So my understanding is a black hole results when a very large star burns up its hydrogen/helium and the nuclear fires can't keep the mass from contracting into a singularity from which light can't escape. Now if there was all that mass in the star in the first place to create a well of gravity from which light can't escape (and presumably more at the start), why does it have to wait until it shrinks to singularity sized? Why doesn't this massive star have the same gravity in star form?

And can there be stellar objects so big that they both undergo fusion and have so much gravity that the light they produce can't escape?

It's only a black hole if the mass is concentrated inside the Schwarzchild radius. Generally, the temperatures from the fusion reactions raise the gas pressure enough to maintain the star outside the critical radius. Once the fusion stops (and it goes all the way to iron, not just hydrogen and helium), then the star collapses. And probably produces a supernova, although some extra heavy stars may skip the explosion part, and then either a neutron star or a quark star or a black hole as the remnant.

It does have essentially the same gravity, it's just that the mass is much more concentrated.

Could any natural process pack together enough hydrogen to make a black hole? I have no idea. A black hole containing mostly fusible hydrogen is a frightening thought. I don't think it can interact with the outside universe, but it's certainly going to be exciting inside.

Good question. I'm sure other posters will want to stir the pot.

I don't think a black hole could have fusion taking place inside the schwarzschild radius, mainly because as the mass of hydrogen (or other fusible element) contracted,it would start fusing and creating an expanding force before the object became a black hole. This would halt expansion, any aditional gravity would just accelerate fusion (more gravitational force to overcome coulomb repulsion between atoms of fusion fuel) so the star would burn out quicker. Besides,after the star burns out, and starts contracting under gravity it has to overcome electron degeneracy to become a neutron star and then neutron degeneracy to shrink further to a black hole so I think any fusible (or whatever the term for them is) atoms would have decayed to neutrons, or (more likely) quarks by the time the star starts approaching a black hole stage.
That's what I think, anyway, but I stand to be corrected...

Right, just a quantitative addition to my previous post. I've done a bit of rough calculation for schwarzschild radius of a 9 stellar mass black hole (ball park of cygnus x1 black hole candidate, I believe), simplified hydrogen element to its ionized state (i.e. proton) and found the number of atoms in the 9M mass. Then I calculated volume of a proton, the volume (in context of schwarzschild radius) of the black hole, and divided the latter by the former to find how many hydrogen ions would actually fit into the black hole volume under normal circumstances. The difference between this number and the number calculated from the mass is about 35-36 orders of magnitude (unless I made a mistake in calculation and you're welcome to correct me) more particles due to mass than would actually fit in the volume under normal circumstances. Considering the proton is the simplest hydrogen model (i.e. smallest in terms of volume) and i did the calculation assuming all black hole volume was taken up (disregarding any empty space due to the shape of the ions), this is the minimum difference, the actual difference would probably be even larger. A very crude calculation, but I think it illustrates that hydrogen atoms would not survive a gravitational collapse into a black hole intact and in any shape to start fusion. Appologies for using classical physical reasoning but I'm in no way competent enough yet to take general relativity and quantum effects into practical considerations.

Ok, now I'm confused (not impossible, just difficult).

So the schwarzchild radius exists in supermassive stars, just somewhere in the core? So what's to keep the rapidly moving matter from rapidly moving beyond the schwarzchild radius in a matter of months, if not hours? For every chunk of matter that passes this radius, it's no longer able to contribute to keeping the star expanded!

Intuition tells me that there's more to this than meets the eye... I'm beginning to believe that supermassive stars don't become black holes over centuries, but rather quickly once a swarzchild radius grows to a sizeable enough diameter, and far more rapidly if the star is eating matter either from planets, gas clouds, or another star.

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Your confusion might be cleared up if you start thinking about stars and black holes in terms of density.

