A team led by Australian National University astronomers discovered the star, formed long before the planets came into existence soon after the big bang 13.7 billion years ago. But it could still be spotted in southern skies by anyone with a good-quality backyard telescope.

Project leader Stefan Keller said not only was this the oldest star discovered, it was the oldest ever likely to be found.

Dr Keller said the first generation of stars after the big bang were thought to have been huge and lived for very short periods, but his team's discovery was a second-generation star, formed from a single supernova.

"This is as far back as we can take stars. We believe it's the end of the line as far as stars that we can observe go, and that's pretty exciting," he said.

Dr Keller said each star was like a time capsule of what the universe was like when it was formed, so by studying the latest find, scientists could begin to explore the earliest years of the universe.

The discovery might help resolve any outstanding parts of the big bang theory. He said when bushfires swept through Canberra in 2003 his team lost the telescope it planned to use to map the southern sky, and rebuilding had taken a decade. With the new, better SkyMapper telescope, the researchers imaged the skies every night and used the brightness of the stars to calculate their age.

Dr Keller said the universe started life as a combination of hydrogen, helium and some lithium, but everything else had been formed since then, inside stars.

The age of stars could be measured by the amount of iron inside them, he said, but this star had no measurable iron in it at all.

"It's the first time that's ever been seen and that tells this star was put together when this universe was very old," he said.