Q1: sun	Q2: trigger	Q3: iron	Q4: photodisintegration	Q5: heavy elements	Q6: degenerate

Stellar Death Match II: Supernovae

• Iron ignites:
  A cutaway of a highly evolved massive star shows a series of layers.
  
  The outer layer is a non-burning hydrogen envelope. Inside of this is a hydrogen burning shell, then a helium burning shell, then carbon, then oxygen, neon, magnesium, and silicon. In the center of it all is a core of iron ash. Iron is special. It is the lightest element for which fusion absorbs energy rather than releasing it. In other words, it takes more energy to cause iron to fuse than comes out in the reaction. Once the iron ignites, the gas will cool, because the burning iron will take heat out of the system!
• Core collapses:
  As the core cools, gravity begins to win, and the core begins to collapse.
• Temperature rises:
  The collapsing core causes the temperature to rise again, to 10 billion degrees Kelvin.
• Photodisintigration begins:
  When the temperature reaches 10 billion degrees Kelvin, the light has enough energy to destroy the atoms! All of the elements in the core---helium, iron, carbon, etc.---are destroyed. This reverses over 10 billion years of susion activity in less than 1 second. Like the burning of iron, this process absorbs energy, so that the core can collapse even more...
• Core collapses:
  Core collapses still more, and is now composed of only protons, neutrons, electrons, and light. As the core collapses the density rises.
• Charge destruction:
  When the density reaches ~10¹² kg/m³, the protons and the electrons become fused into neutrons and neutrinos. Recall that neutrinos are very high energy particles that can penetrate several light years of light as though it wasn't even there... These neutrinos have no difficulty in leaving the core of the star, and carrying away energy into the Universe.
• Core collapse:
  Now that there is no charge, the core can collapse even further, untill the density passes 10¹⁵ kg/m³.
• Neutron degeneracy:
  The neutrons are degenerate at these densities, and resist being packed any more closely. However, because the infalling material has gathered momentum (an object in motion tends to stay in motion until a force acts to stop it), it overshoots the degenerate density, and the core density rises to as much as 10¹⁷-10¹⁸ kg/m³.
• REBOUND:
  Just like a ball hitting a wall, which gets closer to the ball than it would actually "like" to be, then rebounds, the material in the core rebounds from the dense, degenerate center.
  The entire process, from start of collapse to rebound, takes less than one second!
• Shock waves:
  Shock waves are produced by the rebound which travel outwards at high speed, picking up material as they go. These shock waves carry material from the envelope away from the core. In other words, the star explodes.