More black hole questions

[jajrussel]

What causes a nuclear bomb to explode? I am under the impression that the explosion is the result of pressure. Is this impression correct? I ask because I am just starting to try and understand how a black hole forms. To this point my understanding is a star collapses the pressure causes an explosion. The star then collapses again, except this collapse is unable cause enough pressure to create another explosion.

I have tried to think my way around the next questions. Why doesn't the first explosion blow away enough mass that the formation of a black hole can not occur? Why isn't there a continuous chain of explosions?

[insane_alien]

yes, its pressure caused by extreme temperature.

 

in a supernova that creates a black hole there IS an explosion. the pressure created forces the core inwards on itself and eventually the collapse will be unable to stop itself with degeneracy pressures.

[jajrussel]

Wikipedia – Says – During collapse, an electron-degenerate gas forms in the core, providing sufficient degeneracy pressure as it is compressed to resist further collapse. Above this mass limit, a neutron star(supported by neutron degeneracy pressure) or a black hole may be formed instead.

My impression when I read this is that the black hole forms around the core. Is my impression correct?

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The article is about degenerate matter. I have yet to figure out how to place the link into the forum.

[toastywombel]

Its also good to keep in mind that some scientists think different kinds of black holes form differently. While stellar black holes (smaller ones) seem to form by the collapse of a dying star, the supermassive galactic black holes are thought to have other ways of forming. This is because there is a dearth of intermediate sized black holes compared to the amount of stellar sized and galactic sized. This might imply that they have separate ways of forming.

[Moontanman]

What causes a nuclear bomb to explode? I am under the impression that the explosion is the result of pressure. Is this impression correct?

 

No it is not, a critical mass can form with out the extreme pressures of a bomb. The pressure makes the bomb more efficient resulting in a more complete fission of the uranium or plutonium.

 

I ask because I am just starting to try and understand how a black hole forms. To this point my understanding is a star collapses the pressure causes an explosion.

 

What happens depends on the mass of the star, a sun like star just collapses into a white dwarf.

 

The star then collapses again, except this collapse is unable cause enough pressure to create another explosion.

 

No, not quite, the initial collapse both explodes the outer layers and and compresses the inner layers at the same time into a neutron star or black hole depending on the mass.

 

I have tried to think my way around the next questions. Why doesn't the first explosion blow away enough mass that the formation of a black hole can not occur? Why isn't there a continuous chain of explosions?

 

Sometimes the initial explosion does blow away enough mass to prevent a black hole. Sometimes you get a white dwarf or a neutron star or a quark star, a black hole can only form if the original star is above a certain mass when they process starts, IE it depends on how much mass you start out with. There is no continuous chain of explosions because there is no energy source for them, fusion is over by this time, all reactions past iron consume energy they do not produce it. Gravity overcomes any rebound type energy.

[jajrussel]

What prevents the inner layers that are collapsing from bouncing back from the center? It would seem that the black hole already existed.

Am I brought back to degenerate pressure?

Edited January 9, 2010 by jajrussel

[Moontanman]

What prevents the inner layers that are collapsing from bouncing back from the center?

 

I don't understand, what would bounce and off of what?

 

It would seem that the black hole already existed.

Am I brought back to degenerate pressure?

 

No, the black hole is formed by the infalling matter, beyond a certian density it never stops falling inward, time slows as it becomes a black hole but the infall or collapse never stops.

[jajrussel]

Moontanman, I'm thinking I missunderstood part of your answer in post #5. I was thinking that as the star blew apart the innerlayers collapsed and pressure would build up and push back. My thoughts still are not clear but I am reading up on it when I can.

Thank you.

[Moontanman]

Moontanman, I'm thinking I missunderstood part of your answer in post #5. I was thinking that as the star blew apart the innerlayers collapsed and pressure would build up and push back. My thoughts still are not clear but I am reading up on it when I can.

Thank you.

 

Gravity prevents any push back, either you get a white dwarf, a neutron star, or a black hole. Once a certain density is reached there is no push back.

[Janus]

What causes a nuclear bomb to explode? I am under the impression that the explosion is the result of pressure. Is this impression correct? I ask because I am just starting to try and understand how a black hole forms. To this point my understanding is a star collapses the pressure causes an explosion. The star then collapses again, except this collapse is unable cause enough pressure to create another explosion.

I have tried to think my way around the next questions. Why doesn't the first explosion blow away enough mass that the formation of a black hole can not occur? Why isn't there a continuous chain of explosions?

 

Start with a massive star. At the beginning, the pressure at the center causes it to fuse hydrogen into helium, this releases enough energy to hold the stars upper layers from collapsing further.

 

After a time, this helium ash start to "clog up the center" and the fusion drops off. The star starts to collapse a little. The pressure builds up in the center until it becomes high enough to cause the helium to fuse. Now you Have helium fusing at the core and a layer of Hydrogen fusing above that.

 

The Carbon from the Helium fusing in turn clogs the center. More collapse, more pressure, and the carbon starts to fuse. This continues until Iron forms at the core.

 

Up to now, every stage of fusion produced extra energy to help hold the star up. Though each stage produces less energy than the last. However, Iron is different, it takes a net input of energy to fuse Iron. The Iron just sits there building up, with fusion taking place in the layers above.

