Jocelyn Bell Burnell: How to find a pulsar

In this edition of Titans of Science, Chris Smith sits down with Jocelyn Bell Burnell, the astrophysicist who's work detecting cosmic radio waves helped discover the existence of pulsars...

Chris - The thing that you are best known for is what you labeled on one of those charts. You had miles of that. You wrote LGM, Little Green Men, as a joke. I mean, it was a joke, wasn't it? But what was that and why did that stand out to you, that signal?

Jocelyn - Once you've used the radio telescope for several weeks and seen all the kinds of phenomena it picks up, you get to say, yep, that's a quasar, that's radio interference, that's somebody with a badly suppressed car or a pirate radio station or something like that. There was on occasion another signal that didn't fit in either of those categories. It just looked different. You can explain it in terms of Fourier analysis, but we won't go there I think in this <laugh>. It looked different. And the first few times I logged it with a question mark and carried on because I've got miles of this paper to plough through. And by the third or fourth time I saw it, my brain said, 'you've seen something like this before, haven't you? You've seen something like this before from this bit of sky, haven't you?' And then it's easy because I'm storing the rolls of paper according to which latitude on the sky I'm observing, in shoe boxes. So I found the shoebox that covers that latitude strip. Now this is the one I've just seen, wasn't there last time we looked at that bit of sky, but the two, maybe three times, before it was there and it's from the same bit of sky. What's going on? You probably know, you see different constellations in the summer and the winter sky. But this phenomenon wasn't changing with the date. It was keeping its place amongst the constellations apparently. And I showed this to my thesis advisor and he was quite convinced I'd done something wrong. So there was a famous occasion where he came out to the observatory at the time this thing was due to be passing overhead and watched me set up for the observations and saw the pulses coming in himself and he began to take it more seriously than <laugh>.

Chris - What did they look like? What were those pulses and why were they leaping out at you saying 'this is not car interference. There's something a bit different here.'

Jocelyn - The thing was going blip, blip, blip, blip. Now quasars do a certain amount of blip, but they go blippety blipblip, blipblipblip.

Chris - That's the twinkling, isn't it? That's the variation you mentioned.

Jocelyn - Yes, that's right. Whereas this thing had a steady beat and it really looked manmade, it looked artificial. And we put a lot of effort into trying to explain it as something artificial, but it moved round the sky with the stars and this thing, whatever it was, was keeping its place amongst the stars and getting four minutes earlier each day.

Chris - Did you think for a second, I've found aliens?

Jocelyn - It was unlikely to be aliens and that idea disappeared as soon as I found a second and then a third and a fourth all in different parts of the sky.

Chris - Ah, so that would argue that whatever it was that was doing it where you first saw it, that phenomenon was obviously being repeated across the sky and other patches of the universe. And it was then a question of, so what's causing it?

Jocelyn - Yeah. And the interesting thing was the subsequent ones that I found were similar but pulsed at a different rate. Each one had its own rate. One of them had a pulse rate of a quarter second. It was really zinging along <laugh>, it was quite hard to catch it, it was going so fast. The other three were all one point something seconds apart. So it was going blip, blip, blip at a reasonable rate.

Chris - You knew then once you started seeing these things coming from elsewhere in the sky and also tracking with the stars in that way, you knew you had to be onto something. So how did you pursue that then? Because you were in your PhD, there's only a limited amount of time and resources in a PhD. So what did you decide to do?

Jocelyn - Well, most of the effort went into establishing that this was real because they were so extraordinary. We couldn't publish and then discover, 'oh, we forgot to check such and such and that's what's causing it.' So we had to make sure we didn't fall into that trap. So the first job was to ask a colleague and his graduate student, they also had a separate radio telescope and separate receiver, on the same site but separate kit, to ask them very quietly, 'could you take a look at this particular spot of the sky please and tell us what you see?' And I vividly remember that particular test. We were working with a grad student called Robin Collins. My telescope saw the thing shortly before Robin's telescope was due to see it, and mine showed it was pulsing nicely. And then we went and stood by Robin's chart recorder and nothing happened.

Chris - So then what?

Jocelyn - Well, the two academics started walking down this very long laboratory discussing 'now what could it be that shows in our radio telescope not yours? Could it be No, it can't be that because you're right. What about no, it can't be that.' And it's quite a long laboratory and I was padding along behind them. Robin stayed by his pen recorder and we got down the end of this long laboratory and suddenly there's a shriek from Robin 'here it is!' And we went charging back up. Robin had miscalculated by five minutes when his telescope would see it. I mean thank goodness it wasn't 25 minutes or we'd all have gone home <laugh>. But that was fantastic. It's not some fiction of my equipment. It's being seen by a separate radio telescope with a separate amplifier and a separate pen recorder, a totally separate system. It's something way beyond the observatory, whatever it is.

Chris - What did we have in our kind of repertoire of understanding of physics astrophysics at that time that could account for that? Or was it a complete blank sheet?

