I was watching a video (https://www.youtube.com/watch?v=IZ59_akUUBs) about massive explosions and came across 2007bi. The video stated that this SN happened due to gamma-ray driven antimatter creation.

Apparently, its core being made mostly of oxygen began releasing energetic photons which decayed into electron/positron pairs. Their mutual annihilation caused the core to collapse and triggered the supernova.

I have a couple of questions concerning this.

  1. Pair-instability supernova happens when a star is about 130 solar masses, but the star here was only at 100 solar masses.... (per Wiki "These stars are large enough to produce gamma rays with enough energy to create electron-positron pairs, but the resulting net reduction in counter-gravitational pressure is insufficient to cause the core-overpressure required for supernova. Instead, the contraction caused by pair-creation provokes increased thermonuclear activity within the star that repulses the inward pressure and returns the star to equilibrium. It is thought that stars of this size undergo a series of these pulses until they shed sufficient mass to drop below 100 solar masses, at which point they are no longer hot enough to support pair-creation. Pulsing of this nature may have been responsible for the variations in brightness experienced by Eta Carinae in 1843, though this explanation is not universally accepted.") [https://en.wikipedia.org/wiki/SN_2007bi ]

Is it more likely the size of the star was wrong, or the that it can happen at lower mass or possibly there was something else at work here?

  1. Why wouldn't the extra energy from the electron/positron annihilation add more energy to the star's core? It seems counter-intuitive that adding energy reduces the internal supporting pressure. Can someone explain this?

Pair instability supernovae are thought to end the lives of stars with initial masses $>130 M_{\odot}$.

For SN2007bi, the relevant paper by Gal-Yam et al. (2010) deduced that they had seen the explosion of a $100M_{\odot}$ helium core and they infer that such a massive core would have arisen in a star with an initial mass of $\sim 200M_{\odot}$. So there is no contradiction here between theory and observation.

The core of a very massive star relies on radiation pressure as a means of support. Pair production removes gamma ray photons that were providing pressure support and replaces them with the rest masses of (relatively) slowly-moving electrons and positrons, which do not.

What matters here (for the pressure) is the kinetic energy density, not the total energy density - which is roughly constant. Pair production has the effect of turning the pure kinetic energy density of photons into the rest-mass of electrons and positrons, thus reducing the pressure.

Of course, the matter and anti-matter also annihilate giving back the photons, but it is an equilibrium process such that once a population of (albeit short-lived) matter/anti-matter pairs are created, they reduce the radiation pressure. The idea is that in the cores of particularly massive stars this is a runaway process, with core contraction leading to greater pair production, more contraction... And ultimately a supernova that blows up the whole star.

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  • $\begingroup$ Your answer was very helpful. Thanks 1 final question: So would 2007bi shed the extra 30+ solar masses as ejections? IF so, how long would this period last? $\endgroup$ – Rick Mar 3 '19 at 13:00
  • $\begingroup$ @Rick It has probably shed more than that if it started as a 200 solar mass star. The only mass estimate here is of the core of the star that exploded. $\endgroup$ – Rob Jeffries Mar 3 '19 at 14:59
  • $\begingroup$ @Rick These stars have lifetimes of a few million years, but I would think most mass loss would be in the final ~million years or less. $\endgroup$ – Rob Jeffries Mar 3 '19 at 18:15

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