The Naked Scientists

The Naked Scientists Forum

Author Topic: How can we define a black hole accretion zone?  (Read 1940 times)

Offline jeffreyH

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 3927
  • Thanked: 55 times
  • The graviton sucks
    • View Profile
How can we define a black hole accretion zone?
« on: 28/04/2014 22:12:24 »
Black holes grow by accreting matter but is there a method for determining where this accretion zone starts?


 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4130
  • Thanked: 249 times
    • View Profile
Re: How can we define a black hole accretion zone?
« Reply #1 on: 29/04/2014 10:53:27 »
Accretion disks can form around black holes, neutron stars, white dwarf stars and young stars.

An accretion disk has two components:
  • Some matter is robbed of its angular momentum, and it spirals into the central body
  • The angular momentum is transferred to some other matter, which spirals further from the central body, becoming less dense. This makes the outer edge a bit hard to define.

Some matter remains in the disk, forming planets (if it is cool enough) or a warm gas/dust cloud (glowing in infra-red), or a hot plasma (glowing in X-Rays). The glowing edge forms a boundary which could define the accretion disk (but the location of this edge will differ depending on what sort of telescope you use to observe it, at which wavelengths).

See: http://en.wikipedia.org/wiki/Accretion_disk
 

Offline jeffreyH

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 3927
  • Thanked: 55 times
  • The graviton sucks
    • View Profile
Re: How can we define a black hole accretion zone?
« Reply #2 on: 02/05/2014 00:01:57 »
Accretion disks can form around black holes, neutron stars, white dwarf stars and young stars.

An accretion disk has two components:
  • Some matter is robbed of its angular momentum, and it spirals into the central body
  • The angular momentum is transferred to some other matter, which spirals further from the central body, becoming less dense. This makes the outer edge a bit hard to define.

Some matter remains in the disk, forming planets (if it is cool enough) or a warm gas/dust cloud (glowing in infra-red), or a hot plasma (glowing in X-Rays). The glowing edge forms a boundary which could define the accretion disk (but the location of this edge will differ depending on what sort of telescope you use to observe it, at which wavelengths).

See: http://en.wikipedia.org/wiki/Accretion_disk

I would imagine that the angular momentum of an approaching mass need not be particularly extreme to escape being drawn into a black hole/neutron star as it will be traveling perpendicular to the direction of the gravitational field. My main question concerned the point at which matter would be stripped from the surface of a celestial body. The point at which the gravitational field of both masses cancel exactly at the surface of the approaching body. This distance should increase with a decrease in mass of an approaching object. The larger the mass the nearer it can get to the compressed object before donating mass to the accretion disk.
 

Offline jeffreyH

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 3927
  • Thanked: 55 times
  • The graviton sucks
    • View Profile
Re: How can we define a black hole accretion zone?
« Reply #3 on: 02/05/2014 00:18:19 »
BTW The increase in angular momentum of outer objects could be due to the reduction of the radius of the collapsed object. It is much less disturbed by the external gravitational fields. Whereas the orbiting bodies experience an increase in the gravitational effect. This balance in effect should be proportional to mass size and radial distance.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Re: How can we define a black hole accretion zone?
« Reply #4 on: 04/05/2014 18:03:57 »
The original question does not have a precise answer because in the absence of any othe material the gravitational field of an object extends right out to infinity. 

You therefore have to define something else to allow you to set sime sort of size to the accretion zone.

In the case of a black hole the smallest distance friom the centre is the event horizon which is about one mile for each soar mass that the black hole contains so a typical 10 stellar mass black hole is about ten miles across which is pretty small.  The billion solar mass black holes at the centres of large elliptical galaxies are only about as big as the solar system. 

The event horizon is not really a useful limit though because that defines the smallest size that we could possibly identify. 

A more useful limit might be the point at which cold accreting gas would heat up due to the gained gravitational energy being turned into kinetic energy to glow like a star.  For a stellar mas black hole this would be approximately the size of a normal star with the same mass and colour as the temperature chosen.

This is OK for small black holes but there are no such thing as billiion solar mass stars around so there is no easy bench markfor sars of this size and the relationship is not a simple linear one like size of the event horizon. and this limit is notreally discussed in most texts however ther is one futher fact that could be used.  That is the fact that if billion solar mass lump of gas was coollapsing it is possible for it to collapse directly into a black hole without ever becoming a star and synthesising new nucleii so the maximum size of te acretion zone of a black hole is probably about the size of the solar system  which at the distance of galaxies is a pretty small thing. 
 

Offline jeffreyH

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 3927
  • Thanked: 55 times
  • The graviton sucks
    • View Profile
Re: How can we define a black hole accretion zone?
« Reply #5 on: 05/05/2014 01:25:21 »
The original question does not have a precise answer because in the absence of any othe material the gravitational field of an object extends right out to infinity. 

You therefore have to define something else to allow you to set sime sort of size to the accretion zone.

In the case of a black hole the smallest distance friom the centre is the event horizon which is about one mile for each soar mass that the black hole contains so a typical 10 stellar mass black hole is about ten miles across which is pretty small.  The billion solar mass black holes at the centres of large elliptical galaxies are only about as big as the solar system. 

The event horizon is not really a useful limit though because that defines the smallest size that we could possibly identify. 

A more useful limit might be the point at which cold accreting gas would heat up due to the gained gravitational energy being turned into kinetic energy to glow like a star.  For a stellar mas black hole this would be approximately the size of a normal star with the same mass and colour as the temperature chosen.

This is OK for small black holes but there are no such thing as billiion solar mass stars around so there is no easy bench markfor sars of this size and the relationship is not a simple linear one like size of the event horizon. and this limit is notreally discussed in most texts however ther is one futher fact that could be used.  That is the fact that if billion solar mass lump of gas was coollapsing it is possible for it to collapse directly into a black hole without ever becoming a star and synthesising new nucleii so the maximum size of te acretion zone of a black hole is probably about the size of the solar system  which at the distance of galaxies is a pretty small thing.

Thanks for the reply. Some of the points you made are very interesting. I agree that for stellar mass size black holes the region would be equivalent to the original size of the star. Your point about billion solar mass black holes is also a very good one. There is nothing to compare too. I am pondering the role of central black holes within galaxies. The orbital profiles currently need dark matter to explain them. What if the galaxies are acting as if part of a unified system and contained within the scope of a confinement region? If we could find the extent of the black hole's uncompressed volume would this encompass the whole galaxy?
 

The Naked Scientists Forum

Re: How can we define a black hole accretion zone?
« Reply #5 on: 05/05/2014 01:25:21 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length