COCKE: It pretty well convinced people and convinced us, too, that pulsars were
indeed not white dwarfs but rather neutron stars. And then finally—I
think maybe a month later—the discovery of the radio pulses from
the general direction of the Crab Nebula was announced.
DISNEY: What was more, they discovered that this pulsar was pulsing about 30 times
a second. So, it was even faster than the one they discovered in Australia.
COCKE: People were talking about it, wondering what in the world these things
were, and why. And everyone was being rather astounded at the apparent
connection with the Crab Nebula.
DISNEY: And
the reason why everybody was excited was because it looked as if pulsars
were the first actual sight of something which people had been prognosticating
for thirty years—namely neutron stars. That's to say, objects which
are made out of incredibly dense material—so dense that the usual
analogy is that a teaspoonful of it would weigh a billion tons.
COCKE: Well, this was what really pinned it down for us, because what we then
did was, we went to do a rather more thorough study of the Crab Nebula
ourselves to see if we could pinpoint where, within the Nebula, the pulsar
might be.
DISNEY: Radio telescopes don't have very good directional resolution. And, in
fact, the uncertainty was so great that it could have been literally thousands
of stars. So before we could look for a particular place, we got to make
some guess—do some detective work as to where we should look. And,
it seemed the logical place to start off was to look right in the center
of the Nebula. I mean—what's supposed to happen in a supernova explosion
is that once upon a time it was a star, and then the center of it collapsed
to this neutron star and the outside is completely blown off.
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| Crab Nebula; note that the picture alternates between two images made nearly 30 years apart, which illustrates the expansion of the Nebula with time. |
COCKE: As it turns out, at the center of the Crab Nebula, there's a double star.
And Baade's Star is the South Preceding component of that double star,
and precedes it in its motion across the sky as the Earth turns. And we
found that Baade's Star was indeed a very peculiar object in its own right.
It had a continuous spectrum in the optical, and no absorption or emission
lines of any sort, and had been presumed by Baade and by Minkowski to
be the core left over from the supernova explosion that produced the Crab
Nebula. So, what we did was—we said, "Well, why don't we make
some observations of Baade's Star itself, to see whether or not it might
be pulsing at the same period that the radio pulsar had been discovered
to pulse at."
I remember asking a couple of very well known, very prominent astrophysicists, asking them whether or not they felt that the pulsar would ever be detected optically. And they all were very, very negative about it, and they said, "Oh, no, I doubt very much that will ever happen."