The Third European Summer School on Experimental Nuclear Astrophysics A. Tumino, M. Busso, R.G. Pizzone, C. Rolfs and C. Spitaleri (eds) EAS Publications Series, 27 (2007) 25–39 STELLAR NUCLEOSYNTHESIS AND CHEMICAL EVOLUTION OF GALAXIES C. Chiosi 1 Abstract. We present the basic theory of nuclear reactions in stars and sketch the general rules of stellar evolution. Then we shortly review the subject of supernova explosions both by core collapse in massive stars (type II) and carbon-deflagration in binary systems when one of the components is a White Dwarf accreting mass from the compan- ion (type Ia). We also present elementary notions of s- and r-process nucleo-synthesis. Finally, we shortly review the topic of galactic chemi- cal evolution and highlight some simple solutions aimed at understand- ing the main observational data on abundances and abundance ratios. 1 Introduction How are elements made? What is the origin of the element abundances in the so- lar system, in the solar vicinity, in the Halo, the Bulge, and the Disk of the Milky Way, in external galaxies of different morphological type, and finally in the Universe as a whole? Abundances in the solar system and solar vicinity provide the richest information. In addition to 1 H(X ≃ 0.76), little 2 H and 3 He, primor- dial nucleosynthesis (Big Bang) does not go beyond 4 He (Y ≃ 0.24) and traces of 7 Li and 7 Be. 2 Elementary Theory of Nuclear Reactions Why nuclear reactions and nuclear energy generation in stars? To build up all the elements heavier than 4 He and to provide an energy source able to survive for about 10 Gyr. Basic Principle is the Einstein relation ∆E =∆Mc 2 where ∆M =[ZM p +(A - Z )M n - M Nuc ]. The binding energy of a nucleus is E B = ∆M and the average binding energy per nucleon E/A (A is the mass number) is ≃7 Mev. The maximum E B is at A = 56, being lower than this for lighter and heavier nuclei. As a consequence of it, reactions fusing light elements will be eso- energetic up to Fe and endo-energetic above. There are two basic types of nuclear 1 Department of Astronomy, University of Padova, Vicolo dell’Osservatorio 2, 35122 Padova, Italy c  EAS, EDP Sciences 2007 DOI: 10.1051/eas:2007142 Article published by EDP Sciences and available at http://www.edpsciences.org/eas or http://dx.doi.org/10.1051/eas:2007142