| Part I of this thesis discusses weak interactions in the presupernova evolution of large stars. Weak interactions set the timescales of the later burning stages of these stars and are important in setting the electron fraction per nucleon, and the average entropy, crucial quantities in core collapse calculations. The evolution of an 18 {dollar}Msb{lcub}odot{rcub}{dollar} star is sketched, showing where these interactions have their effects and what nuclei have the largest role in the events.; Current treatments of weak interactions, particularly electron capture and {dollar}beta{dollar} decay reactions, and their implementation in stellar evolution codes are discussed. These treatments only use nuclei with A of 60 or less. Several {dollar}beta{dollar} decay parents which are larger than this need to be included in the calculations. Recent (n,p) experiments indicate that the electron capture rates currently used are probably too large by factors of two to six, and even larger. These calculations tend to overestimate the Gamow-Teller strength of nuclei, leading to electron capture rates which are too large. This effect becomes larger as the density, and thus the electron chemical potential, increases.; Part II of the thesis studies explosive nucleosynthesis after core collapse. Successful, independently verified, core collapse calculations have not yet been performed, and two methods of generating shock waves are typically used. In one approach, the temperature of a piece of the star is highly elevated, while in the other approach, a piece of the star is given a large outward velocity. In either case a shock wave is generated, but how similar are they? We find that the resulting peak temperature distributions are quite similar from the oxygen shell of the star outward. But in the silicon shell, where the ejected nickel originates, one must be very careful about where the shock was started. If the shock is started too close to this region, the temperatures will be unphysical because the shock wave has not had time to form properly. Therefore one cannot use such methods to reliably eject just the 0.07 {dollar}Msb{lcub}odot{rcub}{dollar} of nickel seen in SN1987a without fatally compromising the temperatures in this region. |
