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The Question

(Submitted October 05, 1996)

What information is available regarding the excessive amount of X-ray emissions surrounding a black hole? What is the significance of the X-ray emissions around a black hole?

The Answer

The first place we suggest you look are the black hole pages in Imagine the Universe! under the Basic and Advanced sections. There you can find out information on Black Holes and more specifically X-ray Binaries (which, we believe, is what you are asking about).

Many stars are observed to be in binary systems, where two stars are orbiting each other (as the Earth orbits the sun). Another thing to know is that, the more massive a star is the faster it uses up its nuclear fuel (mostly hydrogen); therefore the sooner it "dies".

If we happen to have a binary star system, and the more massive of the two stars explodes as a supernova and it leaves behind a neutron star or a black hole, then it will result in a binary star system with a normal star and a compact object orbiting each other. All these things working out is rare, but there are over a billion stars in the galaxy, so even rare things happen fairly often.

Now, imagine that the "normal star" then runs out of its fuel. The first thing it will do is expand as it enters its "red giant phase", as our Sun will about 4,000,000,000 years from now. Then, some of the star's outer atmosphere will spill over onto the black hole. It will eventually fall in, and in the process become very hot. We can observe this hot gas with X-ray telescopes, so we call this an X-ray binary.

As far as the significance of the X-ray emission, it is to let us observe the effects of the black hole, and therefore learn something about it. Black holes do not emit light, in fact they are so dense that they trap it. Therefore, the best way to learn about them is by observing the material they affect. Observing X-rays from an X-ray binary is one effective way of doing this.

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