EDMONTON — It took a decade, but two scientists have solved the mystery behind a chunk of radioactive rock the size of a small city that has been floating in space.

It turns out the material, discovered by astronomers in 1999, is the core of a supernova, or exploding star, that occurred 11,000 years ago, but only became visible 330 years ago.

Craig Heinke, a physics professor at the University of Alberta, along with Wynn Ho of Southampton University in the United Kingdom, finally figured it out.

"I'm pretty pumped. It's been absolutely great," said Heinke in an interview with The Canadian Press. The duo's findings are being published in the Nov. 5 edition of Nature.

"This one has been a real puzzle for about 10 years since other astronomers detected this object first. We have been able to figure out what it is. We are able to show conclusively that this is a neutron star, something that was not entirely clear before," Heinke explained.

Neutron stars are produced when massive stars explode, an event called a supernova. Neutron stars are the remnants left behind and are the densest objects in the universe.

Researchers now have access to the complete life cycle of a supernova and can learn more about the role exploding stars play in the makeup of the universe. Most minerals found on Earth are the products of supernovae.

The remnant in this case was difficult to identify, partly because of its age. "It was an infant neutron star with an unusual wrapping if you like," offered Heinke.

It is the youngest neutron star ever identified. Scientists were thrown off track because it was basically in disguise -- covered with a 10-centimetre layer of carbon. Neutron stars are usually covered with hydrogen, which gives off an entirely different radioactive signature.

Heinke and Ho developed a model using X-rays to determine the makeup of the mysterious space rock.

"Whatever is sitting on top of the neutron star changes the radiation that comes out, so if you look at the X-rays you basically have an imprint of whatever is the element on the surface ... and if it's hydrogen it gives you one imprint. If it's carbon, it's something rather different," said Heinke.

Heinke suggests the carbon coating developed because the surface temperature of the young neutron star was much hotter than the norm.

"At extremely high temperatures of millions of degrees, there can be nuclear fusion on the surface ... turning the hydrogen and helium into carbon," he said.

"We think we've shown there is nuclear fusion on the surface of a neutron star and we think this is the first time we've seen this in the evolution of a neutron star."

Heinke suggested the neutron star will develop a hydrogen coating more recognizable to scientists once it cools.

It's a career highlight for Heinke.

"I started reading about neutron stars in about first grade and I just thought they were the most fantastic, exotic objects ever. Somehow I managed to be able to study these for my livelihood and it is incredibly fun."