The Evolution of Helium Shell-burning Stars

The realization that the nuclei of planetary nebulae are stars which evolve with a time-scale of 10⁴ yr (O’Dell,Seaton) has prompted several theoretical investigations of the late stages of stellar evolution (Vila, Divine, L’Ecuyer). Vila has assumed that all nuclear burnings have ceased in these stars, and that the energy release is entirely gravitational. He showed that the inclusion of neutrino energy-loss processes based on the universal Fermi interaction of Feynman and Gell-Mann reduced the theoretical evolution time by two orders of magnitude to a value comparable with the observed time. His theoretical models, however, have effective temperatures greater than those observed by a factor of six.