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Black Hole Size


Corkeyno2

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I've been working on a 3D size comparison in blender, and I want to compare the sizes of black holes. However, I can't find a website that will tell me the radius or diameter of one, only their masses. If anybody could recommend a website or source I would be very grateful.

Thanks,

Corkey

 

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Just now, johnfosteruk said:

I thought a black hole had no size. It's a singularity, point etc and that's where our understanding goes wobbly. The accretion disk has a size though. No idea what though off the top. 

Sorry for not being clear enough. I'm looking for the event horizon size.

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From hubblesite.org:

 

The size of the event horizon (called the Schwarzschild radius, after the German physicist who discovered it while fighting in the first World War) is proportional to the mass of the black hole. Astronomers have found black holes with event horizons ranging from 6 miles to the size of our solar system. But in principle, black holes can exist with even smaller or larger horizons. By comparison, the Schwarzschild radius of the Earth is about the size of a marble. This is how much you would have to compress the Earth to turn it into a black hole. A black hole doesn't have to be very massive, but it does need to be very compact!

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2 minutes ago, SilverAstro said:

The event horizon is only the same size as the Schwarzschild radius if the black hole is not rotating.

 

Do you have any info or links for rotating black holes SilverAstro?

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2 minutes ago, Corkeyno2 said:

I thought it was 3km for the sun

Ooops !

The surface at the Schwarzschild radius acts as an event horizon in a non-rotating body that fits inside this radius (although a rotating black hole operates slightly differently). The Schwarzschild radius of an object is proportional to its mass. Theoretically, any amount of matter will become a black hole if compressed into a space that fits within its corresponding Schwarzschild radius. For the mass of the Sun this radius is approximately 3 kilometers and for the Earth it is about 9 millimeters. In practice, however, neither the Earth nor the Sun has the necessary mass and therefore the necessary gravitational force, to overcome electron and neutron degeneracy pressure. The minimal mass required for a star to be able to collapse beyond these pressures is the Tolman-Oppenheimer-Volkoff limit, which is approximately three solar masses.

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5 minutes ago, SilverAstro said:

 

The next question is - what proportion of black holes in the universe are not rotating ?

 

I was going to ask that, but I'm already out of my depth.... ;) 

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As most black holes are formed from collapsed stars, they would continue to rotate due to conservation of angular momentum. These rotating (Kerr) black holes may slow with time but would never completely stop rotating. These are likely to be the most common type of black hole.

The 'Schwarzschild' black hole, a simple singularity and event horizon, is going to be less common. What the percentage is I have no idea!

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40 minutes ago, Stu said:

I was going to ask that, but I'm already out of my depth.... ;) 

and me ! But I like poking at things :)

and to discuss with like-minded happy pilgrims is to learn ( here endeth the sermon !) and if Corkey had not asked the question then, although I knew previously that the relation was proportional, I did not know, off-hand, in real km what the magnitude was. I would not have blundered betwixt the Sun and Earth and so I would not have, for ever more!,  the 3km in ma'heed ;)

Did you find the diagram in that ref for the rotating black hole ?  - how can anything that has no size form a ring which does have a size ? depth-limit squared :)

If we visit a black hole and it is rotating we can enter the ergosphere and escape, so we know then that it is rotating, so we carry information away with us and the black hole should become less massive and shrink   - Soooo does this mean that Hawking Radiation is not the only way a black hole can evaporate ? Where is Stephen when we need him ??

 

 

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Its all gone quiet :D , that was fun :)

 

2 hours ago, SilverAstro said:

If we visit a black hole and it is rotating we can enter the ergosphere and escape, so we know then that it is rotating, so we carry information away with us and the black hole should become less massive and shrink   - Soooo does this mean that Hawking Radiation is not the only way a black hole can evaporate ? Where is Stephen when we need him ??

Oh wow ! and (Hugh) golly-gosh (/Laurie) I was right! It can lose mass as we enter and then carry information away via the ergosphere, but ONLY until it is reduced to its Invarient Mass (aka Rest Mass) Once it has (we have) evaporated its relativistic mass the ergosphere then collapses to the Schwarzschild radius as the rotation is reduced to zero. Pity the last man/spacecraft through :happy6:

If anyone wants my paper-trail see below (at your peril !) , else tear here and discard :) :-
------------------------------------------------------------------------------------------

Black hole information paradox : https://en.wikipedia.org/wiki/Black_hole_information_paradox

"Hawking remained convinced that the equations of black-hole thermodynamics together with the no-hair theorem led to the conclusion that quantum information may be destroyed. This annoyed many physicists, "

https://en.wikipedia.org/wiki/No-hair_theorem

The no-hair theorem postulates that all black hole solutions of the Einstein-Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum.

Fair enough, they say we can carry away information about its rotation, I should have cottened-on earlier, because we know that we can obtain information about its (rest)mass without reducing it, by observing the orbits of stars etc round it.

Then from Barkis' @barkis picture, I needed to know why the photons cannot get further in towards the Sw radius than that,  

Photon sphere https://en.wikipedia.org/wiki/Photon_sphere

"Spherical photon orbits around a Kerr black hole
In contrast to a Schwarzschild black hole, a Kerr (spinning) black hole does not have spherical symmetry, but only an axis of symmetry, which has profound consequences for the photon orbits. "

 so onwards to learn more about the Kerr metric :-

Kerr Metric  https://en.wikipedia.org/wiki/Kerr_metric
"Objects between these two horizons must co-rotate with the rotating body, as noted above; this feature can be used to EXTRACT ENERGY from a rotating black hole, up to its invariant mass energy, Mc2."

 

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