Expert: Courtney Seligman - 5/24/2010

Question
I just saw a show on astronomy, specializing in black holes. It stated that for a star to escape a galaxy, it had to get very near a black hole. This is what surprised me: this phenomenon requires a binary star system. I do know that binaries will impart extra energy, but - A:where does the energy come from just because it's a binary? (It has to be enormous)And B: why can't a single star escape though perturbations in a very crowded environment? I admit I'm confused. Thank you.

Answer
To answer part B of your question first, yes, in a sufficiently crowded environment, single stars could escape, as a result of very close gravitational interactions with other single or multiple star systems. But such situations only exist in our galaxy in globular clusters, and even there, such escapes are very rare.

As far as part A is concerned, the usual way in which a star achieves escape velocity is to be the less massive member of a binary star system, in which the more massive star is so heavy that it ends its life as a supernova. If the supernova explosion causes more than half the total mass of the system to "disappear" into interstellar space, the gravity of the system will no longer be large enough to hold the two stars together, and they will each head into interstellar space, with whatever their orbital velocities were, prior to the explosion.

The high velocity required to escape the galaxy does not come from the explosion itself. All that does is allow the star to go off into space, instead of continuing to orbit the other star. But if the total mass of the system is very large, and the stars are relatively close together, their orbital velocities around each other can be very high.

I should note that although the star that supernovas might (and probably would) become a black hole, it is also conceivable that it could become a neutron star, or in rare cases, simply blow itself to smithereens, leaving nothing behind, at all.

Not quite finally, the statement that the star had to get close to a black hole implies that getting close to a black hole might somehow eject a star from a galaxy. But if a star passed close to a black hole, although it would gain a huge velocity in doing so, it would lose all the speed it picked up, as it moved away from the black hole. So it isn't getting close to a black hole that causes the ejection. It's the mass lose involved in creating a black hole (or other stellar remnant) that allows its companion to escape.

And to end, I might note that when the one star goes off into space, the "black hole" or whatever the other star has become also does so, but in the opposite direction. So although the two might be close together at the moment of the supernova, after that, they get farther and farther apart, for the rest of eternity.