146 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1924 



distance apart. A comparison between their distance apart, as cal 

 culated in kilometers, and their angular distance apart, as observed 

 in the sky, leads at once to an estimate of the distance of the nebula 

 to which they belong. It is gratifying to find that estimates of 

 nebular distances made in this way are in good agreement with 

 estimates made in other ways. The calculation of the masses of 

 these condensations leads to a still more interesting and significant 

 result. In every nebula for which the calculation can be made, the 

 calculated mass of a single condensation proves to be approximately 

 equal to the mass of the average star. 



This gives, I believe, the key to the evolutionary process we haA e 

 been considering — we have been watching the creation of the stars. 

 In Plate 1, Figure 1 we saw the raw material — a gaseous mass of ex- 

 treme tenuity, already moulded, as a result of shrinkage and conse- 

 quent increase of rotation, to the stage at which disintegration is 

 about to commence. Further shrinkage takes place, and in Plate 1, 

 Figure 2, and Plate 2, Figure 1 we see the ejection of jets of gas 

 from which the future stars will in due course be made. In Plate 

 2, Figure 2, and Plate 3, Figure 1 individual stars are beginning to 

 form, although at present only as vague condensations in what is 

 still a continuous nebular mass. Finally, the outermost parts of 

 Plate 3, Figure 2, and Plate 4, Figure 1 show us the finished prod- 

 uct — separate masses, although still far more tenuous than ordinary 

 stars, starting off on their independent existences. Each of these 

 masses will go through the changes we have already briefly described. 

 It will contract, getting hotter in doing so, until it reaches a maxi- 

 mum temperature just as the gas laws are beginning to fail, after 

 which it cools and contracts into a dead dark mass. 



The family of stars born out of a single nebula may be millions 

 in number. They may either mingle with the general mass of the 

 stars or, if the original nebula was sufficiently remote from the main 

 universe of stars, may form a separate colony by themselves. In 

 illustration of the former alternative, numbers of groups of stars 

 are known — e. g. the Pleiades, the stars of the Great Bear — in which 

 all the members have a common velocity and, generally speaking, 

 similar physical constitutions also. All the stars of any such group 

 are voyaging through space together, and have obviously done so 

 since they first came into being. The alternative possibility of a 

 family of stars forming a detached colony by themselves is perhaps 

 exemplified in the so-called "globular" star-clusters, such as the 

 well-known cluster in Hercules (pi. 5). These are globular only in 

 name, for Shapley has found that they are of an elliptical structure, 

 showing symmetry about a plane precisely as might be expected if 

 they were the final product of a rotating nebula. 



