Nucleosynthesis, mass loss and stellar evolution

Stars are formed from material of the interstellar medium with a given composition: X (the fraction of hydrogen), Y (the fraction of helium), Z (the fraction of the sum of the heavy elements, which can be splitted in Z1, Z2, Z3,... representing these heavy elements). The produced stars evolve, i.e. as a function of time the mass, radius, luminosity, temperature and density of the different stellar layers, from center to surface, change; due to nuclear reactions also the chemical composition of the different layers, varies. A part of the stellar matter is, during the evolution, restituted to the interstellar medium, whether or not altered by the nuclear processes. In material of deep layers, the composition is changed by nuclear reactions, hence if such material is restituted, it has a chemical composition different from the initial one, hence X', Y', Z'. Low mass stars pass through a planetary nebula stage, where a small fraction of the stellar matter is transferred to shells, which resolve afterwards, so that this matter returns to the interstellar medium. The stars evolve later into white dwarfs. Massive stars pass through a supernova stage, where most of the stellar matter is restituted to the interstellar medium