When Big Stars Die

by Bryan Gaensler

The following article is a popular account of my work which I wrote for The Bulletin magazine in May 1999. The article is ©1999 Bryan Gaensler, and may only be reproduced or distributed for private use unless my explicit permission is given otherwise.

Picture at right: An image of the barrel-shaped supernova remnant G296.5+10.0, made using a radio telescope. The image is in "false colour", meaning that the colours are not real, but correspond to how bright the source is. The remnant is about the same height as three full moons stacked end-to-end, is approximately 4000 light years (40 000 000 000 000 000 km) from us, and is the remains of a star which exploded about 10 000 years ago. Note that the little stars all over the picture are not stars at all, but are actually radio emission from distant galaxies, each containing billions of stars. [Credit: Dr M. Kesteven / Australia Telescope National Facility]

Astronomers are waiting for a supernova explosion - one of the most violent events in the universe. These massive conflagrations occur every hundred years or so, and with the last one in our galaxy thought to have been in 1670, a supernova is long overdue.

Like all other working astronomers, I just hope the Big One happens in my life time.

To grasp the magnitude of this colossal event, you must first understand how things usually work in the life of stars.

Most stars are something like our Sun. It's almost one million kilometres in diameter and its centre is so hot that hydrogen burns there to form helium. It's a massive hydrogen bomb, but a rather ordinary star. And it won't last forever.

The sun's light and heat have sustained life on Earth for almost five billion years. But in another five billion, it'll sputter and hiccup, and with its hydrogen having run out, it'll swell to 300 times its size, swallowing up the Earth in the process. Finally, it will gently puff off its outer layers, forming a glowing ring known as a "planetary nebula".

The centre of our dying Sun will remain intact to become a "white dwarf" - a small star, initially very hot (1 000 000 C), but which will eventually cool to float dark and dead through space.

One in a 100 stars are different though, and are destined to become supernovae. These stars are big - around 10 times heavier and four times larger than our Sun - and their hydrogen burns furiously, making them incredibly hot.

Instead of lasting 10 billion years, these giant stars gobble up their fuel in just 10 million - a long time in human terms, but an astronomical blink of an eye.

Like any star, these giants are a balance between heat, which pushes gas up to the surface of the star, and gravity, which pulls everything back towards the centre.

When all their fuel is used up, their heat supply gone, the only thing left in these giants is gravity, and that causes a dramatic collapse.

The collapsing star shrinks until its atoms are squeezed together. Then it explodes, and like the release of a tightly coiled spring, the outer layers of the star are thrown outwards at up to 100,000,000 kph, releasing more energy in a second than the star released in its entire life.

This star is now a "supernova", an explosion so bright that if a star near us went supernova, we'd be able to see it during broad daylight for weeks.

Space is full of lumps and clouds of gas. When material from a supernova explosion slams into the surrounding gas, everything in the area heats up and begins to glow, forming an expanding ring known as a "supernova remnant".

While supernovae happen every century or so, their remnants last for up to a million years. When we look at the sky through telescopes, we see hundreds of these giant smoke rings.

Some went off just a few hundred years ago and are bright and expanding quickly. Others are barely visible tendrils of gas, remains of ancient explosions.

After becoming involved in astronomy, I soon focussed on the patterns made by supernova remnants. While you'd expect these patterns to be symmetrical, like circular smoke rings, they turn out to be infinitely more complicated. They form arcs, ovals, tangled knots and even overlapping rings and figures-of-eight.

I began wondering what controlled the shapes these remnants assumed? What stopped them being nice and round? Several possibilities seem reasonable: If the supernova explosion itself were lop-sided rather than spherical, that could cause an oddly-shaped remnant; or, if the gas surrounding the star formed complicated patterns, then the remnant could be distorted and misshapen from colliding with these surroundings.

Just which factor makes the difference is not clear, and while this conjecture may seem very esoteric, the death of a star, and its aftermath, are part of a vast and important cycle.

The gas cloud created by the explosion eventually collapses to again form new stars, restarting the process.

Our sun, our planet, every atom in our bodies, was once gas hurtling outwards from a supernova explosion. Understanding supernova remnants therefore helps us understand where we've come from, and what else might be out there.

