In this book, we have analyzed the contribution of the magnetic stress energy to the supernova bounce. A calculation of Maxwell stress tensor is proposed that leads to the expression of the magnetic force density. The magnetic field which produces magnetic pressure and magnetic pressure are associated with the electromagnetic vacuum to a perfectly magnetically permeable one. We find a generic result of the surface magnetic field, B . 1018 G and the magnetic pressure, Pmag 1033 2 cms g which is responsible for the bounce of falling material. By making the link between the magnetic force density and the Maxwell stress tensor we found the magnetic pressure, which support the star from gravitational collapse and we found the strength of magnetic field at the surface of the star.