figure 1

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.

expanding crab nebula
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."

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