There are several independent methods of determining the time when the first stars appeared and stellar nucleosynthesis started. Each method is model dependent. The use of radioactive isotopes, e.g. 232Th, 238U and 235U, relies on their initial abundances, obtained from models. Improvements in the accuracy of measurements and refinements of r-process models are "pathways" to a more reliable radiometric cosmochronology. Cluster isochrons in the H-R diagram rely on stellar models.

However, even though each method is far from perfect, the various independent approaches give a similar age for the earliest nucleosynthesis, —14 Gyr, which is indistinguishable from the Hubble time. There is little doubt that the galaxies, at least the Milky Way, and the earliest stars were formed very soon after cosmological nucleosynthesis, but the exact delay cannot be precisely quantified at present.

From self-consistent model estimates of the age of the most ancient r-elements an exponentially decreasing rate of production of these elements with time is derived. The exponent, — 2, agrees with models of galactic chemical evolution, to be discussed in the next chapter.

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