Stingray In The Sky Sparks New Theory

May 07, 1998

Australian astronomers have put forward a new theory to explain a weird stingray shaped remnant of a giant exploded star, or supernova.

"Space is full of peculiar shapes like ducks, guitars, speedboats or jellyfish. Trying to explain this "stingray" shape may have led to a new way to explain how supernova remnants are formed," says University of Sydney PhD student Bryan Gaensler.

Mr Gaensler and his team are studying a supernova remnant, G309.2-00.6, which is shaped like a stingray, using radio telescopes of the CSIRO's Australia Telescope National Facility.

When a massive star runs out of fuel and dies, there is a huge explosion called a supernova. The outer layers of the star are thrown off in all directions, while the core of the star collapses into a pulsar or black hole.

After the explosion the outer layers of the star plough at great speed into gas surrounding the star and they begin to 'glow' in the radio spectrum. This slowly expanding smoke-ring is called a "supernova remnant".

"Now, ideally, you'd expect a supernova remnant to be nice and spherical. But it almost never works out this way - there are all sorts of shapes: rings that are bright on one side, loops, barrels, arcs etc," Mr Gaensler says.

It is thought these shapes could be caused by the "interstellar medium" (the thin gas between the stars) as it has dense clumps, tunnels, walls, cavities and so forth that shape the gas as it moves. So this means that the weird shapes of supernova remnants reflect the complicated environment into which they expand.

"In fact, since the interstellar medium is largely invisible, supernova remnants are a great way to learn about the structure and composition of this gas between the stars," Mr Gaensler says.

However some supernova remnants such as the "stingray nebula" show patterns which can't be explained by the complicated structure of the interstellar medium alone, causing the team to look for a new theory to explain their shape.

G309.2-00.6 was discovered in 1974, but only with modern telescopes has it become possible make a sharp enough picture of it to see just what is going on.

The team proposes that to create the rather distorted stingray shape there is an object such as a black hole or pulsar sitting in the centre of the supernova remnant, shooting out jets of material. These jets smash into the two sides of the expanding supernova remnant, brightening it and distorting it to form areas that look like the head and rear-end of the stingray. Furthermore it appears that one of the jets has actually blasted through the shell, and has then flowed through a tunnel in the surrounding gas to make the long "tail".

"We still don't know what the source is in the centre that's doing all this. A group in the USA will shortly be looking at it with X-Rays to try to find out, and we expect their data will reveal the central object which is having such a dramatic effect on the supernova remnant," Mr Gaensler says.

This isn't the first supernova remnant to have these jets. One very well studied supernova remnant called "W50" has a strange object called "SS433" in the centre of it. SS433 is shooting out jets in opposite directions, which smash into W50 and create two lobes, just like in the stingray nebula.

"The difference is that W50 is very old - about 100,000 years since the supernova explosion, while G309.2-00.6 is just a youngster - only about 4000 years since the explosion. You can see the difference too - in G309.2-00.6 the head and rear-end protrude a little bit beyond the shell of the remnant, but in W50 the jets have been ploughing into the supernova remnant for so long that they've almost ripped it completely apart," Mr Gaensler says.

For a long time astronomers thought there was only one of these objects in the Galaxy. Finding a second one led to the question of whether or not there were more. When seeking the answer the team looked at all the known supernova remnants in the Galaxy (about 220 of them in total).

"We think we've found about 10 whose shape is distorted like G309.2-00.6 and W50, and maybe they too might have a black hole or pulsar with jets shooting out in the middle of them. If turns out to be so it would mean we then know exactly where to look if we wanted to study these bizarre compact objects - in the centre of supernova remnants which are distorted like the stingray nebula," Mr Gaensler says.

Bryan Gaensler has recently been awarded a prestigious Hubble Postdoctoral Fellowship, one of only five Australian-based students to have received this award. He will travel to the Massachusetts Institute of Technology in September to take up the fellowship.

CSIRO Australia

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