Jocelyn Bell Burnell: Surveying the radio sky

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 - You joined the laboratory of Anthony Hewish. He was your supervisor, wasn't he? So what problem was he trying to solve and what challenge did he set you for your PhD?

Jocelyn - There had been a grad student called Margaret Clark, who had left just weeks before I arrived. Her particular project had been surveying the radio to sky, looking at what objects were giving out radio waves. And she noticed that some of the objects that gave out radio waves, the signal wasn't steady, it fluctuated, it changed brightness. She noted that these were the objects that they thought were extremely compact objects. My thesis advisor, Tony Hewish, took over this idea and thought maybe if we look for fluctuating radio sources, they'll turn out to be the very compact ones, which at that stage were suspected all to be quasars. There weren't many known at that stage. It was a really hot topic. So finding more quasars was my particular project.

Chris - And what is a quasar?

Jocelyn - We didn't know what it was at that stage. We now know it's got a massive black hole in its middle, maybe several hundred million times the mass of the sun. And some rather more normal material around the black hole. Probably in the process of falling into the black hole. A net result is these objects are very strong radio sources, often quite bright in the optical as well, but they're known as quasars, which is an abbreviation for 'quasi stellar radio source', hence Quasar. There was still considerable uncertainty about what quasars actually were. We now know that they're the same kind of mass as a galaxy, a hundred thousand million stars, much more compact and with a massive black hole at their centre, which drives lots of things.

Chris - How were you trying to find them?

Jocelyn - It's a rather peculiar radio technique. Because they are extremely compact and because we're viewing them through the gubbins that are in space between the Sun and the planets, it's called the solar wind, the solar wind means that the signal from these compact objects will fluctuate, twinkle. So basically I'm looking for twinkling radio sources, which probably are quasars.

Chris - And how do you actually find them? What's the antenna you have to erect in order to detect these signals?

Jocelyn - You do it at quite low radio frequencies. The wavelength was about 12 foot, which is quite a big wavelength. You're looking for a twinkling, so you can't take long exposures, which means you need a very, very big telescope to pick up lots of signals because you can't average the signal to noise.

Chris - Right. Okay. And what's making that radiation? Where are those signals coming from?

Jocelyn - We now know that quasars have massive black holes in their centres and it's stuff around the black hole that generates the primary radio signal. Then as it travels through the space between the sun and the planets to reach us, the solar wind, that gas blowing out from the sun, affects it and alters its strength. So it fluctuates. So I'm looking for a fluctuating, randomly fluctuating radio signal, which means there's a very compact object out there and it must be pretty strong if I can see it.

Chris - You therefore start to get this data having got the detection system working, when did it become apparent to you that there might be something interesting to see?

Jocelyn - I actually spent the first two years sledge hammering. I built the equipment along with about half a dozen other people and it took us two years.

Chris - With a sledgehammer?

Jocelyn - Oh yeah, yeah, yeah. I literally did a lot of sledgehammering. I could swing a sledgehammer. I was playing field hockey at the time and I could hit the hockey ball from one end of the pitch to the other, which my teammates never learned to, you know, start running <laugh>

Chris - Did suddenly everyone on the hockey team want to be a radio astronomer because clearly this is the path to greatness on the hockey pitch.

Jocelyn - It was useful on the hockey pitch, particularly if you're playing full back because it shifts the ball to the far end of the pitch and you and the other full back can stop and have a chat and watch the others doing things at the far end of the <laugh>.

Chris - But what were you actually sledgehammering?

Jocelyn - We were building a radio telescope and it had to be a very large area radio telescope because we couldn't do these long exposures. So if you're operating with short exposures, you need a large area.

Chris - Were you thinking then flipping heck, I've done two years of this, a PhD's only three years long. Were you getting a bit nervous then thinking at what point am I actually going to get some data to work with?

Jocelyn - I think I'd been told it would probably take two years, but I did feel it was rather a lot of manual labour for an academic qualification <laugh>.

Chris - Because the thing is you can't publish as a thesis building a radio telescope because that's not advancing knowledge. So you must have been a bit nervous thinking, well when am I going to get something I can actually publish.

Jocelyn - I don't remember being nervous about that. What was really good was the first time we switched it on, it worked and it's probably the first radio telescope in history to work the first time you switch it on. Normally you switch it on and something's not working, so there's nothing coming in <laugh>.

Chris - What was the readout? What was the display then? How were you showing yourself what it was seeing, this telescope?

Jocelyn - We had pen chart recorders. You know, rolls of paper that rolled under a pen. And the pen did this continuous squiggle along this rolling sheet of paper and I ended up with several miles of paper after six months of observing.

Chris - And is that one of those pictures that Joy Division used as their album cover? Is that one of those charts?

Jocelyn - It's snippets from several charts taken and lined up.

Chris - And so when did you, then you say it worked straight away. Well how did you know it was working, but when did you then say, 'oh look, there are some interesting signals in here. I am seeing these twinkling phenomena. I was going after'

Jocelyn - The purpose of the exercise was to find more of these quasi stellar radio sources and they became very clear as soon as I got the thing on and running. So that was my main thesis job. And in fact, we couldn't, as Tony said, change the title of the thesis. So the majority of the thesis was on these quasars and how they twinkled. The twinkling is actually caused by plasma blowing out from the sun, solar wind it's called. And it's not smooth. It's a wind with clouds in it and the clouds disrupt the radio signal from the distant quasar and make the quasar appear to change brightness. It's actually not itself changing in brightness. It's because of the mess made by this solar wind that you pick up a fluctuating signal. Whereas other kinds of radio objects in the sky are much more extended and you see them through several different bits of the solar wind at the same time. And so the solar wind effect averages out. So it's a neat technique for picking up compact objects like quasars.

Chris - It's a bit like when you've got the fog lights on on the car, the red you can see through the fog, it doesn't scatter much. But the white light gets scattered all over the place. He's an Irishman, Tyndall. John Tyndall discovered the Tyndall effect.

Jocelyn - That's right. Yes. John Tyndall was Irish. Yes. In the days when we didn't distinguish between North and South Ireland. <laugh>.