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i 8 Indian Neck Lane, Peconic, N Y. 1 1958 
50 percent more massive than the sun, 
yet no larger than fifteen miles in di- 
ameter. In the immediate aftermath 
of its birth, a pulsar may shed energy 
at a fantastic rate, sufficient to propel 
and illuminate the surrounding nebula 
of the supernova. From the fact that 
no bright star was seen when Cas A 
exploded, Shklovsky deduced that no 
pulsar formed. If this conclusion is 
correct, the possibility remains that 
a so-called bare neutron star — one 
lacking the high magnetic field and 
fast spin of a pulsar — was formed. 
In that case, the supernova would be 
dim, but the bare neutron star, starting 
with an initial surface temperature of 
millions of degrees Celsius and cooling 
very slowly over the centuries, would 
still be observable today as a compact 
X-ray source at the center of Cas A. 
Since no compact X-ray source ex- 
ists in Cas A, Shklovsky concluded 
that not even a bare neutron star was 
left from the supernova. This position 
was recently reinforced by an X-ray 
image of Cas A, made with unprec- 
edented sensitivity by the Einstein sat- 
ellite, which confirms that there is 
no hint of a compact X-ray source 
at the center of Cas A. This leaves 
two likely possibilities concerning the 
Cas A explosion: either the presuper- 
nova was totally disrupted or the core 
of the presupernova collapsed to form 
a black hole. 
Since there is no sure way of di- 
rectly observing a black hole, we have 
to resort to indirect evidence that may 
bear on the question of whether a 
black hole formed in the supernova. 
In particular, the chemical composi- 
tion of the knots may be relevant. 
Nuclear reactions inside a star con- 
vert the gas there, which consists 
mostly of hydrogen and helium, to 
successively heavier elements. By the 
time a massive star has reached the 
developmental stage of a type II su- 
pernova, its central core should consist 
mostly of iron. The high speeds of 
the outward-flying knots in Cas A in- 
dicate that they must be fragments 
of the exploded star. Thus, if the core 
was disrupted along with any overlying 
layers, the knots should contain no- 
table amounts of iron. As mentioned 
previously, however, the spectra of the 
knots lack the strong emissions of iron 
that would then be expected. Because 
of this circumstance, Shklovsky de- 
cided that the core of the presuper- 
nova star did not shatter and that since 
it did not form a neutron star, it must 
have become a black hole. Although 
this argument seems logical, it has 
not been generally accepted by as- 
tronomers. Some, for example, believe 
that the composition of the knots can 
be accounted for by nuclear trans- 
mutations during the supernova ex- 
plosion itself. 
I don’t know how bright the su- 
pernova would have been if a black 
hole had formed, as theorized by 
Shklovsky. However, Chevalier’s pro- 
posal that the supernova explosion that 
produced Cas A was smaller than 
usual at least predicts the supernova 
might have been visible to the naked 
eye as a dim star. 
An astronomer active at the time 
of the explosion and therefore in a 
position to record it was John Flam- 
steed, England’s first astronomer 
royal. He systematically tabulated the 
stars visible to the naked eye during 
the latter part of the seventeenth cen- 
tury and issued a catalog of them. 
Even today, many stars continue to 
be known by their “Flamsteed num- 
bers,” as assigned in his catalog. On 
August 16, 1680, Flamsteed observed 
what he later described as a sixth- 
magnitude star that he named 3 Cas- 
siopeiae. There is no indication that 
he believed the object to be at all 
different from the many other stars 
that he was surveying. By means of 
a sextant, he determined the angular 
separations of the object from each 
of two bright stars: Scheat, in the con- 
stellation Pegasus, and Algol, the fam- 
ous Demon star in Perseus. From these 
measurements, he later calculated the 
position of 3 Cassiopeiae. 
As far as is known, neither Flam- 
steed nor anyone else ever saw 3 
Cassiopeiae again. There is no star 
at the position he posited, so at various 
times astronomy historians have con- 
cluded that Flamsteed’s August 1680 
observations were erroneous. But three 
scholars within the past two years have 
pointed out independently that Flam- 
steed’s derived position for 3 Cassio- 
peiae is very near the deduced ex- 
pansion center of the knots in Cas 
A, suggesting the possibility that the 
dim star he observed may have been 
the supernova. In that case, the 
supernova would have exploded in 
1680 rather than in the deduced 1657 
to 1671 period. This discrepancy is 
not surprising, given the uncertainties 
in measuring the motions of the Cas 
A knots. In addition, there is the pos- 
sibility that the expansion has slowed 
down since the explosion occurred, 
which would lead us to deduce an 
80 
