Date: January 8, 2009
Podcaster: Stuart Lowe
Description: In 1967 a young postgraduate student – Jocelyn Bell – spotted an annoying bit of scruff in data from her radio telescope. After careful investigation, Bell and her supervisor established that the signal was coming from over 200 light years away. Initially a mystery, that observation marked the discovery of the first pulsating neutron star. Today, on the 5th anniversary of the discovery of the first “double pulsar”, we talk to astronomers to find out what pulsars are, what they have told us and how they might be used to measure ripples in the fabric of space-time.
Organization: The Jodcast from Jodrell Bank Centre for Astrophysics
Bio: Stuart Lowe works on instrumentation for the European Space Agency’s upcoming Planck spacecraft. In his spare time, Stuart co-creates an astronomy podcast – the Jodcast – along with other astronomers at The University of Manchester’s Jodrell Bank Centre for Astrophysics.
Today’s Sponsor: American Astronomical Society (AAS)
In 1967 the young University of Cambridge postgraduate student, Jocelyn Bell, was looking at chart recordings of a patch of sky. She was looking for radio emission from quasi-stellar objects but her eye was caught by what she later called an annoying bit of scruff. When she looked more closely she saw that the scruff was actually a signal repeating every 1.337 seconds. After careful investigation, Bell and her supervisor Anthony Hewish had established that the repeating signal was coming from outside the solar system at a distance of over 200 light years. Here’s Professor Jocelyn Bell-Burnell…
Prof Jocelyn Bell:
At the time we were having trouble making sense of this. Radio Astronomers are aware, at the back of their minds, that if there are other civilizations out there in space, it may be the radio astronomers that first pick up the signal. [It's] not obvious why they’d communicate at 80MHz – which frequency we were at – but just possibly, may be it’s another civilization. Not a very intelligent civilization if it is because the frequency of the repetition rate of the signal is dead constant. So if they’re sending us a signal it’s in the amplitude of the pulses and that’s a daft way to communicate across space. So it didn’t make total sense but, you know, just faintly, possibly. And this is where the nickname Little Green Men came from. So we nicknamed that source “Little Green Man”. It subsequently became little green men number 1 when I found numbers 2, 3 and 4.
So if it wasn’t little green men, what could cause a repeating signal from outside the solar system? Dr Tim O’Brien of Jodrell Bank Centre for Astrophysics…
Dr Tim O’Brien:
Well there were a number of suggestions. One was that it could be some sort of binary star system and as the stars orbited one another like eclipses or some sort of interaction between the stars was causing the pulsing. Another was that it was a pulsating star. So the star was sort of growing and shrinking and that was causing the change in brightness. Another was that it was some sort of rotating star so there might be a bright spot of radio emission on the surface of the star and as the star spun that was what caused the flashing. Now the real problem for any of these interpretations was the speed of the pulsations. And for the rotating star model the star would have had to be very small indeed in order for it to rotate fast enough for it to explain the high speed of the pulsations that were being observed.
One candidate was an idea that had been first proposed in the 1930s by two astronomers – Fritz Zwicky and Walter Baade. This was something called a neutron star; an object that they predicted would be formed at the end of the life of a massive star when it ran out of nuclear fuel at the core, the core would collapse in on itself under its own weight. The electrons and protons that were in the core of the star would be crushed together to form neutrons. And they predicted the existence of this incredibly dense, compact object that may be only about the size of a city – 20 km across – but weighing as much as 1.4 times the mass of the Sun.
It’s got a very strong magnetic field. That magnetic field isn’t aligned with the spin axis of the star. So much like with the Earth where the north magnetic pole is not in the same place as the geographical north pole – it’s offset to one side. As the star spins the magnetic poles circulate around and around the star. And it seemed that these magnetic poles were producing the radio waves in two beams of radio light that were shooting out from the magnetic poles in opposite directions. Imagine now as the star spins, those beams sweep around the sky, sweep through the galaxy much like a giant, cosmic lighthouse and as these beams sweep past the Earth you see a flash, flash, flash, flash as the star spins.
The crucial observation that supported this idea was the discovery of a pulsar in the heart of the Crab Nebula.
Dr Tim O’Brien:
The Crab Nebula is a supernova that exploded in 1054 and seen by Chinese astronomers amongst others. Now we see this very bright nebula with a rather unusual star very close to the core. It was discovered that the star itself was indeed a pulsar.
