Aussies explain why dying star sent mixed messages

Australian astronomers have explained why a dying star sent out mixed signals about its identity.

A cosmic explosion seen in 2001 looked like one particular kind of dying star, but then mysteriously seemed to change into the death-throes of another kind.  

Dr Stuart Ryder of the Anglo-Australian Observatory and his colleagues now say this odd behaviour was caused by a ‘companion’ star that still lurks at the scene of the explosion.

Called supernova 2001ig, the explosion took place 37 million light-years away in a galaxy called NGC 7424, and was first spotted in 2001 by a renowned Australian amateur supernova hunter, Bob Evans.

After studying the explosion, Ryder and his colleagues predicted the existence of the companion star. They recently found it, using the Gemini South telescope in Chile.

This is only the second time a companion has been found where a star has exploded.

But Ryder thinks companion stars could explain much that has puzzled astronomers.

"We've been putting supernovae into different categories for decades, but it's been a mystery how some of the different types are related to each other, and what's actually going on physically."

"In this case, the companion star and the one that exploded would have been orbiting each other. We think the companion helped strip hydrogen gas from the other star before it exploded," he said.

"There was a trace of hydrogen in the supernova early on, but it soon disappeared. So at first it looked like we had one kind of star exploding, but later on it looked quite different."

Two lines of evidence led the astronomers to suspect a companion was at work.

This supernova (called SN2001ig) was similar to another rare one found in 1993 (called SN1993J). Astronomers found a companion at the site of the 1993 supernova, suggesting that this supernova might have one too.

And observations made with a CSIRO radio telescope near Narrabri, NSW, showed that there were thick patches of gas and dust in the space around the star.

"That gas and dust was shed by the star before it died," explained Ryder. "We think the orbiting companion swept it into a spiral shape: the bumps and dips in our radio data fit such a spiral. And people have seen these spirals around other stars with companions."


Detailed media release


Galaxy NGC 7424 and the supernova field

Spirals of gas and dust around Wolf-Rayet stars, imaged by Dr Peter Tuthill, University of Sydney

More information

Dr Stuart Ryder, Anglo-Australian Observatory (Sydney, Australia)
+61-(0)2-9372-4843 (office)
+61-(0)419-970-834 (mob.)

Media assistance

Helen Sim, Anglo-Australian Observatory (Sydney, Australia)
+61-(0)2-9372-4251 (office)
+61-(0)419-635-905 (mob.)


A paper on the observations, "A post-mortem investigation of the Type IIb supernova 2001ig", by Stuart Ryder, University of Tasmania graduate student Clair Murrowood and former AAO astronomer Dr Raylee Stathakis, was published online in Monthly Notices of the Royal Astronomical Society on May 2. It is also available at