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The Question
(Submitted April 25, 1997)
I am a senior in High School
and was wondering if you could take a couple minutes from your very busy day to
answer some questions concerning black holes for my Astronomy class.
1. What exactly happens to the material absorbed into black holes ?
2. Wouldn't the black hole finally fill up ?
3. Can life be maintained within a black hole ?
4. Can you see the back part of a black hole ?
The Answer
1. You probably have heard about Einstein's famous equation, E =
mc2, which
gives the energy associated with material of a given mass m. When material
falls into black hole, a process called accretion, usually about 10% of the
mc2
energy gets radiated away as the material approaches the black hole. The other
90% gets absorbed into the blackhole and simply adds to its mass. In some
cases, the material won't have a chance to radiate much energy and essentially
all of the mass goes right into the blackhole.
2. Actually, a black hole is already essentially a geometric point, with
effectively infinite density. There is no inherent limit to the mass of a
black hole. There is a region around black holes called the event
horizon. Once anything, including light, crosses the event horizon, it can
never escape. This is what gives the black hole its name. The size of the
event horizon gets bigger as the black hole gets more massive. This allows the
black hole to "grow", in a sense, as more mass falls in. There is
very strong
evidence that some galaxies have black holes as massive as a billion Suns at
their centers (one example is the Sombrero galaxy... you can see a picture of
this galaxy at
http://lheawww.gsfc.nasa.gov/users/ptak/agn/liner_llagn.html.
3. No, anything that falls into a black hole will get heated to very high
temperatures (this is how the 10% of the energy gets radiated away... the
material gets very hot, in a process similar to how meteors and space debris
burn up as they enter the Earth's atmosphere). Also, once the
material gets very close to the blackhole, tidal forces will stretch it very
thin (just think about the effect that a Moon has on the Earth's oceans, and a
typical blackhole is likely to be much more massive than the Moon).
4. By definition, you can't see a black hole at all... again not even light can
escape from within the event horizon. Interestingly, though, black holes warp
space so much that if you could orbit a black hole close to the event horizon,
you could see the back of your own head... light reflecting from the back of
your head would get bent around the black hole so that you could see it. You can
see some movies that demonstrate this and similar effects for neutron stars at:
http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html
Andy Ptak
for the Ask an Astrophysicist team
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