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9 July 1997


Exploded stars in our Galaxy line up with the Galaxy's magnetic field like cosmic compasses, a student working with CSIRO's Australia Telescope has found.

University of Sydney PhD student Bryan Gaensler described his discovery to the annual meeting of the Astronomical Society of Australia (taking place at the University of NSW this week). Bryan is studying supernova remnants - the remains of exploded stars - which can often be seen with radio telescopes such as the Australia Telescope.

"When a star explodes, it blasts a giant 'cinder' of gas and other stuff into the space around it," explains Bryan. "After a few years it starts glowing at radio wavelengths, and we can make a picture of it.

"You'd expect this wreckage to be a round symmmetrical ball, but it's usually not. Up to seven times out of ten it's barrel-shaped, stretched out."

Bryan has now found that the long axes of these barrel-shaped remnants line up with the Galaxy's magnetic field - a completely unexpected find.

"I was trying to prove the opposite, actually," he says. "Conventional wisdom has been that the force of the exploding star is so strong that nothing outside it could affect it - just like a steamroller can run over a bunch of daffodils without being affected."

But it seems the daffodils do pack some punch. Bryan thinks that the key is the gas shed by the stars before they explode. "This gas could line up along the magnetic field lines, making a sort of stretched-out bubble. Then when the star explodes it is contained somewhat within this envelope of gas.

"Other people have tried doing this kind of work but only recently have observations made clear the shapes of many of these remnants," says Bryan.

Our Galaxy's magnetic field is known to be lined up along the spiral arms of gas and stars. Other spiral galaxies show the same pattern. But why galaxies have magnetic fields at all is still a bit of a mystery.

Sixty to 200 light-years across in size, the supernova remnants Bryan studies are 10 000 to 50 000 years old.

[Available pictures: 3 radio pictures of supernova remnants, as computer files or displayed on screen. A series of pictures of supernova remnant 1987A, which shows it turning from a fuzzy blob into a much better defined object. This sequence, available as a computer 'movie', illustrates how improvements in the resolution of images can make the shape of the object clearer.]

More information from:

Helen Sim (ph) 02-9372 4251 or email: hsim@atnf.csiro.au

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