But how do we determine the forces that shape a supernova remnant? The answer is to make detailed pictures of large numbers of these remnants, and search them for common properties.

My PhD thesis revolved around this idea and I made many observations of supernova remnants using the Australia Telescope Compact Array, in Narrabri, NSW.

Normal telescopes that focus and magnify visible light are not much good for viewing supernova remnants. Supernova remnants are best "seen" through radio telescopes which make pictures of invisible radio waves from space - the same sort of radiation which carries radio and TV signals.

Once the telescope has received the radio waves, and a computer has converted the waves into a "normal"-looking picture of a supernova remnant, you can then start thinking about what it means.

I decided to look at supernova remnants which are shaped like the walls of a barrel (see figure). Up to 70 per cent of supernova remnants are stretched out in this way, so obviously something fundamental is going on.

Astronomers have been arguing over the causes of barrel-shaped supernova remnants for 40 years, but no-one has had a good answer.

Making observations of new barrel-shaped supernova remnants, and looking at images of remnants made by other people, I made a stunning discovery.

I was trying something a little silly, namely, measuring the direction along which barrel remnants "point" to see if there was a pattern.

If you look at the remnant in the picture at the top of the page. you'll see that it's pointing straight up and down. To my great surprise, I found that almost every barrel remnant points in the same direction!

Our galaxy, the Milky Way, is magnetic, although at a level about a million times weaker than the Earth. If you flew through the Milky Way with a super-sensitive compass, it would always point in a particular direction, "Galactic North" if you like.

Unbelievably, I found that all barrel remnants point to Galactic North, and that supernova remnants act like giant compasses.

This was certainly a surprising finding because supernova remnants are huge expanding bubbles, and Galactic magnetism is so weak that it should hardly be noticed. It certainly shouldn't affect a supernova remnant.

The answer seems to be that the Galaxy's magnetism doesn't affect the actual remnant, but it does acts on the gas floating around the star before it goes supernova.

This gas just sits there for millions of years before the star explodes, and magnetic forces gently stretch it out along the north-south direction, forming giant tubes or sausages in space.

When the star does explode, the remnants expand into this sausage-shaped tunnel, taking on its shape and making a barrel remnant which points to Galactic North.

So not only does this theory explain why supernova remnants aren't round - the original question I set out to answer - but it tells us lots of other things we hadn't realised before. Like that Galactic magnetism has a real effect on interstellar gas, and that space might be full of sausages and tunnels all pointing in the same direction.

This theory has to be tested of course. The first thing is to try and make pictures of the gas around the barrel remnants to see if there really are tubes and sausages into which the remnants are expanding.

Together with some fellow astronomers from Australia, Argentina and the USA, I've already started doing this, and the news is good. It seems barrel remnants are indeed expanding into giant stretched-out tunnels, just like my theory predicts.

What I'm working on now are the particulars of how the Galaxy's magnetism actually stretches interstellar gas. How long does this process take? How strong must the magnetism be for it to work? These questions can only be answered after detailed calculations and simulations.

Finally, we need to check the possibility that it isn't a huge fluke that all the barrel remnants in our galaxy point in the same direction. The way to do this is to conduct the whole experiment again, but this time in other galaxies.

Our Milky Way is just one of uncounted billions of galaxies out there, each magnetic, and each filled with supernova explosions and their resulting remnants. If we can make pictures of barrel remnants in other galaxies, we can see if they too line up with their galaxy's North Pole.

The problem is that we can't do this just yet. Other galaxies are too far away, and our telescopes don't have sharp enough vision to pick out individual supernova remnants.

This work will have to wait for the Square Kilometre Array (SKA), a huge telescope planned for construction in around 2010.

The SKA, which may well be built in Australia, will be able to see things in 10 000 times more detail than the telescopes I presently work with. It will allow us to make incredible pictures of supernova remnants, and of everything else out there.

The Southern Cross, the Saucepan, and other constellations seem just like they were when humans first came to this continent 40 000 years ago, and night after night, we look up at the stars and nothing in the sky seems to change.

But as we now know, stars do change, and die, and even become us. Imagine the Southern Cross with one of its stars missing. It's only a matter of time. It's just that the time scale involved is much longer than we can imagine. Truly astronomical.

Last updated: 13-Jan-2001
Bryan Gaensler