»k y Reporter 
in 1572, but no “new star” was found 
in that constellation during the period 
from 1657 to 1671 nor, indeed, has 
one been found at any time since. 
That Cas A exploded without a 
“new star” being seen presents an 
enigma that has interested many sci- 
entists. According to one often dis- 
cussed theory — proposed long ago by 
another scientist, but revived and re- 
vised recently by Roger Chevalier, an 
astrophysicist at Kitt Peak National 
Observatory in Tucson, Arizona — the 
exploding star was much smaller than 
usual for a type II supernova because 
it somehow shed its voluminous outer 
layer before the explosion occurred. 
This would make the supernova about 
7 magnitudes dimmer than previously 
calculated, with an apparent magni- 
tude of +4.5 at the brightest. A star 
- of that magnitude is visible although 
not conspicuous to the naked eye and 
thus could have been overlooked. 
A second explanation was advanced 
by Iosif Shklovsky, a world authority 
on supernovae at the Space Research 
Institute in Moscow. He believes (as 
I do) that the light from a type II 
supernova is not produced just by 
the explosion. Instead, the supernova 
luminosity may be largely powered by 
energy from a newborn pulsar formed 
when the star collapses and explodes. 
Thus, the size of the presupernova 
star would be less important in de- 
termining the supernova brightness 
than the properties of the new pulsar. 
A pulsar is a highly magnetized, 
rapidly spinning neutron star, about 
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