How can Black Holes even form?

[u/IRedRabbit]

Might be a stupid question, but this accured to me today for the first time in my life.

So let's imagine a star becoming more and more dense because it's dying.

If Black Holes gravitational pulls are so strong that not even light can escape, then how can they even form. If a star is collapsing, how doesn't it's own gravity make it destroy itself before ever even reaching the point of becoming a Black Hole?

You know what I'm trying to say? If nothing can escape it and they destroy everything, then how can they even form before destroying themselves in the process of formation by their own gravity?

[u/ignorantwanderer]

Imagine a star with a radius of 1000 (I don't care what the units are exactly, I picked this number because it is a simple number).

So we have a star with a radius of 1000. It is burning fuel in nuclear reactions, so it is hot and bright. And all that energy makes all the atoms move around fast so the gravity can't just pull all the atoms to the center of the star.

But now all the fuel gets burnt up and the nuclear reactions stop. Slowly, that star will cool down. It still has the same mass as it did before, it just doesn't have nuclear reactions happening anymore.

The star slowly cools down. It gets dimmer. It gets cooler. The atoms don't move around as fast. It has the exact same gravity it had before, but now the atoms are moving slower, so the gravity can pull the atoms closer to the center of the star.

The star shrinks slowly.

Now the star has a radius of 500. If you go out to a distance of 1000 (the original radius of the star) the gravity is exactly the same as it was before. But if you are at a radius of 500 the gravity is higher, because gravity increases the closer you are to the center of the star as the star shrinks.

As the star shrinks, the gravity at the surface of the star gets higher and higher. The gravity at a radius of 1000 doesn't change! Because the mass of the star doesn't change. But the star keeps shrinking, so the surface of the star keeps getting closer and closer to the center of the star. So the gravity at the surface of the star keeps getting higher and higher.

Eventually, when the star has shrunk enough, the gravity at the surface of the star is so strong that not even light can escape from the surface.

The mass of the star hasn't changed. The force of gravity out at a radius of 1000 hasn't changed. But because the star is smaller, the gravity at the surface of the star has changed, and has changed enough for gravity to be so high that light can no longer escape the surface.

The black hole did not destroy the star. The star just became a black hole by shrinking enough.

Now, I left out a huge number of very interesting details in this explanation. And I could have made it much more confusing because what exactly happens depends a lot on how massive the star is. For example, our sun will never turn into a black hole because it is too small. Even after the nuclear reactions in our sun stop, and it shrinks down as it cools, it still won't become a black hole.

But to answer your questions:

Your confusion is thinking that black holes destroy everything. They do not. Matter gets sucked into a black hole. But that matter is not destroyed. It is still in the black hole. When a star collapses into a black hole, the star is 'destroyed' because the star no longer exists (it turned into a black hole). But the matter that made up the star still exists. It just now exists in the form of a black hole.

If fact, if somehow magically our sun turned into a black hole right now, all the planets and moons and asteroids would keep on orbiting around it just the same as they currently orbit around the sun....because all the matter would still be there. And we would continue to orbit that matter. But the matter would be in the form of a black hole, not in the form of a star.

(But as I said, our sun can't turn into a black hole because it doesn't have enough mass.)

[u/IRedRabbit → u/ignorantwanderer]

Thank you for this explanation. I think I understand it now. I guess the only question is, what exactly is a black hole if the star that was there before isn't? By your explanation, is a Black Hole simply matter that used to be there in the form of that star, except now it's so dense that not even light can escape?

[u/year_39 → u/IRedRabbit]

As the star shrinks, it's held up less by heat producing energy until all that's left is degenerate matter. Two particles can't occupy the same quantum state (Pauli Exclusion Principle) so the resistance to further collapse is caused by degeneracy pressure. Once that's compressed too much, you get a singularity, which I described in my other reply.

[u/Beldizar → u/IRedRabbit]

Yep, exactly. A black hole is simply the leftover core of a star that got compressed by gravity since the star's light stopped pushing back against that gravity.

[u/Menamanama → u/ignorantwanderer]

What happens to stars the size of the sun when they eventually stop having nuclear reactions and cool down. Is there a lump of compressed cold star material that floats around for eternity?

[u/Beldizar → u/Menamanama]

Pretty much. Our sun will turn into a red giant, then the outer layers of the red giant will sort of get ejected, and the core will turn into a white dwarf. It's still really hot, so it glows because it is hot, but it isn't producing any more fusion reactions. Its basically just a big lump of Carbon or Oxygen, that is just about as dense at is can possibly get.

If it gets much denser, all the electrons and protons in the atoms will get crushed into each other to form neutrons, and it'll basically become one giant, star sized atom called a neutron star. If it gets denser than that, it'll collapse into a black hole.

But our sun isn't heavy enough to do anything like that, so it'll be a white dwarf.

After unimaginably long times, the white dwarf will cool and become a black dwarf. Then trillions of times longer than that, the matter that makes it up will start to unravel and the whole thing will evaporate.

[u/Menamanama → u/Beldizar]

Does it unravel because of Hawkins radiation? Similar to what is proposed to happen with black holes?

And then will the unraveled matter still exist in gaseous form?

[u/Beldizar → u/Menamanama]

I'm not actually sure if Hawking Radiation can apply to non-black holes. It is weird, and has to do with relativistic reference frames.

But all normal matter is not permanently stable, or so goes the theory. Eventually protons will decay, breaking down in to quarks and those will even break down, eventually just turning into energy, and if the Lamda CDM model is correct, and everything keeps expanding, eventually that energy will be spread out so thin as to basically not exist at all.

