Of Dead Stars and Cosmic Lighthouses

by Bryan Gaensler

The following article is a popular account of my work which was published in the Sep/Oct 2000 issue of Newton magazine (pp 113-116). This article is ©2000 Bryan Gaensler, and may only be reproduced or distributed for private use unless my explicit permission is given otherwise.

Picture at right: Cassiopeia A is the glowing ashes thrown out by a star whose explosion could have been seen on Earth 320 years ago. It's about 10,000 light years away from Earth and is 10 light years across. Astronomers have long wondered what happened to the star itself. Calculations on the star's original size, based on its remnants, indicate that the collapsed core of the star should still be sitting somewhere near the centre of the remnant - but nothing has been found. That was until the Chandra X-ray Observatory, a satellite sent into orbit last year, trained onto it. The image it recorded shows a tiny bright point near the centre - the missing neutron star may have been found. [Credit: NASA / CXC / SAO]

For tens of thousands of years, humanity has gazed in wonder at the night sky. Some find spirituality and religion in the patterns they see, others seek to understand the reason behind it all. But whatever our reasons for doing so, we all share a sense of fascination when we gaze at the stars.

Stars live for an imaginably long time, and their positions in the night sky change at an imperceptible rate. Look at the sky tonight, and the Southern Cross you see will look the same as it did to the convicts and soldiers of Sydney Cove in 1788, and to the Aboriginal people who lived in the same spot for many thousands of years before them.

And, like our predecessors, we comfort ourselves with the belief that, whatever happens on Earth, the stars will always be the same.

But the stars do change. Once or twice a century, one of the 200 billion stars in our Galaxy, the Milky Way, suddenly brightens, occasionally so much that it can be seen in broad daylight. Then, over months or years, it will slowly fade away. The ancients knew these as `guest stars', and saw them as omens or warnings. We now know them as supernovae, the vast explosions produced when massive stars, much bigger than our puny sun, run out of fuel.

The power of these explosions is unimaginable. In just a few seconds, more energy is released than in the millions of years of the star's life. Hurtling out into space at tens of millions of kilometres per hour, the glowing shell of gas and dust - the supernova remnant - can be seen for up to 100,000 years after the explosion. What's left behind is the star's collapsed core, which, we believe, often becomes a neutron star, a bizarre object that's only about 20 kilometres in diameter, yet weighs a fantastic 3000 trillion trillion tonnes.

With a magnetic field 10 trillion times stronger than the Earth's, and spinning on its axis up to 60 times per second, this neutron star generates two beams of intense radio waves that shoot off in opposite directions, sweeping across space like beams from a lighthouse. These can be picked up on Earth as a series of blips, the interval between each blip corresponding to the time the star takes to rotate.

I'll never forget the first time I heard a pulsar (as these cosmic beacons have become known). It happened one night during 1993, while I was working as a student on the radiotelescope at Parkes, in New South Wales. I trained the receiver dish on a well-known pulsar some 1500 light years away, in the constellation of Vela, which had been made by a supernova that exploded some 12,000 years ago. As the telescope swung towards the pulsar's coordinates, I hooked the incoming signal up to a loudspeaker, and at first heard just the popping and crackling of interstellar static. Then, dramatically, as the dish locked on to the coordinates, there it was, thudding away like a jackhammer, 11 times a second - a pulsar!

When you hear a pulsar for the first time, it's difficult to believe that this is a natural phenomenon. Something tells you that something so regular must be artificial - a signal from an alien civilization, perhaps? Indeed, when pulsars were discovered, in 1967, baffled astronomers briefly considered precisely this possibility. But three decades on, while it's only fair to point out that we don't understand every detail of these strange objects, there is now a solid picture that our Galaxy is swarming some 150,000 of these remarkable pulsars, of which more than 1000 have now been detected through that characteristic stram of radio-wave pulses.

From the moment I heard that bizarre cosmic clock over the loudspeaker at Parkes, I was hooked. And although I've worked on lots of different things during the last seven years, I always find myself coming back to pulsars.

What particularly fascinates me is what we can learn about pulsars from the massive explosions that formed them. Just a few thousand years after a star explodes in a spectacular supernova, its glowing remnant becomes visible as it slowly spreads out across space. And if our theories about the deaths of stars are right, there should be a pulsar thumping away, at the centre of that supernova remnant.

And while in a few cases we have indeed located pulsars in supernova remnants, in the vast majority we've found nothing. This leaves a glaring hole in our beautiful theory, and tells us that we don't really understand what is going on. Perhaps sometimes when a star dies in a supernova, its massive explosion doesn't leave a pulsar after all but something more exotic, like a black hole. Or perhaps nothing at all is left behind. Or maybe we have to consider a few subtleties, for example that the beams from a pulsar won't always come near the Earth, and so there are some pulsars we can never see.

This is what I am working on right now. Where do we find these missing pulsars? Are we looking for them in the right way, or should we be looking for some other, even stranger sort of object? Or is there just nothing there to find? With my colleagues, at the Massachusetts Institute of Technology and elsewhere, I use telescopes all over the world - and sometimes even in space - to study supernova remnants and pulsars, and attempt to understand where they come from and how the whole picture fits together.

We're getting exciting results in particular from the Chandra X-ray Observatory, a new telescope that makes images of the X-ray emissions from various celestial objects. The very first image from Chandra was of a familiar supernova remnant called Cassiopeia A, long regarded as a prime example of a remnant that is missing its pulsar. And what Chandra revealed has, but a little point of light, never seen before, sitting inside the glowing shell of Cassopeia A (see the figure at the start of the article).

We think this object is a neutron star, but haven't detected pulses from it. Maybe we're not looking hard enough, or maybe it's a neutron star which for some reason isn't pulsing. More studies of it are needed before we can work out what's going on.

As esoteric as studying exploding stars might seem, supernovae are part of the cycle of star death and re-birth. Incredibly, the ashes of a star remnant combine and collapse again to form new stars. It is humbling and inspiring to understand that our Sun, our planet, everything around us, and every atom and molecule in everyone who ever lived, was once part of an exploding star.

When the ancients looked up in wonder at a shining supernova, they could never have imagined they were witnessing a similar process to that which, billions of years earlier, began life on Earth. We still look up in wonder, but now we are doubly fortunate because we can now also begin to understand.

Last updated: 13-Jan-2001
Bryan Gaensler