170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



chosen, as it now appears that the "late" types, if anything, are 

 younger in age. This conclusion is based by the German astrophysicist 

 von Weizacker on dynamical arguments, by Baade and others on the 

 content. Baade, at Mount Wilson, has superposed on Hubble's classi- 

 fication a distinction between two types of stellar population: Type 

 I, associated with spiral arms, consists primarily of gas clouds and 

 large, hot, blue stars which are thought, from considerations of stellar 

 evolution to be young; type II population, associated with the ellip- 

 tical nebulae and the cores of spirals, consists primarily of cooler 

 stars, which may be quite as old as the earth and sun — a good 3 

 billion years. Hubble's three classes can be arranged in a kind of 

 sequence, from the globular (circular appearing) elliptical nebulae, 

 through the more and more elliptical types, through a transition type 

 neither elliptical nor spiral, and then, in two parallel "branches," 

 through the later and later types of spirals, both normal and barred. 

 If the more recent ideas are correct, spiral evolution is taking place 

 in the reverse direction; late-type spirals are collapsing to the final 

 stage of globular nebulae. 



One aspect of the content of extragalactic nebulae has received but 

 scant attention until recently. Just as in our own galaxy, there is 

 interstellar gas and dust in the other nebulae. The dust shows up 

 as dark streaks on photographs ; the gas emits light of the character- 

 istic colors of hydrogen, oxygen, nitrogen, and other gases. In the 

 extragalactic nebulae we have an excellent opportunity to study the 

 distribution and physical conditions of these interstellar gases, and 

 work now in progress at the McDonald Observatory is directed to 

 this end. 



We have already seen how one component of the motion (the radial 

 velocity) of nebulae can be measured by the Doppler effect, and how 

 their distances can be estimated by the brightness method. In 1925, 

 Hubble and Humason at the Mount Wilson Observatory noted a 

 correlation between these two, in the sense that velocity of recession 

 for nebulae on all sides of us increases with increasing distance. The 

 later work of Hubble and Humason has shown a remarkable relation 

 which holds as far as the spectra of nebulae can be observed: the 

 velocity of recession on every side is proportional to the distance, and 

 increases about 100 miles per second in each million light-years. 



At first sight, this observation seems to leave us — or our galaxy — 

 in a central and highly repelling position, with all the rest of the uni- 

 verse "running away from us." A moment's reflection shows, however, 

 that the "velocity-distance law" implies a symmetrical view from any 

 other nebula ; an observer there, considering himself "at rest," would 

 see the others "running away" from him, and with velocities propor- 

 tional to their distances. As for explanation of this strange behavior, 



