TIME SCALE OF OUR UNIVERSE — OPEK 217 



STABILITY OF STAR CLUSTERS AND DOUBLE STARS 



The dynamical stability of clusters has been investigated repeatedly, 

 with the result that most galactic clusters will dissolve, either under 

 the tidal action of the galactic center or through encounters with field 

 stars or other members of the cluster, in time intervals of the order 

 of 1,000 million years (46, 47) . Although this statement refers to the 

 future and, theoretically, is compatible with an unlimited past, the 

 probability of simultaneous occurrence of a great number of old clus- 

 ters which just now have come to the verge of disruption is very small. 

 We may expect an average cluster to be observed in the middle of its 

 lifetime, and assume, therefore, that the age of most clusters is some 

 1,000 million years or less. Yet, most of them contain early-type 

 stars which cannot be very old. Consideration of the dynamical sta- 

 bility of clusters confirms thus the youth of their members, and adds 

 another argument in favor of the theory that stars are being born 

 continually. Apart from that, no new criterion of age for the galaxy 

 is forthcoming — clusters which are older than their stellar content 

 cannot be observed. 



The situation is similar with wide double stars. The distribution 

 of the distances between their components (48, 49) indicates that 

 equipartition of energy caimot have taken place (50), and that the 

 binaries could not have been subjected to encounters with field stars 

 for longer than, say, 5,000 million years (4). On the other hand, the 

 statistical material from which tliis conclusion is drawn is based chiefly 

 on the relatively luminous A-type binaries wliich, according to the 

 preceding, cannot have lived to so great an age, anyway. 



Thus, conclusions as to age based on the dynamical stability of 

 clusters and double stars are overruled by the shorter lifetime of their 

 components, and can be used only to reaffirm the short time scale of 

 stellar evolution. 



THE RED SHIFT OF EXTRAGALACTIC NEBULAE 



The observations by V. M. Slipher, Hubble, and Himnason, if in- 

 terpreted in a straiglitforward manner, indicate a recession of the 

 extragalactic nebulae proportional to distance, or an expansion at 

 a uniform rate of the visible portion of the universe. 



Eecent developments have shown, in a manner that leaves practically 

 no doubt, that Hubblc's scale of distances should be at least doubled. 

 The distances of the nearest nebulae were determined by Hubble from 

 the period-luminosity relation of the long-period cepheids. The zero 

 point of this relation depended upon space absorption in low galactic 

 latitudes, and was known to be inaccurate, but, for lack of better data, 

 it was accepted and used during the past quarter of a century as a basis 

 for work on the structure of the universe. Some cosmological theories 