Jocelyn - It had to be something physically very small because it's pulsing quite fast, responding quite fast, but it's also quite strong. Whatever it is, it's got a sizable radio signal. So it's small and it's big. Doesn't quite make sense. We were really very puzzled as to what it was.

Chris - If you've got something small, but it's powerful. That sort of star regime. Would that put it into the neutron star kind of regime then? Because you're talking about something incredibly dense, incredibly massive, lots of material in there, but very compact.

Jocelyn - Yes, although we didn't see at that stage how a neutron star could produce blasts of radio waves or beams of radio waves. It has to be a highly magnetised neutron star spinning quite fast with its magnetic axis offset from the spin axis. So you get a beam coming out of the magnetic poles, which then swings around as the star spins.

Chris - Well, what's wrong with that? Why can't that be the case?

Jocelyn - Well, nobody had ever seen anything like that before.

Chris - They'd never seen LGM either.

Jocelyn - Well, some people probably thought they had seen LGM, but nevermind that <laugh>.

Chris - I know what you mean. But no one had seen what you saw. And so what was wrong with saying, well, this is what we think it is?

Jocelyn - I guess we were still trying to make sure we believed this. An important stage was when I found a second and then a third and a fourth and the fourth one pulsed much faster. But astronomical objects are big things. How can they go bung, bang, bung, bung, bung, bung, bang at that rate? It doesn't make sense. But we had to publish. You couldn't sit on the result any longer, <laugh>.

Chris - What was the reaction like? So you put this out there and you say, 'we've got this, we've found more examples of it.' Did you speculate in that paper as to what it might be, or did you just leave it as a blank canvas?

Jocelyn - I can't honestly remember without going back. We published the first one first and said we've got some more, so this is not alone. And then the other three followed in another paper a few weeks later. People were quite cross that we wouldn't give them the details of numbers two, three, and four right away because they wanted to follow up the observations. But the reaction to the first paper was phenomenal. There are umpteen stories of senior radio astronomers saying to somebody, 'I know you're scheduled on the telescope tonight. You're not, we are having the telescope to go and observe this fantastic new thing.' So a lot of people were jumping on the bandwagon very quickly, very interested.

Chris - What did it unlock and how were you able to then pursue that observation to then solve the problem?

Jocelyn - I had another three of these that I had discovered. I found the first four and we published numbers two, three, and four in a subsequent paper. And the fourth one was a good deal faster. So that's obviously going to constrain what kind of object it was. There were various ideas floating around about what they might be. My supervisor Tony Hewish had one. When he gave the colloquium announcing that discovery in Cambridge, Fred Hoyle was in the audience. Tony thought it was a type of star called a white dwarf, which was probably oscillating, breathing in and out and launching shockwaves into the atmosphere with each breath. When he'd finished his colloquium, Fred Hoyle was the first person to speak and in his best Yorkshire accent, 'I don't think it's an oscillating white dwarf. I think it's a neutron star.' Neutron stars being good deal smaller, and at that stage talked about by some mad theoreticians, but nobody really believed in them.

Chris - No one believed neutron stars were real at that time?

Jocelyn - They were really rather extreme. But this was the proof that they existed.

Chris - Well, that argues, well they exist. But how did you explain the phenomenon that put you onto them in the first place, which was this pulsation? How did you then reconcile the fact we've got this giant ball of neutrons, very compact, very small, very energetic, but it's pulsing. How does that hang together?

Jocelyn - The pulsing is an observational effect. It's a bit like a lighthouse. The lighthouse beam is steady, but it swings around the sky and you only see it when it's shining in your face. So you see pulse, pulse, pulse, pulse. And it's the same with these neutron stars, or pulsars as they're now called. There's beams coming out of the north and south poles. And as this swings around, um, the beam may shine on your radio telescope or it may miss Earth totally. But if it shines on your radio telescope, you see pulse, pulse, pulse, each revolution.

Chris - You were lucky it shone on your telescope. What was the reason that the pulse swings round though? Is that magnetic,

Jocelyn - The pulse is shaped by the star's magnetic field and the star is spinning. So you can think of a beam coming out of the north and south magnetic poles of this star, a bit like a lighthouse beam. And if the magnetic axis is offset from the spin axis like it is on Earth, magnetic north is not at true north. Similarly for these stars, the beam is coming out of the magnetic north. The star is spinning around true north. And so the beam cones around the sky, and if Earth happens to lie on that cone, you get a pulse.

Chris - And my mind is boggling at the idea of one of these things spinning around four times a second, which is what you spotted in one of the examples, isn't it? How does it do that?

Jocelyn - Yeah, actually they go much faster than that. That's by no means the fastest. You can certainly get 30 times a second. They are very compact stars. They're only about 10 miles across, but they weigh the same as the Sun, more than the Sun <laugh>.

Chris - Do any of them have planets?

Jocelyn - We suspect that at least one does. Yes. Planets marginally upset the spin of the star. And so we've got such fantastic accuracy with these pulsars that we can see slight wobbles in the rotation rate, which are probably due to planets. I don't think we could ever prove that, but that seems the most likely explanation for a bit of a wobble in the pulse period.