Right, I think what your talking about (a layer of mass inside the schwarzschild radius of a giant star) requires you to integrate the gravitational force on it at that point since it has to be treated as an extended body now, as opposed to just taking the centre of mass as a point particle. Alternatively, think of it this way, imagine you are a particle at the surface of the star. All its mass is located on just one side of you so it's all dragging you in the same direction (towards its centre of mass since any gravitational force tangential to the surface is symetric but acting in opposite directions and therefore cancelling out).
Now imagine you are a particle inside the star at the schwartzschild radius. If the mass of the star mass was a point particle at the centre the force you would feel would be much greater because you would be closer to the mass. However, the star is an extended object so if you were a particle inside the star you would see the centre and most of the mass to your one side but now there would be some considerable mass (the outer layer of the star you had to go through to get tothis point in the star) on your other side as well. This would have 2 effects. Firstly now there is less mass pulling you towards the centre of the star (smaller gravitational force) and secondly there is now also some mass pulling you in the opposite direction so in effect the force towards the centre you feel at that point is lesser than if there was a point particle of the star's mass at the centre. In other words, from your point of view the schwarzschild radius has receeded away from you.
I dunno if this makes sense to you and it's a simplified model, I'm sure, but I think that's why the core of a star doesn't turn into a black hole.
As far as what halts the gravitational collapse of a burning star, as the particles in a star are forced together and start fusing, their temperature skyrockets. That means they now have much more internal energy and move around much more quickly (kinetic theory of gases). Therefore they now collide much more often and with more force, causing the overal volume to expand (Charles' Law). You can try this at home with a blown up balloon in a freezer. You'll see that whenit's cooled it will have a smaller volume than at room temperature.
Hope this helps.

But a sphere with a radius equivalent to the Schwarzschild radius of the whole star doesn't contain anything like the mass of the whole star when the star is going about its usual fusion business. So the escape velocity from the surface of that small sphere is well below the speed of light: it can only reach lightspeed when the mass of the whole star is enclosed within the predicted Schwarzschild radius.

Grant Hutchison

You're right.
It turns out that if you're somehow embedded in a spherically symmetrical mass-distribution, the gravitational force you feel is easily calculated. You feel only the gravity of the matter that lies at a smaller radial distance from the centre than your own distance.
All the matter lying farther out than your radius exerts no net gravitational force on you -- a consequence of the fact that the gravitational force within a spherical shell cancels down to zero at all points enclosed by the shell.

Grant Hutchison

Could sufficient iron (and heavier elements) be assembled to create a black hole with only a minor implosion? Neil

Mass is not the most crucial factor. That's density. What happens is that as the star burns its fuel its core contracts more and more, until you get a huge amount of mass all packed into a relatively small volume. In other words, the density of the core increases tremendously. Eventually, it's so dense that light cannot escape it, and you have a black hole.

As I explained above, what matters is the density, not the mass. In very dense objects, though, I don't think ordinary fusion is possible. You might want to do some research on neutron stars, which are below black holes in terms of density.

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Technically, the black hole is a singularity (infinite curvature of spacetime, or an object with all the mass concentrated in an infinitesimally small volume), and the event horizon (i.e schwartzschild radius) kind of protects the rest of the universe from all the weird things going on inside by not letting the rest of the universe exchange information with it. So if you are trying to compare the size of a star before collapse to after, you are technically comparing to something with no volume. Talking of density seems to make more sense.
As for iron or other elements shrinking directly into a black hole, I don't think that's possible (though I'm not an expert) mainly because as the object is shrinking, the colapse is momentarily opposed by electron degeneracy pressure where the electrons in he object are being forced into atomic orbitals and spins (I think) which they are not allowed in (look up Pauli exclusion principle), which is what stops the collapse of a white dwarf. If the gravity force is stronger, The electrons are actually forced to 'combine' with the protons and make neutrons (a neatron star). At this point neutron degeneracy (which is simmilar to electron degeneracy except with neutrons) kicks in and for a black hole to form the gravity has to overcome this, too. So there's a lot of steps in the gravitational collapse of iron or anything else involved, and you'd probably see a big difference between an iron object and an event horizon defined black hole.