We can turn it into a sound and rather than look at the signal on a chart recorder as Jocelyn Bell would have done in the 1960s, we can actually listen to that sound through loud speakers.
<sound of Crab pulsar>
That was the drill like sound of the Crab pulsar. Since the announcement of the discovery of the first four pulsars in the journal Nature on 24 February 1968, many more pulsars have been found. Although many of those have periods similar to that first discovery, some are quite different. Tim O’Brien…
Dr Tim O’Brien:
The Crab pulsar at the heart of the Crab supernova remnant we know is just less than a 1000 years old and, in fact, its period is about 33 ms; it spins around 30 times a second. So the idea is that pulsars are actually born as fast rotaters, they’re spinning quite fast, and they radiate away energy and slow down over time.
As they slow down, they gradually lose energy and eventually they switch off so they’re no longer visible as pulsars. The longest period pulsar we know is around 8 seconds. In fact we started to see old pulsars with very short periods; the so-called millisecond pulsars. These have periods of only a few milliseconds. The fastest of these in fact spins at around 716 times a second; a period of 1.4 milliseconds. If we turn the signal we receive from one of these millisecond pulsars into a sound then it’s actually quite painful to listen to.
<sound of pulsar B1937>
That was the rather ear-piercing sound of pulsar B1937 So how can a pulsar end up spinning so fast that the surface of the star is spinning at an appreciable fraction of the speed of light?
Dr Tim O’Brien:
Well, its that we believe that they are actually in binary systems and as material from the companion star is transferred onto the pulsar, it can spin it to these incredibly high speeds.
Five years ago today saw the announcement of the discovery of a very special binary system; one with two pulsars orbiting each other. Here’s Professor Michael Kramer of Jodrell Bank Centre for Astrophysics.
Prof Michael Kramer:
The double pulsar is an amazing system. We discovered it in 2003 and it’s the only system where we have two radio pulsars orbiting each other which is quite unusual. We’ve never had this before and the orbit is very quick – only 145 minutes – with velocities exceeding 1 million kilometres per hour. It allows us to test predicitions of Einstein’s theory of gravity and it’s the best test we have of General Relativity so far in strong gravitational fields.
We see the same effects that we observe in the solar system with Mercury that the orbit of Mercury is slowly changing its orientation in space. We see this in a much much faster rate in the double pulsar. So for instance, for Mercury it takes 3 million years for the orbit to be in the same orientation again. For the double pulsar it only takes 20 years and that just indicates how relativistic that system is compared to what we can do in the solar system.
As well as continuing to study the objects we already know about, astronomers are now looking for more extreme examples.
Prof Michael Kramer:
So the next holy grail as we call it is of course the discovery of a pulsar orbiting a black hole. That would be the next superb step towards testing GR in extreme conditions. So by using a pulsar around a black hole we can use it as a probe that measures out the orbit and hence the properties of black holes as they are predicted by GR or other theories.
So what does the future hold? Astronomers are now starting to use observations of many pulsars to look for ripples in the fabric of space-time called gravitational waves. Here’s Dr George Hobbs of the Australia Telescope National Facility.
Dr George Hobbs:
The aim is to detect gravitational waves; these are waves predicted by the general theory of relativity. As big masses like big black holes move in the universe they emit these gravitational waves, the gravitational waves come through space, and they affect the signal that we receive from various pulsars. So a timing array project is to observe a large number of pulsars all around the sky and to use the signals that we receive from those pulsars to try and detect these graviatational waves.
So somehow we’ll need to combine data from the Parkes telescopes with the various European telescopes and the North American telescopes to have enough pulsars where we can do that. On the longer term we want to really study this background and understand the gravitational waves and where they’re coming from and what are their properties and to do that we’ll probably need hundreds of pulsars. For that we’ll need a completely new telescope. The Square Kilometre Array which is this telescope that might be built in 20 years or so will be an absolutely ideal telescope for really studying the gravitational wave signals.
Over the years since their discovery international teams of researchers have found around 1800 pulsars. These have only been the ones that are bright enough to detect and that were pointed towards us so there may be 100,000 pulsars in our galaxy alone. As we’ve heard, pulsars are unique and invaluable tools that allow us to test our basic understanding of fundamental physical laws. They look set to do so for many years to come. Jocelyn Bell…
Prof Jocelyn Bell:
It’s still a very exciting, dynamic, rapidly changing field. You’d think after 40 years it would’ve settled into an interesting but mature phase. Blow me, is it heck. It’s fascinating.
365 Days of Astronomy
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