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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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