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A black hole is defined by its event horizon which starts from what is called a crease set, which is a set of events*, which are isolated in the sense that no messages can be sent between any of the events in the set.

A heuristic way of thinking about it is that a bunch of "mini-black holes" will form instantaneously at a set of points that are isolated from each other, each "mini black hole" expanding outwards from that point before merging together. In relativity whether two isolated events happen at the same time is subjective, so it is a moot question as to whether they form at the same time. For a collapsing star, effectively the merging happens very quickly and in the very early lifetime of the black hole.

Though this describes the formation of the event horizon, it is not correct to think of the singularity also forming at the crease set. All we can say about the singularity really is it is contained within the event horizon.

*An event is a point in spacetime, i.e. something with an exact location in both space and time.

https://www.worldscientific.com/doi/10.1142/S0218271811019220

This is an interesting time to be asking that question, considering something that was just observed.

>The hypermassive star was produced by the merger of two smaller neutron stars. Normally such collisions result in neutron stars so massive that they collapse into a black hole almost instantaneously under their own gravity. But the latest observations revealed the monster star hovering in view for more than a day before it faded out of sight.

So, according to the observation, two neutron stars (which are always quite close to the critical mass for a black hole) merged, bringing the total mass much higher than the needed limit, but it took a significant amount of time for the observable neutron stars to 'fade away' into the black hole.

That is so cool. Wow

Im assuming there are no alternative material types for neutron strars that would make them lese dense?

Nope. Neutron stars exist in a narrow window of masses and densities and these characteristics have been confirmed in observations of them, at least the sort that we can detect (ones that are pulsars). The neutron star itself does have some structure, though, with various layers containing slightly different densities, the different forms being called "nuclear pasta". It has a gnocchi, spaghetti, lasagna, anti-spaghetti, and finally a 'swiss cheese' layer.

I'm glad that scientists get to indulge in whimsical naming once in a while. (the truth and beauty quarks got re-named top and bottom unfortunately).

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The "material" of a neutron star is just that neutrons. All of the electrons in the atoms are pressed into the protons, turning them into neutrons. I forget is this is neutron or electon degeneracy pressure... But it ceases to be made of atoms at that point.

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