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How Do Black Holes Grow?

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#1 Brian Albin

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Posted 25 July 2013 - 07:47 PM

Does the large Black Hole at the center of a Galaxy start as a small Black Hole from the collapse of a single star? If so, What is the mechanism of it's growth?
It seems that perhaps stars falling into the hole give the hole their energy and this in some way causes the hole to grow. But how is this done?

#2 deSitter

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Posted 25 July 2013 - 08:03 PM

By assimilating the physical and cultural uniqueness of the civilizations they encounter.

Wait, that's how the Borg gets bigger. Sorry!

-drl

#3 deSitter

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Posted 25 July 2013 - 08:07 PM

Sorry I couldn't resist :)

People like BHs because they are simple. For example, the size just depends on what's inside, the mass (and the charge for rotating ones, whatever that means). So the more goes in, the bigger it gets. Strangely, the bigger it gets, the less dense it need be. A really huge one need not be any denser than the air, or even the intergalactic medium.

Since these things are supposed to have eating disorders and consume everything nearby (this is never quite shown, just assumed it seems to me) they grow more and more rapidly. The bigger it gets the more stuff is attracts. So it's not really any different than the formation of a planet from the stellar nebula.

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#4 GlennLeDrew

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Posted 25 July 2013 - 11:59 PM

Black holes 'grow' simply by consuming more mass.

I can't picture a configuration whereby a collection of matter having such a low density as our atmosphere (let alone the ISM or IGM) can result in an event horizon. Unless this is another way to express the visibility horizon of the Universe. But then would this mean we exist inside a black hole? I can't see this as being the case.

Perhaps someone could chime in with the result of this calculation:

For a *uniform density* of that of air (1kg/m^3), what is the radius of a spherical volume which has a surface gravity of C?

#5 derangedhermit

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Posted 26 July 2013 - 01:03 AM

Does the large Black Hole at the center of a Galaxy start as a small Black Hole from the collapse of a single star? If so, What is the mechanism of it's growth?
It seems that perhaps stars falling into the hole give the hole their energy and this in some way causes the hole to grow. But how is this done?


Yes, it apparently can start from the collapse of a single star. There are a couple more mechanisms proposed also. The star's mass does not disappear, so other mass is still be drawn toward it by its gravity - other stars, gas, whatever. The mass keeps increasing, so the gravity keeps increasing.

No one has ever seen the creation of a black hole. It would be nice if we could find, say, 1,000,000 candidate stars and monitor them. The more we could monitor, the quicker we might see it happen.

#6 Jarad

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Posted 26 July 2013 - 07:41 AM

It's a good question.

As others have pointed out, they can grow the same way a planet or star can grow.

The big question though is how they grow so large so fast. Some of them weigh billions of solar masses. And since they are so small and compact, the question is how to squeeze so much gas in so fast?

That's why some people think there may be some other way that a large black hole can form.

For Glen's question, I think I can do that calculation:
The Schwarzchild radius is 2MG/C^2. The constant 2G/C^2 simplifies to 1.48*10^-27m/kg. The volume of a sphere is 4/3pi*r^3. For the density of 1kg/m^3, that's also its mass. So the schwarzchild radius for a density of air would be about 1.27*10^13 meters, or about 12.7 billion km. It would weigh about 8.6*10^39 kg, or about 4.3 billion solar masses.

Jarad

#7 Pess

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Posted 26 July 2013 - 08:34 AM

Keep in mind that a BH is just condensed mass. Any mass compacted dense enough will eventually have enough graity that light won't escape from its surface.

But people tend to think of BH's as these all-devouring things. That simply is not true. If some power were able to compress our own star down to about 4 miless across. Its gravity field would be exactly the same as it is now. The planets would go on orbiting unchanged.

It would just be that, because the gravity is so intense close in, that light could not escape from the surface.

So our own Sol BH would not devour our galaxy anymore than regular Sol does.

Suns, like Sol, are always taking in dust, rocks, out-of-fuel satellites Earth sent etc... So our Sol 'grows' with the slow addition of this mass. If it became a BH through a Harry Potter magic wand wave That BH would also gather in dust, rocks etc at the same rate.


Pesse (Without radiating any light though, it might cure global warming.) Mist

#8 Charlie B

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Posted 26 July 2013 - 09:43 AM

If it became a BH through a Harry Potter magic wand wave That BH would also gather in dust, rocks etc at the same rate.


Not exactly! If angular momentum is conserved, objects would form an accretion disk because of the greatly reduced volume, which would slow the in-fall. We might need to survive on X-ray illumination.

Charlie B

#9 Jarad

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Posted 26 July 2013 - 09:45 AM

If it became a BH through a Harry Potter magic wand wave That BH would also gather in dust, rocks etc at the same rate.



Actually, it would accumulate slower. Right now if a rock or comet or something orbits close enough to hit the sun's surface, it gets captured. If it compressed down to a black hole, the object would have to orbit down close to the event horizon to get caught. Objects passing by where the surface is now would keep orbiting.

