Slide 8 of 13
Luckily, black holes are rarely formed in complete isolation. There is almost always
other matter around. For example, binary star systems, which contain two stars
in close orbit, are very common. This slide is an artist's rendition of what it would
look like of one of the stars in a binary system collapsed into a black hole
The intense gravitational field of the black hole sucks matter off of the
companion star. The matter does not fall directly into the black hole. It
swirls around and spirals in, much like water down a bathtub drain. As this matter
fell towards the black hole it gains energy, and heats up to the point where
it emits a great deal of radiation (x-rays in fact). This radiation is emitted while
the matter is still relatively far from the black hole, so it can escape and this is
what we detect. The evidence is somewhat circumstantial, since the same
sorts of x-rays would be emitted even if the collapsed star was
some other compact object such as a neutron star or white dwarf. However,
if we can measure the masses of the two stars, and the collapsed star is heavy enough,
theoretical arguments force us conclude that the x-rays are being emitted by matter falling in to a black hole.
