A star is a large and massive ball of gas, but that gas is hardly like anything we know here in earthly laboratories. It is something like the gas in a fluorescent bulb, or the hot gas in a bolt of lightning, but it's much more dense and under a lot of pressure, especially deep within the star.

It is more like a liquid than a gas, but at depth within the star it is more like a solid ... and it is highly electrically conductive.

The field of helioseismology is where researchers study the transmission of sound waves through the Sun and, thereby, deduce much about its internal structure (in very much the same way that seismologists on Earth use sound waves to determine the structure of the Earth's subsurface layers and its core.

It is said that if the Sun weren't so hot that it would incinerate you, that it is so tenuous at its outer layers (the photosphere, for example) that you couldn't stand on it ... you would sink into it as if you were trying to stand on water. It is only much deeper in toward the core that things get really dense.

Even so, in the outer layers photons can't travel very far without striking another particle. This means that the particles in the gas (plasma) are highly interactive. Once you get to the level of particle interactions in physics, gravity plays very little role in the outcome of particle-particle interactions. At those short distances, electromagnetic and nuclear forces are much stronger than gravity.

But, in the aggregate, the tremendous (in comparison to the Earth) bulk of the star causes it to warp spacetime in its vicinity. For all practical purposes, the influence of gravity on the particles of which the Sun is composed breaks down within the photosphere, well "above" the core.

What happens in a supernova is that the core collapse doesn't generate enough pressure (due to gravity) to cause the internal matter to degenerate, so the outer atmosphere rebounds from compressed neutron matter near the core. This liberates the atmosphere (in a titanic explosion) and leaves the core in a state of shambles. Think about what happens to a sinking ship when it is crushed by the pressure of the overbearing seawater at depth: any air pockets within the ship explode and the bits of the metal and wood, etc., holding the ship together fly apart with the explosion.

The creation of a black hole is different. In this scenario, the core collapse is so sudden (practically instantaneous) and the mass of the star so great (and therefore the slope of the curve of spacetime so steep) that all that mass comes barreling down onto the core's inner region and smashes the neutrons together with so much force -- in such a short time -- that collapse proceeds right on through rebound. In other words, there isn't time for the core neutrons to rebound. The only possible outcome of this scenario is the creation of a black hole ... that is, the collapse continues beyond the point where spacetime closes in on itself.

It might help you to envision that gravity and the curvature of spacetime are one and the same. That is, as you increase the curvature of spacetime, you're likewise increasing the "hold" of gravity. You can't separate the two. So, the math here is that if you could find a way (in physics) to create a slope greater than a certain threshold, no normal matter could withstand the pressure.

If you can grasp that idea, then the only remaining step is to realize that this scenario is possible in physics, and results in a slop so steep that no amount of energy can overcome it, and that leads to a closing off of spacetime into a black hole.

iulianm:

... I am not able to see how the curvature of the space inside the sphere of the gas is generated by the all the particles from that sphere of gas - at the end of the star's "normal" life.

I think this is where you went astray: It's not the gravitational attraction between individual particles that causes the slippery slope of the curvature of spacetime -- it is the aggregate of all of them acting together (the sum of the individual masses).

Another part of the model we haven't discussed is that when the core starts to fuse iron, the process is different from the fusion of all other elements that came before: fusion which creates iron doesn't release energy, it consumes it. This has the effect of "pulling the rug out from under" the stellar atmosphere. It instantly runs out of the pressure of radiation that had been flowing outward from the core (that is what keeps a star in equilibrium). This is what leads directly to the collapse -- there is no longer anything "holding it up".

This is why I mentioned the model of "attraction forces" being "straightforward".

Yes, that's true for large objects. It is not true at the particle level inside the star, however.

 I wished an explanation of the implosion itself being generated by the existence of a "curvature": the outside grav.field does not explain the implosion, so only the curvature of the space between the iron nuclei can explain it.

No, it's a consequence of the combination of lack of continued radiative equilibrium (the shoring up from below) and the pressure of the overburden of the outer layers of the star (they want to collapse, under the curvature of spacetime).

... If it is a body, then why there is a curvature "inside" it ?

Yes, it acts more like a body than like a collection of gas particles. But it is a very special kind of body: a stellar core surrounded by an atmosphere (which the core leaves behind in a hurry at implosion time).

It might also help to remember that the black hole's new radius was actually there inside the star all the time. It was just filled with much less dense matter prior to the implosion.

I just went back through the earlier posts in this thread and noted that I have negelcted to provide you some important references:

• Black Holes & Time Warps: Einstein's Outrageous Legacy, by Lawrence Krauss
• Death by Black Hole, by Neil deGrasse Tyson

You should read one or the other of those if you have the chance. Krauss's book has more depth and detail, as well as history of the theory of black holes. Tyson's is more dramatic and is a lighter read. Both are excellent summaries of current thinking.

Before starting to "struggle" to connect your last answers:

1. I graduated from a technical faculty, not from a physics one; but, I am as curious as one can be;

2. I bought recently all the books from a "Science" collection of a press house in my country: Barrow and other authors, even Hawking's  "Short history of time", etc....

 ...and, not to forget,  Wikipedia articles: stars, black holes, gravity.

3. Unfortunately the titles you mentioned aren't translated - no chance for the near future either( I heard about them, though)

4. ...but none of these books seem to answer to my questions; maybe I err somehow;

5. the "attraction forces" part I use it as a counterpoint only; I believe in the validity of the empirical observations on light bending - Eddington, etc; but it appears if someone wants to extend this model of gravity, of spacetime curvature, - at least from my pov - there are difficulties; maybe I am wrong. 

The example with the droplet is interesting.

iulianm:

5. ... it appears if someone wants to extend this model of gravity, of spacetime curvature, - at least from my pov - there are difficulties; maybe I am wrong. 

That would be many someones ... the current thinking is that once you start the process of gravitational collapse, it can continue -- given enough mass and density -- right on through the "breaking point" to a singularity.

Using the example of the droplet, consider it a series of snapshots in time:

• At the top, everything is normal. There is just slight warpage of spacetime here and there as distributed clumps of mass dictate, and there is a dimple where the star lies. This star is just massive enough to form a black hole if it collapses.
• Collapse begins and the curvature of spacetime near the hole (the top of the "column", where it begins the slope downward from "normal" spacetime (first arrow).
• Each successive arrow (moving downward) points to a different stage in the gravitational collapse, which is proceeding more and more rapidly as the core shrinks. As the core shrinks, if it doesn't lose mass, then the core becomes more and more dense. Its radius is diminishing rapidly.
• Things proceed rapidly to the point (next to bottom arrow) where degeneracy is passed and spacetime completely closes off the remaining stellar mass. The radius has shrunk as far as it can go via known physics -- effectively to zero (since we can't see inside the black hole, we can't be sure). The event horizon forms (around the point where spacetime closes off). This is Hawking's realm and he does a good, if somewhat technical, job of explaining what happens here.
• The black hole is (almost forever) sealed off from normal spacetime now. It resides inside the sphere (the droplet), which is of unknown -- but theoretically infinitesimal -- radius.

If you consider the core collapse as a continuous -- though quite rapid -- series of events, you could put them together in a curve that looks very much like this. The surface tension of water is a fair analogy -- water can be stretched up to a certain point, then it snaps ... Just as spacetime can be stretched by a dense mass until the density passes a threshold from which rebound is impossible. Imagine you were a bug walking on water. Everything would be fine until you became heavy enough to break the surface tension -- at which point you would rapidly fall "through" the surface of the water and would be lost to it (you'd sink). The black hole is "sinking" out of normal spacetime.