In the studies described in this paper we make quantitative use of the radio-active decays of uranium and thorium in cosmochronology in much the same manner as these decays have been employed in geochronology. The paper is divided into two quite different parts representing different views as to the immediate source of the material that now constitutes the solar system. Model 1. The Autonomous Galaxy. From its origin, the Galaxy has been an autonomous system—no further important additions of material from intergalactic space have taken place at subsequent times. Star formation, stellar evolution, and nucleosynthesis have declined at a steady exponential rate over the whole lifetime of the Galaxy. The duration of nucleosynthesis is found to be independent of this rate over rather wide limits. Model 2. Steady-State Cosmology and Galactic-Intergalactic Exchange of Matter. The abundance of the elements in intergalactic matter has reached a steady state through interchange with galaxies in which stars produce elements beyond hydrogen. As a consequence of this same point of view, the Galaxy acquired significant quantities of intergalactic material at various times. This occurred particularly about one billion years before the sun and solar system were formed. Except at epochs of addition of new gas, stellar activity has declined exponentially, as in Model 1. Consideration of the decay of the radioactive isotopes Th^(232), U^(235), U^(238), according to Model 1, leads to the conclusion that the age of the Galaxy is 15^(+5)_(−3) × 10^9 years. Similar considerations according to Model 2 lead to the conclusion that the expansion time scale of the universe (the reciprocal of the Hubble constant H) is 11 ± 6 × 10^9 years. The error can be reduced to ±2 × 10^9 years if the present thorium-uranium ratio is chosen to give a Pb^(208)/Pb^(206) age for the solar system concordant with that given by Pb^(207)/Pb^(206), namely 4.5 × 10^9 years. Applications to the chronology of the Galaxy can no longer be made in a simple manner but the age found in the Model 1 calculations would seem to be a lower limit.

© 1960 Elsevier Inc. Supported in part by the joint program of the Office of Naval Research and the U. S. Atomic Energy Commission.