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## Main Question or Discussion Point

*I feel like this could go in quite a few of the*Physics

*subforums (*Quantum Physics

*,*Beyond the Standard Model

*,*Special and General Relativity

*, or*High Energy, Nuclear, Particle Physics

*) instead of*Astronomy and Cosmology

*, but hopefully this will work. This is my first question I've posed here, btw, so please forgive (but feel free to correct) any missteps.*

**(Background: B.S. in physics, pretty good grasp of GR and SR, wide familiarity with astrophysics, less robust grasp of QM; this question is personal interest alone.)**

As far as I've been able to tell, the sequence of events in a massive star's collapse aren't very well-understood yet. It's not known whether the iron core collapses first to an unstable neutron star and then into a black hole, or if the collapse goes directly to a black hole. Moreover, there's a gap between the largest theoretically possible neutron stars (~3 M

_{☉}) and the smallest observed stellar-mass black holes (~5 M

_{☉}); presumably, this would be the result of some aspect of the collapse sequence, but that's not really known either.

There is a great deal of discussion about the theoretical minimum-mass black hole, usually in the context of quantum gravity and the potential for quantized black holes. The entropic argument for black holes (that the event horizon of a black hole must be quantized to the Planck area in order to limit information to discrete bits) doesn't seem to hold up well and has been generally dismissed by the larger physics community, but at the same time it seems quite reasonable to assume that some minimum to the mass of a black hole must exist. If there was no lower limit on the mass of a black hole, then the power output of Hawking radiation would go to infinity, which means either there's some minimum on black hole mass or the Hawking radiation equations break down at quantum levels.

On the other side of things: all the discussions/models/depictions I've ever seen of stellar-mass black hole formation feature a macroscopic event horizon, with the Schwarzschild radius and the surface radius growing closer (either because the Schwarzschild radius is increasing due to added mass or because the surface radius is shrinking due to increased density) until they finally meet and the black hole is formed.

However, in the collapse of a massive star where neutron degeneracy pressure is insufficient to arrest collapse and a black hole forms, it doesn't seem that there would be anything preventing the density at the center from increasing without bound. The only pressure would be radiation pressure from the massive amounts of energy received, but due to the gravitational field the incoming radiation would be dramatically blueshifted while outgoing radiation would be dramatically redshifted, so that the pressure gradient goes to infinity as you approach the center.

**Is it possible, then, that the exponential increase in density at the center will reach the necessary conditions for the formation of a minimum-mass black hole before the conditions for a black hole are met at a larger radius, such black holes originate at the center and propagate outward?**

If so, that might lead to some interesting predictions about the rest of the black hole formation sequence, including a possible explanation for why the gap exists between neutron stars and black holes as well as a more promising basis for black hole quantization.

Of course, I fully recognize that trying to model a minimum-mass black hole using GR and the characteristics of Hawking radiation may well be comparable to trying to model a thermonuclear explosion using a 2nd-grade chemistry set...but I figure it's worth a shot anyway.