Expert: Tom Whiting - 4/30/2008

Question
Could you please answer this question: Why doesn`t matter (from a star e.g.) fall directly in a black hole, but forms an accretion disk instead? I have already asked this question to an expert and he wasn`t able to answer. Are you able to answer please?

Answer
Hi Aleksandar,
Yes...look at it this way.  If a comet from the outer Solar System
was to suffer an orbit change, (by close pass to a major planet, or
suffer an asteroidal collision) and say, then headed right toward the
sun,  a straight-on perfect hit, then it hits the sun and is
immediately destroyed completely.

BUT, if say,  it's new orbit just misses the sun by a few million miles,
then  it will go into a close orbit AROUND the sun, and in the process, would be ripped apart into fragments which would continue to orbit the sun in a close, high velocity orbit, in effect forming an "accretion disk" of material orbiting the sun.
( It's tidally ripped apart due to getting closer than Roche's Limit of 2.5 radii of the sun.)

A star heading toward a black hole is the exact SAME circumstance.  
And since a black hole has such a small radius, like 10-20  miles,
most stars are NOT on a perfect staight line direct path to the black hole, but just miss it, thus like the comet example orbiting the sun above, it goes into orbit closely around the black  hole at high speed, gets ripped apart by exceeding Roche's Limit,  thus the shredded star material forms an accretion disk in orbit, around said black hole.  The super-heated material gets super-hot and gives off hard X-rays and that's how we can detect the accretion disk material around the black hole, with our X-Ray telescopes up in Earth orbit.   In this process, stellar material is actually in a spiral condition, so some stellar material eventually spirals down into the black hole from orbit, over time.  But not all of it, all at once.  It's a gradual orbit decay of the shredded
stellar material,  over a long time period.

I can't make it any simpler than that.
Hope this helps,
Clear skies,
Tom Whiting
Erie, PA  USA