 

 verse, was a recent achievement of great telescopes. 



On photographs made with such instruments, these nebulae, these 

 stellar systems, appear in many forms. Nevertheless they fall nat- 

 urally into an ordered sequence ranging from compact globular masses 

 through flattening ellipsoids into a line of unwinding spirals. The 

 array exhibits the progressive development of a single basic pattern, 

 and is known as the sequence of classification. It may represent the 

 life history of stellar systems. At any rate, it emphasizes the common 

 features of bodies that belong to a single family. 



Consistent with this interpretation is the fact that these stellar 

 systems, regardless of their structural forms, are all of the same gen- 

 eral order of intrinsic luminosity; that is, of candlepower. They 

 average about 100 million suns and most of them fall within the 

 narrow range from one-half to twice this average value. Giants and 

 dwarfs are known, 10 to 20 times brighter or fainter than the average, 

 but their numbers appear to be relatively small. This conclusion is 

 definitely established in the case of giants, which can be readily ob- 

 served throughout an immense volume of space, but is still speculative 

 in the case of dwarfs which can be studied only in our immediate 

 vicinity. 



The limited range in luminosity is important because it offers a 

 convenient measure of distance. As a first approximation, we may 

 assume that the nebulae are all equally luminous, and, consequently, 

 that their apparent f aintness indicates their distances. The procedure 

 is not reliable in the case of a single object because the particular 

 nebula might happen to be a giant or a dwarf rather than a normal 

 stellar system. But for statistical purposes, where large numbers of 

 nebulae are involved, the relatively few giants and dwarfs should 

 average out, and the mean distances of large groups may be accurately 

 determined. It is by this method that the more remote regions of 

 space, near the limits of the telescope, may be explored with confidence. 



Throughout the observable region the nebulae are found scattered 

 singly, in pairs, and in groups up to great compact clusters or even 

 clouds. Tlie small-scale distribution is irregular, and is dominated 

 by a tendency toward clustering. Yet when larger and larger vol- 

 umes of space are compared, the minor irregularities tend to average 

 out, and the samples grow more and more uniform. If the observable 

 region were divided into a hundred or even a thousand equal parts, 

 the contents would probably be nearly identical. Therefore, the large- 

 scale distribution of nebulae is said to be uniform : the observable re- 



