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lapse to the state of a black hole, 
many astronomers have argued that 
black holes are present at the centers 
of active galactic nuclei. A central, 
massive black hole could then power 
these galactic nuclei through the ac- 
cretion of material from the space be- 
tween the member stars of the parent 
galaxy. 
The basic idea regarding the ac- 
cretion process is that material from 
the galaxy spirals into the central re- 
gion much as water spirals into a drain. 
Some theories concerning the growth 
of black holes picture the material 
spiraling directly into the hole with 
each particle releasing as much as 20 
percent of its mass energy as it dis- 
appears into the hole at nearly the 
speed of light. Other theories of black 
hole accretion picture the material ac- 
cumulating into a disk that rotates 
around the black hole, with matter 
leaking into the hole from the inner 
rim of the disk. In the latter case, 
energy is generated by material falling 
onto the disk as well as by material 
pulled into the black hole. 
Some astronomers believe that all 
spiral galaxies have black holes at 
their centers and that all galaxies are 
active to some degree, depending on 
the rate at which material is pulled 
by gravity into the black holes. Nev- 
ertheless, the inference that the ob- 
servational properties of quasars and 
Seyfert nuclei can be entirely ex- 
plained by a theory involving the pres- 
ence of black holes should not be made 
because active galactic nuclei contain 
some features that are not easily ac- 
counted for in terms of black holes. 
In particular, there are broad emission 
lines in the spectra of these objects 
that can be adequately explained in 
terms of radiation from gas clouds 
moving radially outward at several 
thousand miles per second in a manner 
indicating that the clouds were hurled 
outward in an explosive event. Massive 
black holes do not explode and it is 
difficult to see how an accretion disk 
could explode because it dissipates, 
rather than builds up, energy. 
Perhaps the quasar and Seyfert 
nuclei phenomena are not steady, long- 
term events that occur over periods 
of as much as 100 million years. In- 
stead, they may be recurring explosive 
events. In the latter case, black holes 
cannot be favored over other large 
concentrations of mass merely because 
of their long-term stability. In fact, 
an instability that leads to an explosion 
might be the necessary property of 
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Ancient Carthage, awesome ruins of 
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Dept. NH-2 
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Society Expeditions) 
86 
