Black Holes and the Event Horizon?

In summary: This radius is typically about three times the radius of the event horizon of the compact object. The material spirals inwards, losing angular momentum, until it reaches the surface of the compact object. There it will be heated and emit X-rays, and perhaps cause other effects. As the mass is accreted, the orbiting disk shrinks in size until it disappears when the entire mass of the companion has been accreted. Thus the accretion disk is a transient phenomenon that exists only as long as there is material that is not part of the compact object left in the vicinity
  • #1
TDS
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My questions are these: When a Black Hole is detected, Is it found by its Event Horizon? Is the Event Horizon located in relation to the stars equitorial plane? If I were to approach a Black Hole from its "North Pole" or "South Pole", would I be affected by its Event Horizon?
 
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  • #2
TDS said:
My questions are these: When a Black Hole is detected, Is it found by its Event Horizon? Is the Event Horizon located in relation to the stars equitorial plane? If I were to approach a Black Hole from its "North Pole" or "South Pole", would I be affected by its Event Horizon?
The event horizon is a sphere surrounding the center of a black hole.
 
  • #3
Consider the region of spacetime from which it possible for a photon to escape to (future null) infinity. Now consider he complement of this region, i.e., a region of spacetime from which it is not possible for a photon to escape to infinity. This latter region is the black hole region.

The boundary between the two regions is the event horizon.

Unfortunately, there are no warning makers that tell you when you cross an event horizon.
 
  • #4
I had not though about the Event Horizon as being spherical! What a concept!

Would you happen to know of any books or links where I could read more about this!


MeJennifer said:
The event horizon is a sphere surrounding the center of a black hole.
 
  • #5
So is it safe to say that the Accretion Disk and the Event Horizon are two different things? I had thought that they were the same thing.


George Jones said:
Consider the region of spacetime from which it possible for a photon to escape to (future null) infinity. Now consider he complement of this region, i.e., a region of spacetime from which it is not possible for a photon to escape to infinity. This latter region is the black hole region.

The boundary between the two regions is the event horizon.

Unfortunately, there are no warning makers that tell you when you cross an event horizon.
 
  • #6
TDS said:
IWould you happen to know of any books or links where I could read more about this!

In my opinion, https://www.amazon.com/dp/0316483419/?tag=pfamazon01-20 has the best popular-level treatrment of black holes - I can't recommend it highly enough. Great diagrams.
 
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  • #7
TDS said:
So is it safe to say that the Accretion Disk and the Event Horizon are two different things? I had thought that they were the same thing.

Yes, they are different. The accretion disk lies outside the event horizon.

Sorry, got to run! I'll try and add some more details tonight.

Are asking about evidence for an event horizon that can be seen from afar, or evidence as you cross an event horizon?
 
  • #8
George Jones,

Thanks for this link! My home library has just gained mass!:biggrin:
If you have more links or suggestions, feel free to send them to me. The more that I learn - the better I will be!


George Jones said:
In my opinion, https://www.amazon.com/dp/0316483419/?tag=pfamazon01-20 has the best popular-level treatrment of black holes - I can't recommend it highly enough. Great diagrams.
 
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  • #9
Any information that you can provide on both will be more than I have at the moment. Basically what I am trying to understand is this; everytime you see a depiction of a Black Hole, you see a ring of "light" that signifies the event horizon around the Black Hole. And in the same breath the narrator starts talking about the accretion disk. I would like to understand the difference. As for your questions - both.



George Jones said:
Yes, they are different. The accretion disk lies outside the event horizon.

Sorry, got to run! I'll try and add some more details tonight.

Are asking about evidence for an event horizon that can be seen from afar, or evidence as you cross an event horizon?
 
  • #11
TDS said:
Any information that you can provide on both will be more than I have at the moment. Basically what I am trying to understand is this; everytime you see a depiction of a Black Hole, you see a ring of "light" that signifies the event horizon around the Black Hole. And in the same breath the narrator starts talking about the accretion disk. I would like to understand the difference. As for your questions - both.

