The abundances of heavy elements in EMP stars are not well explained by the simple view of an initial basic ’ rapid ’ process .
In a careful and homogeneous analysis of the ” First stars ” sample ( eighty per cent of the stars have a metallicity [ Fe / H ] \simeq - 3.1 \pm 0.4 ) , it has been shown that at this metallicity [ Eu/Ba ] is constant , and therefore the europium-rich stars ( generally called “ r-rich ” ) are also Ba-rich .
The very large variation of [ Ba/Fe ] ( existence of “ r-poor ” and “ r-rich ” stars ) induces that the early matter was not perfectly mixed .
On the other hand , the distribution of the values of [ Sr/Ba ] vs. [ Ba/Fe ] appears with well defined upper and lower envelopes .
No star was found with [ Sr / Ba ] < -0.5 and the scatter of [ Sr/Ba ] increases regularly when [ Ba/Fe ] decreases .
To explain this behavior , we suggest that an early “ additional ” process forming mainly first peak elements would affect the initial composition of the matter .
For a same quantity of accreted matter , this additional Sr production would barely affect the r-rich matter ( which already contains an important quantity of Sr ) but would change significantly the composition of the r-poor matter .
The abundances found in the CEMP-r+s stars reflect the transfer of heavy elements from a defunct AGB companion .
But the abundances of the heavy elements in CEMP-no stars present the same characteristics as the the abundances in the EMP stars .
Direct stellar ages may be found from radioactive elements , the precision is limited by the precision in the measurements of abundances from faint lines in faint stars , and the uncertainty in the initial abundances of the radioactive elements .