212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 5 



up of the ligliter metal nuclei in superdense stellar cores (27, 28), as 

 well as during the hydrogen explosion of a star that has become hydro- 

 dynamically unstable (24). From this, however, it is a long way to 

 the extreme conditions at which uranium and similar elements of high 

 atomic number can begin to be produced. Although light metals can, 

 indeed, be currentl}'^ supplied by the above-described mechanism, it is 

 doubtful whether the heavy radioactive isotopes could originate in 

 stellar interiors. More likely, these isotopes have come into being in a 

 more powerful "explosion" which involved the whole universe, namely, 

 that which happened at an early stage of its expansion. In that case 

 the age of at least the heavier terrestrial elements would still be synony- 

 mous with the age of the world. 



This leads us to another group of theories which explain the ob- 

 served abundances of all elements, including the heaviest, by their 

 building up from a nonequilibrium, extremely hot mixture (chiefly 

 neutron gas) at an early stage of an exploding mii verse (Alpher, 

 Bethe, Gamow). 



It is possible that the lighter elements (say, those lighter than iron) 

 have originated from two different processes — during the primordial 

 explosion, and currently in stellar interiors — whereas the heavy iso- 

 topes were all created at the "beginning of the world" ; in such a case, 

 as shown above, the "radioactive" age of the universe, or the time 

 elapsed since the big explosion, is about 4.5 thousand million years. 

 However, unless the possibility of formation of the heavy elements 

 in superdense stellar interiors can be definitely disproved, a certain 

 ambiguity will remain attached to the meaning of this figure. 



METEORITES 



The pioneer work of Paneth 20 years ago raised hopes that radio- 

 active age determinations of meteorites, based on their helium content, 

 might yield a clue to tlie age of the solar system at least, or even to 

 that of the whole universe (1). Unfortunately, the meteorites did not 

 come up to original expectations. Paneth's struggle with this problem, 

 which is not concluded yet, led over disappointments and disclaimers 

 of former results; e. g., he announced that all his determinations prior 

 to 1940 were technically unreliable. Paneth's researches are a re- 

 markable example of a gallant fight for the truth, without bias toward 

 his former work, some of which he rejected as soon as it was found 

 that it did not comply with his own high standards. 



The leakage of helium from meteorites to space was one of the many 

 difficulties, and for this reason stony meteorites proved unreliable, so 

 that only data referring to iron meteorites could be fully trusted. 

 From refined analysis of the helium content of the latter Paneth found 

 the ages of meteorites to lie between 100,000 and 9 thousand million 



