Re: How can pieces of stars that collide while entering a black hole 'jet' out

This is a really good question that touches on several very hot issues in astronomical research!

The first thing to know about black holes is that, like for all other masses, the speed that you have to go to escape depends on how far away you are: the closer you are to an object, the faster you have to go to escape from it. For a black hole, if you get close enough to it, eventually the escape speed gets to the speed of light at a place called the "event horizon". From the event horizon inward, nothing can escape from a black hole; but stuff (including light and matter) can escape before it gets to the event horizon.

Thanks to some bad science fiction movies, people often think of black holes as being giant "vacuum cleaners" that go around sucking things in. This is not really the case - far away from the event horizon, black holes are just like other massive bodies (stars, planets, etc.) in their gravitational effects.

So, that's why the matter from the infalling stars can escape - it's far from the event horizon. Why does the debris come out in a jet? The debris doesn't fall directly into the black hole; instead it spreads out into an "accretion disk" and slowly spirals down into the black hole. The jet is formed at the inner edge of the accretion disk where the infalling matter gets very hot. Some material is expelled due to the high pressure; it ends up in a jet because there is a "focussing" effect as the material passes through the gas in the galaxy where the black hole is located. (Apparently you can see the same effect if you take a garden hose and put the nozzle underwater!)

Some books on black holes that you might find interesting: "Black Holes and Time Warps: Einstein's Outrageous Legacy", by Kip Thorne, (W.W.Norton, 1994)

"Prisons of light: black holes", by Kitty Ferguson (Cambridge University Press, 1996)

Any introductory astronomy text book will have info about black holes and accretion disks; a good one is "Universe", by W. Kaufmann (W.H.Freeman, 1994).