IF
BLACK HOLES ARE BLACK, HOW CAN WE SEE THEM?
If you were to take any star in the sky, and squash
it into a small enough space, you would get a black
hole. Once you get close enough to a black hole, the
gravity is so strong that nothing, not even light, can
escape. This point of no return is called the event
horizon; for our Sun the event horizon is about 2 miles,
some 460,000 miles below its surface. We know of two
kinds of black hole. One kind is millions of times heavier
than the Sun, and is found snacking on stars at the
heart of many galaxies, while the other kind is "only"
about 10-100 times heavier than the Sun and is formed
by the supernova death of the very heaviest stars.
Although we can’t see black holes themselves,
we can see their effects on surrounding objects. Most
stars exist in pairs, so if one turns into a black hole
it can feed off the other one: the gravity of the black
hole pulls gas from the companion, forming an accretion
disc of material that spirals into the black hole, feeding
the monster (see picture on page 2). X-ray binaries
with black holes in them can be many times more powerful
than ones with neutron stars because the black holes
are 10-100 times heavier: they can be up to 50 million
times more powerful than the Sun! The giant black holes
at the centres of galaxies perform the same trick, swallowing
nearby gases and stars. In galaxies like ours, the beast
is docile, but in others, the black holes are VERY active,
spewing out great jets of material at a respectable
fraction of the speed of light.
|