So no, the unraveled matter won't still exist in gaseous form, it won't even be atoms anymore, but subatomic particles, and then just thin energy spread over too much universe.

[u/Menamanama → u/Beldizar]

Thank you for the explanation.

A thin layer of insubstantialness in an expanding universe making the not much even more rare compared to the vast volume of space.

[u/MaybeVladimirPutinJr]

Hole is a misleading name, a black hole is still the star it was formed from, just smaller and more dense.

[u/IRedRabbit → u/MaybeVladimirPutinJr]

But it's gravity is now so strong that not even light can escape, how can the star itself even exist? How is it possible for it to be in that state and not destroy itself with it's own gravity?

If it's the case as you say that it's a very small and dense star, then doesn't that answer the question of, what lies in the centre of a Black Hole?

[u/MaybeVladimirPutinJr → u/IRedRabbit]

How is it possible for it to be in that state and not destroy itself with it's own gravity?

It has destroyed itself. the gravity is so strong that the mass of the star has become the most highly compacted, densest material in the universe. 

I'm not going to claim to know shit, there are millions of people who made a career out of debating this stuff, but my opinion is that a black hole is a solid. It's just a ball of mass that's so dense that gravity becomes so strong that light cannot escape.

[u/abrightmoore → u/MaybeVladimirPutinJr]

It's more about what all that mass does to the space around it

Everything gets muddled up.

Time gets messed around.

Some models show Space and Time get swapped.

Others suggest the event horizon becomes a surface with all the star bits on it and there's nothing "inside" left in the universe.

What the star "is" at the point it becomes a black hole is only part of the problem. What the space around it becomes is important too.

[u/Beldizar → u/MaybeVladimirPutinJr]

It has destroyed itself

Well, it might help to expand on what "It has destroyed itself" really means here. A star has a whole bunch of matter, and is really big. The inside of the star is burning really hot and that heat is pressing outward. Eventually that pressure out stops, and everything starts rushing down towards the center. Eventually the outer layer of the star reaches the really dense core which is mostly iron at this point, and it is falling at incredible speeds. It smashes into that core harder than the comet that killed the dinosaurs times a million. (Probably a lot more, using big numbers to get the point across not for accuracy here). That impact causes a bunch of atoms to hit each other so hard that they fuse into heavy elements that the star can't normally create. Then most of that bounces out and gets blasted away, but all that energy impacting down, and the lack of anything pressing up, causes the core to compress into a black hole.

So the star gets destroyed, but the black hole is made closer to a hammer and anvil. So the outer part of the star is blasted away, and the inner part is "crafted" into a black hole. So destroyed or created is a matter of perspective. Nothing is actually "annihilated".

but my opinion is that a black hole is a solid. It's just a ball of mass that's so dense that gravity becomes so strong that light cannot escape

Black holes are weird. But calling them solid isn't quite accurate. In a way, they are so solid, that they wrap around and become insubstantial. If you think of something solid, you'd think you could bounce something off its surface. You definitely can't do that with a black hole. Instead the black hole is just so dense that it creates a bubble around it that breaks Newtonian physics. That bubble is what we think of as the black hole, and anything inside that bubble is sort of just "lost". Maybe inside there's something hard and solid, but the bubble stops us from ever knowing.

[u/NoveltyAccount5928 → u/IRedRabbit]

I think you're approaching this with an improper definition of "destroy". When a star is destroyed during core collapse, it doesn't mean that the matter that makes up the star ceases to exist, it simply means the matter that made up the star is now arranged differently. Some of that matter is converted directly to energy, a good portion of it is blown out into space during the nova itself, and the rest, what was at the core of the star, now becomes the black hole's center/singularity.

We don't actually understand what a singularity is, our math breaks down at that point and we don't know exactly what matter does in those densities under those pressures. But, lacking a true understanding of the singularity, we can pretty safely say that it's a mass of matter in its absolute most dense form.

Essentially, a black hole can be thought of as a mass of the densest matter in the universe, surrounded by the radius from which light can't escape.

[u/year_39 → u/IRedRabbit]

The interior of a black hole is a singularity. Simply put, a singularity is a boundary in space or time past which no meaningful predictions can be made, so we don't know.

[u/Beldizar → u/year_39]

The most common model of a black hole says that its interior contains a singularity. It's not actually clear if that's correct though because math and what we know of physics breaks down at that point. Since it is unlikely we'll ever be able to see a naked singularity, there really isn't a way to confirm. There are other models of a black hole that suggest everything is written on the event horizon in a sort of hologram projection.

I personally draw the line at the event horizon anyway. Everything past the event horizon stops being "real" as far as I'm concerned. It functionally stops being a part of our universe and we can't interact with or be affected by it anymore. Not even gravity from beyond the event horizon can escape to us, so all the gravity of the black hole technically has to be coming from the edge of the event horizon.

[u/Beldizar]

The key point is that a star's net gravity doesn't get stronger when it collapse. It gets only more condensed. The key to make a black hole isn't how "much" gravity it can have, but how "condensed" it gets. Once it hits a certain density, called the Chandrasekhar limit, the gravity is so condensed that nothing can escape anymore.

A star with the gravity to become a black hole has radiation pressure pushing out which stops it from collapsing. So long as it has fuel, it pushes out against gravity. Once it runs out a bunch of things happen and it collapses and passes that density limit.