How Do Accretion Disks Around Black Holes Form and Function?

[vitus988]

How is it formed? How do particles move on it?

 

[bcrowell]

The accretion disk is simply the cloud of matter (if any) falling into the black hole. It forms a disk for the same reason that, e.g., our solar system is planar: conservation of angular momentum. The particles move along trajectories that differ from Keplerian orbits in two ways: (1) the closer you get to the event horizon, the bigger the relativistic corrections to Newtonian gravity, and (2) they collide with one another sometimes. The collisions are the reason the accretion disk is so hot.

 

[vitus988]

The accretion disk is simply the cloud of matter (if any) falling into the black hole. It forms a disk for the same reason that, e.g., our solar system is planar: conservation of angular momentum. The particles move along trajectories that differ from Keplerian orbits in two ways: (1) the closer you get to the event horizon, the bigger the relativistic corrections to Newtonian gravity, and (2) they collide with one another sometimes. The collisions are the reason the accretion disk is so hot.

I am new to the concept or the field. I've read wiki about it but still confused. I would like to get a more detailed theoretical view but don't know where to start. Any book or paper for recommendation?

 

[bcrowell]

I am new to the concept or the field. I've read wiki about it but still confused. I would like to get a more detailed theoretical view but don't know where to start. Any book or paper for recommendation?

What's the level of your background in physics, and specifically relativity?

 

[Phrak]

For an infalling star, it's matter undergoes compression perpendicular to the plane of rotation. I didn't locate a reference for dust, so take it for what it is.

http://adsabs.harvard.edu/full/1994MmSAI..65.1129L"

The second ingredient is an inward spiraling orbit.

https://www.physicsforums.com/showthread.php?t=117893"

 

[vitus988]

What's the level of your background in physics, and specifically relativity?

I am an senior year undergraduate majored in physics in China.
I took an introduction course to general relativity for undergraduate. Knowing some basics about BH. I am trying to get to a deeper insight through English textbooks or papers.

 

[bcrowell]

I am an senior year undergraduate majored in physics in China.
I took an introduction course to general relativity for undergraduate. Knowing some basics about BH. I am trying to get to a deeper insight through English textbooks or papers.

That's good -- it sounds like you have a strong background. Can you be more specific about what point is causing you difficulty right now? Do you just want to learn more about black holes, or is there something related to accretion disks that you don't understand?

 

[vitus988]

That's good -- it sounds like you have a strong background. Can you be more specific about what point is causing you difficulty right now? Do you just want to learn more about black holes, or is there something related to accretion disks that you don't understand?

Here is the Wiki link：
http://en.wikipedia.org/wiki/Accretion_disc

It contains much information, however, without further explanation. How the accretion disk firstly proposed? On wiki it is only :" In the 1940s, models were first derived from basic physical principles." Then there is "In order to agree with observations". What are the observations? Besides, there is "Unsolved problems in physics: Accretion disc jets: Why do the discs surrounding certain objects, such as the nuclei of active galaxies, emit radiation jets along their polar axes? These jets are invoked by astronomers to do everything from getting rid of angular momentum in a forming star to reionizing the universe (in AGNs), but their origin is still not understood." Why jets can't be explained? Is it in contradiction with existing theories, or can't be derived from them?

 

[bcrowell]

It contains much information, however, without further explanation. How the accretion disk firstly proposed?

I think they're simply observed directly. They also help to explain the planar nature of the solar system, and they arise naturally due to conservation of angular momentum.

On wiki it is only :" In the 1940s, models were first derived from basic physical principles." Then there is "In order to agree with observations". What are the observations?

We look through a telescope and see accretion disks.

What may be confusing you is the distinction between two different things: (1) the existence of a disk, as opposed to a spherically contracting cloud, and (2) the fact that material in the disk can actually accrete onto a small object at the center.

As an example of the empirical evidence for #1, we look through telescopes and see the disks. As an example of empirical evidence for #2, we observe that our solar system contains the sun, rather than containing a large disk of hot gas that wasn't able to gather at the center.

Theoretically, #1 is well understood as arising from conservation of angular momentum plus viscosity. #2 is not as well understood theoretically, and that's what the paragraph in the WP article is talking about. I think the story is that basically you can't reproduce #2 without invoking turbulence, but turbulence is difficult to calculate precisely.