TnvIE SCALE OF OUR UNIVERSE — OPIK 211 



equals 5 to 6 thousand million years, less than the double of the age of 

 the earth (1, 16, 22, 23). The sharpest margin results from the 

 uranium 235-lead 207 ratio. The closeness of the order of magnitude 

 of the upper limit to the age of the earth is significant and makes it 

 likely that the true age of the elements does not differ much from that 

 of the earth — a figure of about 4,500 million years appearing to be 

 plausible. 



In these estimates there is some uncertainty from the unknown 

 composition of the earth's interior, which, however, is hardly signifi- 

 cant in view of the exponential law of variation of abundance ratios 

 with time. Even a large error in the present ratio will not affect the 

 order of magnitude of the resulting age. The mere presence of radio- 

 active substances is a proof of the temporal origin of the terrestrial 

 elements. 



Except for meteorites, there are no data available as to the abundance 

 of radioactive isotopes outside the earth; the above-mentioned time 

 limit refers therefore strictly only to the sample of matter represented 

 by our globe. 



Although the relative abundances of the elements (excluding the 

 lightest, which have escaped from small bodies like our planet) in the 

 earth's crust and in the atmospheres of the sun and most stars are 

 very similar, this does not necessarily mean a simultaneous origin for 

 their elements. Only a similar mode of origin is implied. 



Several attempts have been made to explain, with more or less suc- 

 cess, the origin and relative abundances of the elements by equilibrium 

 conditions inside superdense stars (Klein, Beskow, and Treffenberg; 

 Hoyle; van Albade). Supernova explosions inject the mixture into 

 space, whence it condenses again into new-born stars (24). Observa- 

 tions of the Crab Nebula (25), a former supernova, suggest that the 

 product of explosion — the amorphous core of the nebula — is poor in 

 hydrogen, whereas its hydrogen fringe appears to be interstellar gas 

 pushed ahead of the expanding core. We may thus have a double 

 origin for the elements: hydrogen already present in space with an 

 unknown original content of other elements ; and the heavier elements 

 enriching the mixture through sui:)ernova explosions. 



Old stars — those of "Population II" — seem to show, indeed, a 

 smaller metal content than those believed to be more recently formed 

 (26), suggesting a gradual change in the composition of the medium 

 of which stars are built. If this is so, we need not go to the beginnings 

 of the universe to account for the radioactive isotopes on earth : they 

 may be the products of supernova explosions that preceded the forma- 

 tion of the solar system. 



Nevertheless, serious doubts with respect to the latter conclusion 

 are justified. The theory of stellar structure would admit the building 