Jarad

#10 Qwickdraw

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Posted 26 July 2013 - 09:48 AM

They eat stuff

#11 Qwickdraw

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Posted 26 July 2013 - 09:53 AM

Hey, while we are on the subject, since not even light (photons) can escape a black hole does a and photons have no mass, does the energy level of a BH increase due to the "consumption" of light? I know matter and energy are the same thing just in different states so does a photon remain a photon when consumed or does it change into some type of particle with mass?

#12 Pess

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Posted 26 July 2013 - 10:09 AM

Hey, while we are on the subject, since not even light (photons) can escape a black hole does a and photons have no mass, does the energy level of a BH increase due to the "consumption" of light? I know matter and energy are the same thing just in different states so does a photon remain a photon when consumed or does it change into some type of particle with mass?


Well, and I stand to be corrected, when a photon is captured all that means is that the photons 'orbit' becomes circular. Thus the captured photons still exist, they just circle the singularity. However, we can't detect them since no information can get back across.

So, if we use Harry Potters wand again, and stand inside the event horizon somewhere we would be awash in very, very bright bath of photonic radiation as we entered the orbital stream of all these photons.

Way down below we would glimpse the singularity where all the mass comes to a single point. (supposedly)

Pesse (Bring sunglasses & Sunscreen) Mist

#13 Qwickdraw

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Posted 26 July 2013 - 11:08 AM

Hey, while we are on the subject, since not even light (photons) can escape a black hole does a and photons have no mass, does the energy level of a BH increase due to the "consumption" of light? I know matter and energy are the same thing just in different states so does a photon remain a photon when consumed or does it change into some type of particle with mass?


Well, and I stand to be corrected, when a photon is captured all that means is that the photons 'orbit' becomes circular. Thus the captured photons still exist, they just circle the singularity. However, we can't detect them since no information can get back across.

So, if we use Harry Potters wand again, and stand inside the event horizon somewhere we would be awash in very, very bright bath of photonic radiation as we entered the orbital stream of all these photons.

Way down below we would glimpse the singularity where all the mass comes to a single point. (supposedly)

Pesse (Bring sunglasses & Sunscreen) Mist


Thanks Pesse, you go first and tell me what it is like inside.

#14 GlennLeDrew

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Posted 26 July 2013 - 11:17 AM

Jarad,
Thanks for undertaking that calculation! It is worth pointing out that such a configuration, where a considerable volume of such mass can maintain such low and *uniform* density, is most unstable. Self gravity will have gone to work long beforehand, to first make a bunch of stars.

- - -

If we take a neutron star (which is city-sized) very near the required mass, and toss in matter until an event horizon forms, does the neutron then star instantly 'squish' down to a singularity? I don't envisage a singularity, except in the way relativity describes it. If we could 'picture' the object in our non-relativistic framework, I think it would still be a city-sized object lurking just beyond the event horizon.

#15 Pess

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Posted 26 July 2013 - 12:30 PM

Hey, while we are on the subject, since not even light (photons) can escape a black hole does a and photons have no mass, does the energy level of a BH increase due to the "consumption" of light? I know matter and energy are the same thing just in different states so does a photon remain a photon when consumed or does it change into some type of particle with mass?


Stevie tells us that, actually, black holes evaporate over time by losing mass.

It seems virtual particle pairs pop into existence because of vacuum energy. These opposite pairs almost immediately annihilate themselves.

However, if this pair pops into existence next to the event horizon of a BH, then one of the pair can be trapped behind the horizon. Its twin, now free of the pair, becomes a 'real' particle and shoots off into the Universe.

This particle carries away energy...and that energy comes from the mass of the BH itself.

Thus every time this happens the BH loses an infinitesimal amount of mass and over the life of the Universe a BH can, in theory) evaporate away.....

This Hawking radiation is widely accepted by physicists...but not by everyone.....

Pesse (So Black Holes may not be quite so Black) Mist

#16 Pess

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Posted 26 July 2013 - 12:38 PM


If we take a neutron star (which is city-sized) very near the required mass, and toss in matter until an event horizon forms, does the neutron then star instantly 'squish' down to a singularity? I don't envisage a singularity, except in the way relativity describes it. If we could 'picture' the object in our non-relativistic framework, I think it would still be a city-sized object lurking just beyond the event horizon.


Math describes many things that, for practical purposes, don't exist inside our Universe.

For example, a perfectly sharpened pencil balanced on only one atom at its very tip can perfectly be described mathematically but you'd have a hard time finding that in the real world.

Since we don't understand gravity, we know that we have a big chunk of theory missing. Pencils don't balance on their top atoms because that configuration is unstable.

What instabilities arise when mass is condensed down? Does a new force manifest? Does all the mass just combine to form one big giant quarK?

I dunno. The space is still warped so all we know for sure is that the mass is still there.

Pesse (Black Holes play for keeps.) Mist

#17 Pess

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Posted 26 July 2013 - 12:46 PM

If we could 'picture' the object in our non-relativistic framework, I think it would still be a city-sized object lurking just beyond the event horizon.



Pesse (Shhh...you'll give away the location of the Flatland Universe!) Mist

#18 llanitedave

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Posted 26 July 2013 - 08:14 PM

Stevie tells us that, actually, black holes evaporate over time by losing mass.


