COSMOGONY — JEANS 175 



THE LIVES OF THE STAKS 



The stars are almost certainly born in nebulae of the type of the 

 great extragalactic nebulue, such as are shown in Plates 1, 2, and 3. 

 These nebulae show a <zrcat variety of shapes, but a single thread 

 connects them all; they are the shapes of huge masses of gas endow^ed 

 Avith different amounts of rotation. So definitely is this the case that 

 when Hubble recently tried to classify the shapes of these nebulae, 

 deliberately and avowedly shutting his eyes to all theoretical con- 

 siderations, he found that purel}^ observational considerations com- 

 pelled him to classify them in precisely the sequence I had i)iedicted 

 on theoretical grounds some 10 years earlier. 



A huge mass of gas which was entirely devoid of rotation would, 

 of course, assume a strictly spherical shape; rotation would flatten 

 this shape out, just as the earth is flattened by its rotation, until 

 ultimately most of the matter was spread out in a thin disk. We 

 see the process beginning in Plate 1, and it is well advanced in 

 Plate 2. Plate 3 shoAvs a nebula which is probably physically similar 

 to that shown in Plate 2, but viewed from another angle. Now, 

 mathematical theory shows that the thin disklike structure could not 

 remain a mere featureless mass of gas. Just as the cooling of a 

 cloud of steam causes it to condense into drops of water, so the 

 cooling of a cloud of gas causes it to condense into detached masses. 

 We see the phenomenon in progress in nebular photographs; it is 

 a necessary theoretical consequence of the law^s of gases and the law 

 of gravitation. 



Now, the same theory which predicts that the phenomenon must 

 happen predicts the scale on which it will happen. We can calculate 

 how much matter will go to the formation of each " drop," and the 

 calculated masses of the drops come out to be just about the same 

 as the masses of the stars. Indeed, these drops are stars, and the 

 process just described is that of the birth of stars. Unmistakable 

 stars have been observed in the outer regions of many of the spiral 

 nebulai. It is naturally not possible to identify every observed 

 spot of light with a star, but some of them show precisely the same 

 peculiar fluctuations of light as characterize a certain class of 

 variable star, the Cepheid variables already mentioned, and these 

 put the identity of these particular spots of light beyond all reason- 

 able doubt. 



In these nebula, then, we are watching the birth of stars, the 

 transformation of an inchoate mass of gas into an " island universe " 

 of stars. Indeed Hubble found it necessary to end up his classifica- 

 tion of nebulae with clouds of stars. At one end of his continuous 

 sequence is a nebula shaped like a mass of rotating gas, in which not 

 a single star is visible ; at the other end a star cloud in which nothing 



