One distinguishes the main and the weak s -process component. The main component produces heavy elements beyond Sr and Y, and up to Pb in the lowest metallicity stars. The production sites of the main component are low-mass asymptotic giant branch stars. The main component relies on the C neutron source above. The weak component of the s -process, on the other hand, synthesizes s -process isotopes of elements from iron group seed nuclei to Fe on up to Sr and Y, and takes place at the end of helium - and carbon-burning in massive stars. It employs primarily the Ne neutron source. These stars will become supernovae at their demise and spew those s -process isotopes into interstellar gas.