TIME SCALE OF OUR UNIVERSE — OPIK 215 



persistent appearance in globular clusters as an indication of their 

 longevity, a more powerful source of energy must be assumed for their 

 maintenance (35) — either gravitation of their superdense cores, or 

 annihilation of matter. On the other hand, these giants may repre- 

 sent short-lived objects in "statistical equilibrium" with the rest of the 

 stellar population — those which blow up or collapse being replaced by 

 others becoming giants. This latter concept would agree with the 

 calculated red-giant models (39, 40) which are supposed not to draw 

 on miknown sources of energy and are short-lived, their luminosities 

 being abnormally high as compared with their masses. The giants 

 of the globular clusters, as well as the short-period variables which 

 should represent a phase preceding the giant stage, would then cor- 

 respond to stars of more or less similar mass for which the exhaustion 

 of hydrogen has reached a critical limit (35). Taking the observed 

 luminosities with Schwarzschild's models, the limiting mass would 

 be from 3.0 to 2.0 solar mass, indicating for the clusters an age between 

 800 and 2,500 million years. 



The fork-shaped H-R (Hertzsprung-Russell) diagram of the globu- 

 lar clusters represents apparently the result of aging, in contrast 

 to the continually rejuvenated Population I of our galactic surround- 

 ings (the difference in metal content having only a secondary effect). 

 The globular clusters, which are all well outside the galactic plane 

 and are not sharing in galactic rotation, will necessarily oscillate on 

 both sides of the galactic plane, the period of oscillation being less 

 than 100 million years (Oort). Thus, they must have repeatedly 

 gone through the galactic plane. While passing for the first time 

 through the plane, they must have been stripped of all their diffuse 

 matter — which could have been but loosely bound by a gravitational 

 potential of only 1/lOOOth that of the galaxy — through collision with 

 the diffuse matter near the galactic plane; the mechanism is similar 

 to that visualized by Spitzer and Baade (43) for collisions of galaxies. 

 This would have prevented the subsequent formation of new stars in 

 them. The stellar population of the globular clusters must therefore 

 consist of members of almost the same age, which came into being 

 when the galaxy was formed, and represents thus one of the oldest 

 time indicators. The lower branch of their H-R fork appears to join 

 the H-R diagram of Population I at absolute bolometric magnitude 

 + 2 (41) ; this should be the luminosity of old stars which have now 

 arrived at the end of their career as dwarfs. 



The evolution of dwarf stars, without much mixing of their sub- 

 stance, amounts to chemical changes around their central cores, where 

 hydrogen is converted into helium ; the composition of the outer re- 

 gions remains unchanged. Opik (27) has followed the evolution of 

 such stellar models by numerical integrations. From these calcula- 



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