 

Eventually, the Iron core becomes so massive that it can't support the weight of the layers above anymore. It collapses, leaving a hollow that all the upper layers rush to fill in. When they crash into the remnant of the iron core, the forces are enough to cause the whole shebang to fuse in a titanic explosion. This causes forces of it own. In one direction, it forces the outer layers outward. In the other, it pushes inward on the core remnant.(It also produces enough energy to force the fusion of elements heavier than Iron, which is where the heavier elements in the universe come from)

 

If the star is large enough, this pushes the remnant past the point where it becomes a black hole. Some of the star escapes outward while some is absorbed by the Black hole. You get an expanding cloud with a black hole at its center.

[jajrussel]

What I have read makes sense in most ways. What I was having difficulty accepting was how something can become smaller and still have enough mass to be a black hole. Density explains it, the closer things are the more energy it takes to overcome the bond of gravity.

I have read that the larger a black hole gets, the less dense it becomes. I think the explanation had to do with size comparisons. It was smaller now it is bigger therefore its over all density has changed. The changed part of the statement makes sense. A change has occurred, but how would the change affect the overall density? Even applying spin? To me it seems that spin if it had effect would have an equalizing effect on density. Random density samples within the shape would begin to even out in value across the shape. Is this thought wrong?

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It has occurred to me that if a black hole were spinning it would have to be spinning very fast in order for density to change across the shape, gravity would not allow it otherwise, and likely never allow it. The ratio always favors gravity.

[Janus]

I have read that the larger a black hole gets, the less dense it becomes. I think the explanation had to do with size comparisons. It was smaller now it is bigger therefore its over all density has changed. The changed part of the statement makes sense. A change has occurred, but how would the change affect the overall density?

 

Okay, Here's what's happening. Density is mass divided by volume. We can easily find the mass of a black hole, but how do we find it's volume? We can't see anything inside the event horizon. So it is common to define the size of a black hole by its event horizon or Schwarzschild radius .

 

The Schwarzschild radius is the distance from the center of the black hole where the escape velocity equals c. If you compare the Schwarzschild radius with the mass of a black hole, you find that there is a 1 to 1 ratio. If the mass doubles, the radius doubles.

 

Since volume increases by the cube of the radius, as the black hole grows, its increase in volume outpaces its increase in mass, thus we can say that its density decreases.

 

The thing to realize is that the event horizon is not a physical thing, it is just a boundary between what can and cannot escape the black hole. So when we say the density of the black hole decreases, we are comparing the volume defined by the event horizon compared to the mass. it is the "mean" density It says nothing about how that mass is distributed within that volume.

 

We do not consider the mass of the black hole as evenly distributed throughout its volume, but as concentrated at the center. Imagine a lead ball at the center of an inflated balloon. We can inflate the balloon and make it larger, causing the mean density of its interior to go down, but this does not mean that the density of the lead ball itself has changed.

[Mr Skeptic]

What causes a nuclear bomb to explode? I am under the impression that the explosion is the result of pressure. Is this impression correct?

 

No. A fission bomb explodes due to a chain reaction that produces more neutrons than it uses, and is catalyzed by neutrons. Modern fission bombs consist of a partly hollow core that is crushed and compressed by explosives. Because some neutrons are lost, the shape of the fissile material affects the minimum mass required -- called the critical mass. The compression of the explosion reduces the critical mass required, and makes for more efficiency.

 

Modern nuclear bombs also contain hydrogen. The energy and pressure of the fission core explosion are briefly contained, igniting a powerful hydrogen fusion explosion as well. For this both the pressure and temperature are necessary, as fusion is not a chain reaction.

 

I ask because I am just starting to try and understand how a black hole forms. To this point my understanding is a star collapses the pressure causes an explosion. The star then collapses again, except this collapse is unable cause enough pressure to create another explosion.

I have tried to think my way around the next questions. Why doesn't the first explosion blow away enough mass that the formation of a black hole can not occur? Why isn't there a continuous chain of explosions?

 

A black hole forms whenever a certain amount of mass is contained within a certain radius, called the Schwarzschild radius, R = 2GM/c^2. No explosions are necessary. For example, a neutron star can constantly leach matter from an orbiting companion star, eventually acquiring enough mass to form a black hole. No explosions necessary.

[jajrussel]

Thank you for your answers.

More Questions.

I was watching the MIT lecture number two on You Tube, about black holes, and it was said that they had no visible way to detect gravity waves, yet.

My questions are; Wouldn't the red shift or blue shift of a star moving through space be a visible indication of gravity waves?

Wouldn't the galaxy around a black hole be a visible indication?

If we are going to use descriptive words like, fabric of space time, wouldn't we expect the fabric to ripple if we threw another rock in the mix? Or is it just that there is the need to detect the little ripple, so it can be said; yes it is doing what it is supposed to do?

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Wouldn't the star or the rock be viewed as a moving divergence in the whole (not hole) fabric of space time, and what is seen is part of the effect?

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For clarification.

I understand that blue shift might not apply to the star. I just meant the idea.

Edited January 12, 2010 by jajrussel
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