Stellar mass black holes may be produced by the collapse of the cores of massive stars during supernova outbursts. The limit is uncertain, but it is probably 25-35 times the mass of the Sun.
• initially, a smallish remnant forms--a remnant of mass on the order of 1.4 M_Sun--a neutron star
• a shock wave next propagates through the outer layers of the star iginiting nuclear reactions and pushing on the envelope of the star
• depending upon the detailed structure of the envelope of the star, the shock will eject the envelope or it will stall and allow some or all of the envelope to fall-back onto the initial neutron star remnant
• if the material falls-back onto the star, then the mass of the remnant increases
  • neutron stars are like white dwarfs in the sense that as their mass increases, they get smaller (denser) in order to generate the higher pressure needed to counteract the effects of gravity
  • this has the same consequence in that there is an upper limit for the mass of a neutron star
  • the upper limit is not as well-determined as for the white dwarfs as the upper limit depends upon the nature of the nuclear (strong) interaction which is not well-understood under the conditions found in neutron stars -- the upper limit is 2 - 3 M_Sun
• if the mass of the neutron star increases beyond 2 - 3 M_Sun, then the neutron star collapses and forms a black hole -- after the neutron star phase there is no known type of matter which can generate sufficient pressure to overcome the effects of gravity and the star must then contiuously collapse until it compresses to infinite density and zero volume!