A type of nuclear reaction, believed to take place inside massive stars, in which elements heavier than copper are formed by the slow ("s") absorption of neutrons by atomic nuclei. The capture of neutrons occurs on time scales that are long enough to enable unstable nuclei to decay via the emission of a beta particle before absorbing another neutron. This contrasts with the r-process that is thought to operate in supernovae. Prominent s-process elements include barium, zirconium, yttrium, and lanthanum.
