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Seyfert Galaxies 
These peculiar objects resemble quasars in many 
respects, and they are equally puzzling 
by Eugene R. Capriotti 
Next to the sun, the moon, and Sat- 
urn, the spiral galaxies, because of 
their distinctive structure, are the 
most easily recognizable of astronomi- 
cal objects. Billions of these flat pin- 
wheels, each containing hundreds of 
billions of stars, are strewn throughout 
the universe. 
The spectrum of a spiral galaxy is 
generally what one might expect for 
an object containing a myriad of in- 
dividual stars. A small percentage of 
spiral galaxies, however, in addition 
to a composite stellar spectrum, have 
spectral features that cannot be pro- 
duced by a conglomeration of isolated 
normal stars. These nonstellar spectral 
features are produced by hot ionized 
gases having a wide range of density 
and excitation. The first spiral galaxy 
with these special features was dis- 
covered in 1943 by Carl Seyfert, then 
working as a postdoctoral fellow at 
the Mount Wilson Observatory in Cal- 
ifornia. Each similar object that has 
subsequently been discovered has 
been called a Seyfert galaxy. The most 
distinctive structural feature of the 
Seyfert galaxies is that they have very 
small, extremely bright central regions 
called nuclei in which their peculiar 
spectra are produced. 
Little attention was paid to the 
nuclei of Seyfert galaxies until after 
Maarten Schmidt, an astronomer 
working at Palomar Mountain in Cali- 
fornia, showed in 1963 that quasars 
have extremely large spectral red 
shifts, indicating that they are reced- 
ing from us at speeds approaching the 
speed of light. Interest in quasars, on 
the other hand, has practically domi- 
nated extragalactic astronomy since 
the early 1960s, and it is the resem- 
blance of the nuclei of Seyfert galaxies 
to quasars that has generated a great 
deal of interest in the Seyfert galaxies 
as well. 
What makes the quasars so inter- 
esting is that their very large spectral 
red shifts are probably due to the re- 
cessional velocities associated with the 
general expansion of the universe (see 
“Quasars Confirmed,” Natural His- 
tory, February 1980). If that hypoth- 
esis is so, quasars, as a class, are the 
most distant objects in the universe 
because, as a class, they have the 
greatest spectral red shifts. If the qua- 
sars are very far away, then they are 
generating energy at a rate far in ex- 
cess of the rate of energy production 
of a normal spiral galaxy made up 
of hundreds of billions of stars. 
Quasars are also extremely small 
by galaxy standards. The brightness 
of some of these objects varies greatly 
over periods of time as short as a 
few weeks, consequently their diam- 
eters can be no greater than the dis- 
tance traveled by light in the same 
time. This distance is about 100 times 
smaller than the distance from the 
sun to its nearest neighbor star, which 
is about four light-years away. In com- 
parison, the diameter of our Milky 
Way galaxy is about 100,000 light- 
years. 
The problem with the quasars is to 
understand how relatively compact ob- 
jects generate more power than an en- 
tire galaxy of stars. That is also the 
problem with Seyfert nuclei, which in 
many cases produce more power than 
their parent galaxies. Scientists be- 
lieve that if the question of energy gen- 
eration in the nuclei of Seyfert galax- 
ies were to be solved, the problem of 
energy generation in quasars might be 
solved automatically. Because of their 
similarity to Seyfert nuclei, quasars 
are viewed as being extremely bright. 
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