Expert: Jayendra Upadhye - 7/10/2004

Question
What is a black hole's accretion disk and how does it form?

Answer
Hi Charles,
1 - Black holes can form accretion disks only when there is
   a source of matter nearby, such as a star that is more
   massive than them and which they happen to orbit.
   (Cygnus X-1 is a case)
2 - The accretion disk can actually be described as a
   volume of space centered on the black hole, and which
   resembles a disk because it is flattened and circular.

   It primarily consists of gaseous and solid matter that
   is on its way into the black hole.
   
   As the matter approaches the event horizon or the
   boundary beyond which the region of "blackness" starts..
   (mainly because the escape velocity beyond this point
    as one moves inwards is more than the speed of light)..
   the gravitational force "differnce" between any two
   given points of a body (seen radially) is so great that
   it pulverises the body..generating fine dust from
   particulate matter.

   This "pulverising" region starts quite far out from the
   event horizon, increasing in severity of its tidal
   forces as one progresses inwards.

   This crushing actually leads to lot of turmoil and
   eventually the heated matter begins to radiate hard x
   rays!

   This is the "accretion disc" of the black hole.

3 - Actually accretion disks are quite common and may not
   always  belong to black holes.

   The sun had its own accretion disk when the planets
   were forming.

4 - One can say that the rings of saturn are a particular
   case where an icy body enterd this virtual area that
   exists around all astronomical bodies (with more or
   less severity of tidal forces), and filled it out after
   crossing the roche's limit where saturn, (like our
   black hole), broke it up  and has since then prevented
   it from re-forming!    
   read all about it at :-
   http://www.cvc.org/astronomy/roche_limit.htm

   Another case is the asteroid belt..it can technically
   be called as a part of "solar accretion disc".

   Mind you, generally however accretion disks are
   supposed to be made up of infalling matter.

5 - Last but not the least..one neeeds to remember that
   even if an accretion disk is not seen, it can be formed
   anytime once matter is available to populate it! As it
   is just an area of space where the laws of physics
   dictate matter to behave in a certain way!
Jayen