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A Nonconforming 
Supernova 
Cassiopeia A has not behaved 
of its type are supposed to 
by Stephen P. Maran 
Cassiopeia A, an expanding nebula 
from a seventeenth-century supernova 
explosion in the Cassiopeia constel- 
lation, is the youngest supernova rem- 
nant in our galaxy. Unlike other su- 
pernovae of historical times, however, 
it was not readily visible from the 
earth when it exploded, an unusual 
circumstance that has prompted much 
theorizing. There are two categories 
of supernovae: type I, produced by 
relatively low-mass stars; and type II, 
produced by high-mass stars. Accord- 
ing to accepted ideas about type II 
supernovae, the category to which the 
Cas A explosion is assigned, either 
a neutron star or a black hole should 
have formed in the remnant. Neither, 
however, has been found, and some 
astronomers believe that the star that 
produced Cas A must have been to- 
tally disrupted by the explosion. 
Cas A is a powerful radio source, 
a fact that caused its discovery in 
1948 with a rather primitive radio tele- 
scope at Cambridge University in Eng- 
land. In 1951, more advanced equip- 
ment determined the precise location 
of the source, enabling astronomers 
in California to pinpoint it on pho- 
tographs made with the 200-inch tele- 
scope at Mount Palomar. In the pho- 
tographs of the location of the radio 
source, they found a very dim nebula 
composed of many small gaseous 
structures. These have since been clas- 
sified into two groups termed knots 
and flocculi, respectively. Subsequent 
study also shows that the two groups 
of structures are moving out from the 
the way supernovae 
center of Cas A at different speeds, 
and that they also differ markedly 
in chemical composition. The knots 
move at the enormous speed of 3,700 
miles per second; the flocculi move 
at only one-twentieth of that velocity. 
The knots are going so fast that they 
can actually be seen to move on tele- 
scopic photographs made just a few 
years apart. When the motions of the 
knots are traced backward on the pho- 
tographs, you can deduce when and 
where the supernova explosion that 
produced them must have occurred. 
Estimates for the date range from a.d. 
1657 to 1671. Spectra also show the 
chemical differences between the two 
groups. The flocculi have abnormally 
high ratios of nitrogen to hydrogen. 
By contrast, the spectra of the knots 
show no hydrogen or nitrogen, lack 
prominent emissions of iron, and re- 
veal the presence of much oxygen. 
If astronomers had reported seeing 
the Cas A supernova when it exploded, 
we would, of course, know its correct 
date, but they did not. Calculations 
based on the properties of other su- 
pernovae indicate that the Cas A event 
should have been easily visible. Es- 
timates of its brightness differ, but 
range from an apparent magnitude of 
-2.5, brighter than any star of the 
night sky, to magnitude +2, the 
brightness of the North Star. At any 
magnitude in this range, the supernova 
would have been visible without dif- 
ficulty to the unaided eye. Another 
supernova, called Tycho’s star, was 
seen in the constellation Cassiopeia 
H 
78 
