Pair instability supernova pressure and temperature question

[heartofcourage]

I read a scientific journal article that discusses pair instability supernova - I am puzzled about something discussed in the article. If the collision of atomic nuclei and energetic gamma rays produces electron and positron pairs which reduces the pressure inside a supermassive star's core - why is this so? I don't understand this because of the ideas of conservation of mass and energy. Although electrons and positrons are much smaller than protons or neutrons the mass is conserved isn't it - even after conversion from the former to the latter?

 

[mathman]

Pair production involves gamma rays converted to electron-positron pairs. The atomic nuclei enter only to keep momentum balanced. They are otherwise unaffected.

 

[mfb]

The total energy density does not change, but the pressure gets reduced as massive particles lead to a lower pressure per energy compared to photons.

For a gas, pV=NkT, where kT is a typical energy of the particles excluding the rest mass, multiplied with the number of particles we get the total kinetic energy (neglecting the potential energy). A rest mass contributes to energy density, but not to pressure.

 

[heartofcourage]

The way that I understand it is that either electron degeneracy pressure or radiative pressure prevent stars' cores collapsing from the gravity of their mass. If you create more particles pairs that have momentum as well striking atomic nuclei to give them some momentum then shouldn't this create enough energy to cause pressure to resist the force of gravity (which would increase with the additional mass from the particle pairs created)?

 

[mfb]

There is still pressure, but it is lower than pure radiation pressure at the same energy density would be.