That assumes they evaporate faster than they accumulate. The more massive they are, the more slowly they evaporate, but the big ones are also likely to accumulate more. So in practice, I'm not sure that there would be any real-world examples of evaporating black holes. Maybe when the universe has expanded enough to cut off the food supply almost completely...

#19 Brian Albin

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Posted 27 July 2013 - 01:36 AM

So I think I am hearing that the thing labeled as "Black Hole" is not a hole. It is a solid object. Is that right?

#20 StarWars

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Posted 27 July 2013 - 02:41 AM

Does the large Black Hole at the center of a Galaxy start as a small Black Hole from the collapse of a single star? If so, What is the mechanism of it's growth?
It seems that perhaps stars falling into the hole give the hole their energy and this in some way causes the hole to grow. But how is this done?



I think the black hole size depends on the size of the star... :D

A blue super giant could produce a missive black hole..

http://www.enchanted...startypes.shtml

#21 derangedhermit

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Posted 27 July 2013 - 02:51 AM


Stevie tells us that, actually, black holes evaporate over time by losing mass.


That assumes they evaporate faster than they accumulate. The more massive they are, the more slowly they evaporate, but the big ones are also likely to accumulate more. So in practice, I'm not sure that there would be any real-world examples of evaporating black holes. Maybe when the universe has expanded enough to cut off the food supply almost completely...


If things continue as they apparently are already moving, the expansion has already overpowered gravity at great distances; but it will never overpower gravity within a galaxy, will it? I haven't read up on how individual galaxies or galaxy groups may end, but there will be mass left in each galaxy, even when it is dark and cold. I suspect the dissolution of black holes will take a very long time in the end, even on the scale of the age of the universe. I always thought it would be the last thing still "happening".

#22 derangedhermit

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Posted 27 July 2013 - 03:16 AM

So I think I am hearing that the thing labeled as "Black Hole" is not a hole. It is a solid object. Is that right?

I think you will find one hour at Wikipedia, starting with "black hole" and following up on the explanation there by visiting related linked pages, will give you more information than a month asking things like this here.

#23 Pess

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Posted 27 July 2013 - 03:27 AM

So I think I am hearing that the thing labeled as "Black Hole" is not a hole. It is a solid object. Is that right?

I think you will find one hour at Wikipedia, starting with "black hole" and following up on the explanation there by visiting related linked pages, will give you more information than a month asking things like this here.


The only thing 'solid' about a BH is the point singularity at its center.

The Event horizon is nothing more than the distance away from the singularity where the escape velocity of the gravitational field becomes just equal to the speed of light.

I'll be honest, I'm not exactly sure why just adding a tiny bit more mass to a neutron star (pushing it into a black hole) will suddenly cause that neutron star to shrink to a singularity just because the mass edges up enough to create an event horizon.

Pesse (Question: How many Black holes can fit on the top of a pin? Answer: All of them) Mist

#24 GlennLeDrew

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Posted 27 July 2013 - 05:30 AM

About the origin of *very* massive bkack holes. The most massive possible star is thought to contain some 120 solar masses, and possibly as much as 200. But such stars lose a considerable fraction of themselves even before core collapse. And the core itself is but a fraction of the remainder, amounting to just several solar masses.

How to get to the million solar mass regime, such as is often found in galactic centers? It's an interesting recipe, built up from some ir all of the following ingredients.

Galactic centers are very crowded places to start with.

Generate a central bar to dynamically 'funnel' gas from the inner spiral arms. New stars will then continue to form in the nuclear region. The massive ones will add to the BH count.

Dynamical friction causes smaller companion galaxies and globular clusters to gradually spiral in to the central bulge. While most of the outer stars of these systems are tidally stripped away throughout the process, the more robust cores--perhaps with a good size BH within--can survive right down to the galactic center.

And 10+ billion years affords ample opportunity for BH's to form anew, grow, accumulate, and merge.

#25 Pess

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Posted 27 July 2013 - 08:06 AM

If things continue as they apparently are already moving, the expansion has already overpowered gravity at great distances; but it will never overpower gravity within a galaxy, will it? I haven't read up on how individual galaxies or galaxy groups may end, but there will be mass left in each galaxy, even when it is dark and cold. I suspect the dissolution of black holes will take a very long time in the end, even on the scale of the age of the universe. I always thought it would be the last thing still "happening".


I think the consensus is that eventually all the matter in the Universe will be collected in Black Holes. These Black Holes (if Hawking is right) will evaporate leaving a Universe filled with the background radiation shadow of our existence--and nothing more...

There is an interesting line of thought that with the expansion of the Universe there is a corresponding increase in the density of 'Dark Energy' (if you are into that sort of thing) As the Dark energy becomes denser it approaches infinity with a corresponding infinite speed of expansion of every particle in the Universe away from every other particle. Keep in mind that the expansion of space is not limited by 'c'.

But heck, there is more we don't know about Dark matter and Dark energy than we do. After all, Dark matter & Dark energy are ad hoc explanations to explain stuff we might not be looking at or interpreting correctly.

Pesse (We just don't know what we don't know) Mist






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