Spiraling into a black hole? Really?

[kirkster501]

Never quite understood this.  Why would material be "pulled off" a large companion star and then orbit in an accretion disk (as per picture below) and then vanish into a black hole?  If that could happen, why wouldn't the sun "pull" the earth into it?  Stiff that is in orbit is exactly that, in orbit.  So why would matter on a companion star that is in orbit around a black hole have stuff pulled off of it into the black hole?

[andrew s]

Between the black hole and the orbiting star is a point where the pull of one on the other is balanced. For the star in your example it's atmosphere extends to this point and material can spill over into the area of the net pull of the black hole. Then it can spiral on onto the black hole.

For the sun earth the earth's atmosphere and it's solid bulk never gets near the balance point. So it just stays in orbit.

Regards Andrew 

[Tiki]

13 hours ago, kirkster501 said:

  Why would material be "pulled off" a large companion star and then orbit in an accretion disk (as per picture below) and then vanish into a black hole? 

 

In a binary system the more massive component will quite often evolve to a white dwarf/neutron star/black hole before the smaller companion star begins to expand into a red-giant. As the smaller companion star expands it sometimes happens that its outer layers are no longer gravitationally bound and are pulled towards the more massive compact component. Conservation of angular momentum will cause the in-falling matter to spiral into an accretion disc. The particles  in the accretion disc lose energy principally as a result of friction and eventually spiral in to the central compact object.

Try googling Roche limit/lobe/sphere.

I hope this helps.