I'm finishing up a book and in it a physicist Yakov Zeldovitch was explaining how a binary star system oribited each other. He went on to say that if one star collapsed into a black hole the remaining star would continue to circle around what appeared to be empty space.

 

Then he goes on to say that the gravity of the black hole would begin violently stripping matter away from the companion star.

 

I understand the process of a star collapsing into a black hole, but what I don't understand is why the black hole has more gravitational pull than it had when it was a star.

 

I also understand how a black hole keeps pulling in matter and getting more powerful, but where did the binary star get the extra matter.

 

Bettina

It doesn't have more gravitational pull then before. It depends on how far the other star is. If the companion star expands and fills its Roche Lobe, then it will lose matter to the other star, whether it is a black hole or not. Its Roche Lobe is small when the stars are close to each other, and large when they are far.

It doesn't have more gravitational pull then before. It depends on how far the other star is. If the companion star expands and fills its Roche Lobe, then it will lose matter to the other star, whether it is a black hole or not. Its Roche Lobe is small when the stars are close to each other, and large when they are far.

 

Ok, I looked up Roche Lobe, but it doesn't explain it enough except its a region around each star. I think I can picture this region, but the other star collapsed. How does its region change. Shouldn't it be smaller?

 

Bettina

The only thing I can think of would probably be that if some parts of the other star reached into the event horizon of the black hole, that part would indeed be sucked in forever. Within this special radius, the black hole is official a black hole