Accretion disk are swirling matters being pulled into the black hole. They glow as the result of being heated up by the friction. The event horizon is a spherical boundary which you can't escape once you get passed. You can't see the event horizon, at least not directly. If you have a black hole sitting around a region with not much matter, there will be no accretion disk. But the event horizon is always there.
 
  • #12
@pervect,

Thanks for the book link! There goes the mass of my bookshelf again!

@yenchin,

Thanks for clearing up my questions about the Event Horizon and the Accretion Disk!

Now for the really big question. Since the Event Horizon is spherical, is the Accretion Disk also spherical or does it exhist along the equitorial plane of the Black Hole?
 
  • #13
TDS said:
Now for the really big question. Since the Event Horizon is spherical, is the Accretion Disk also spherical or does it exhist along the equitorial plane of the Black Hole?

No, the acretion disk is a disk, i.e, it lies in a plane, for roughly the same reason that the orbits of all the planets lie in a plane.

For accretions disks, I can do no better than to quote from Gravity: An Introduction to Einstein's General Relativity, by James Hartle.

Hartle said:
Consider for example, a black hole or neutron star in mutual orbit with a more normal companion star - one like the Sun for instance. The binary pair can lose orbit energy - by gravitational radiation among among other mechanisms - decreasing the size of the orbit. The orbit can bcome small enought that the outermost layer of the companion is more strongly attracted to the compact object than to its own center. In that case the more normal star will shed mass, which will fall (accrete) onto the compact object. Conservation of its initial orbital angular momentum means that the accreting material does not fall directly onto the compact object but rather forms a disk around it called an accretion disk. Various dissipative mechanisms associated with interactions between the particles in the disk cause them to slowly lose energy and angular momentum and gradually spiral towards the object. They spiral slowly inward on nearly circular orbits until they reach the innermost stable circular orbit, after which they fall rapidly into the compact object. The energy they lose leaves the disk as radiation - characteristically at X-ray wavelengths for compact objects around a solar mass.
 
  • #14
First let me say THANKS to all that has posted answers to my questions on this thread!:biggrin:

I have received more answers in two days on this site than I did in my first semester of Astronomy! This is a good thing for you folks, a sad thing for the school that I went to.

Now for another question.

Would it be possible to observe a Black Hole so long as the probe stayed outside of the Accretion Disk?

I know that the only thing that could actually be observed would be the Accretion Disk unless the probe had a device to detect Hawking Radiation.
 
  • #15
Yes. If you stay sufficiently far away at a safe distance, you can observe the accretion disk. Hawking radiation is too weak to be detected as it is many times weaker than the microwave background radiation.
 
  • #17
@chronon,

My thanks goes out to you as well! If this keeps up I shall need to get another bookshelf!:biggrin:


chronon said:
I've a list of books related to black holes (and in particular Hawking Radiation) at Black Hole Evaporation - useful books
 

1. What is a black hole?

A black hole is a region in space where the gravitational pull is so strong that nothing, including light, can escape it. This happens when a massive star dies and collapses under its own gravity.

2. How are black holes formed?

Black holes are formed when a massive star dies and its core collapses. The gravitational pull becomes so strong that it overcomes the forces that hold the star's atoms together, causing the star to collapse into a single point known as a singularity.

3. What is the event horizon of a black hole?

The event horizon of a black hole is the point of no return, where the gravitational pull is so strong that even light cannot escape. It marks the boundary of the black hole and is the point at which the escape velocity exceeds the speed of light.

4. Can anything escape a black hole?

No, nothing can escape a black hole once it has passed the event horizon. This includes light, matter, and even information. Once something crosses the event horizon, it is pulled into the black hole and can never escape.

5. Are there different types of black holes?

Yes, there are different types of black holes based on their mass and size. The three main types are stellar black holes, intermediate black holes, and supermassive black holes. Stellar black holes are formed from the collapse of a single star, while supermassive black holes are found at the center of galaxies and can be billions of times more massive than the sun.

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