# What is the process that causes a collapsing star to bounce back?

Can someone explain the process by where a star collapses and bounces back. I haven't been able to find a good explanation of the bounce. Edit here, I was wondering specifically where the energy of the bounce comes from. It seems like the mass must decrease in proportion, or the energy must have beens stored some other way. I guess the mechanism is not completely understood, or I would ask that question.

• possible duplicate of Why does a supernova explode Jul 17, 2015 at 22:28
• @KyleKanos This question is not a duplicate. The reason for the "bounce" is reasonably well understood and is only peripherally connected with why a supernova explosion occurs. I will edit the question to make that clear; but the reason for the bounce (the hardening of the EOS subsequent to a dynamical freefall) is not contained in any of the answers to the duplicate either. Jul 20, 2015 at 20:09
• @RobJeffries: OP asked for the reason for the collapse and bounce, which makes it a duplicate (your edit to remove the collapse bit seems a bit too much, IMO). DilithiumMatrix's answer does actually cover the bounce, though the detail of the eos should be included & is not. Jul 20, 2015 at 20:42
• @KyleKanos The casual reader would think that you are claiming that the cause of the bounce and the reason that a supernova explodes are the same thing. Indeed, your now-deleted first comment, made when you marked the question as a duplicate, essentially said just that. The former is well understood, the latter is not. I rolled back the edit, since I agree it would be preferable if the OP decided whether to accept that this was a duplicate. Jul 20, 2015 at 20:58
• @RobJeffries: I'm claiming that the collapse and bounce is the beginning of a supernova event; this is what OP asked about and why it's a duplicate. Jul 20, 2015 at 21:03

The bounce, as opposed to the supernova which is still somewhat mysterious, is caused by a drastic hardening of the equation of state (EOS - the relationship between pressure and density).

Prior and during core collapse, the EOS is dominated by relativistically degenerate electrons - a relatively soft EOS. The beginning of core collapse (initiated either by photodisintegration of iron nuclei or the removal of electrons by electron capture) leads to increasing densities, and causes free electrons to combine with protons to form neutrons (neutronisation) whilst resultant neutrinos escape. This drastically softens the EOS further, leading to a free-fall collapse of the core on a dynamical timescale of about 1 second.

As the number density of neutrons increases, non-relativistic neutron degeneracy pressure becomes important. NRD neutrons are less compressible but, contrary to pop-sci myth, this pressure cannot halt the collapse alone - it was established by Oppenheimer & Volkoff (1939) that the upper limit to the mass that could be supported by ideal neutron degeneracy pressure was $$<0.75M_\odot$$ and all collapsing stellar cores are more massive than that. Instead it is the extreme hardening of the EOS due to the repulsive strong nuclear force between neutrons once they are closely packed (within about $$10^{-15}$$ m) that stops the collapse. Because the collapse occurs dynamically, rather than quasi-statically, the inner core overshoots any kind of equilibrium position (in much the same way as when you compress a rubber ball). Once the collapse halts, the core is then able to rapidly expand outwards, using the stored potential energy in the closely-packed neutrons, towards an equilibrium configuration and this is what is termed "the bounce".