section_path: Current models/Core collapse/Detailed process
section_name: Detailed process
paragraph_text: Detailed process
  
  When a stellar core is no longer supported against gravity, it collapses in on itself with velocities reaching 70,000 km/s (0.23 c), resulting in a rapid increase in temperature and density. What follows depends on the mass and structure of the collapsing core, with low-mass degenerate cores forming neutron stars, higher-mass degenerate cores mostly collapsing completely to black holes, and non-degenerate cores undergoing runaway fusion.
  
  The initial collapse of degenerate cores is accelerated by beta decay, photodisintegration and electron capture, which causes a burst of electron neutrinos. As the density increases, neutrino emission is cut off as they become trapped in the core. The inner core eventually reaches typically 30 km in diameter with a density comparable to that of an atomic nucleus, and neutron degeneracy pressure tries to halt the collapse. If the core mass is more than about 15 solar masses then neutron degeneracy is insufficient to stop the collapse and a black hole forms directly with no supernova.
  
  In lower mass cores the collapse is stopped and the newly formed neutron core has an initial temperature of about 100 billion kelvin, 6,000 times the temperature of the Sun's core. At this temperature, neutrino-antineutrino pairs of all flavours are efficiently formed by thermal emission. These thermal neutrinos are several times more abundant than the electron-capture neutrinos. About 10 joules, approximately 10% of the star's rest mass, is converted into a ten-second burst of neutrinos, which is the main output of the event. The suddenly halted core collapse rebounds and produces a shock wave that stalls in the outer core within milliseconds as energy is lost through the dissociation of heavy elements. A process that is not clearly understood is necessary to allow the outer layers of the core to reabsorb around 10 joules (1 foe) from the neutrino pulse, producing the visible brightness, although there are other theories that could power the explosion.
  
  Some material from the outer envelope falls back onto the neutron star, and, for cores beyond about 8 M, there is sufficient fallback to form a black hole. This fallback will reduce the kinetic energy created and the mass of expelled radioactive material, but in some situations, it may also generate relativistic jets that result in a gamma-ray burst or an exceptionally luminous supernova.
  
  The collapse of a massive non-degenerate core will ignite further fusion. When the core collapse is initiated by pair instability (photons turning into electron - positron pairs, thereby reducing the radiation pressure) oxygen fusion begins and the collapse may be halted. For core masses of 40–60 M, the collapse halts and the star remains intact, but collapse will occur again when a larger core has formed. For cores of around 60–130 M, the fusion of oxygen and heavier elements is so energetic that the entire star is disrupted, causing a supernova. At the upper end of the mass range, the supernova is unusually luminous and extremely long-lived due to many solar masses of ejected Ni. For even larger core masses, the core temperature becomes high enough to allow photodisintegration and the core collapses completely into a black hole.