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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
effect. Observing X-rays from an X-ray binary is one effective way of doing
this.
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