This handout photograph from the Space Telescope Science Institute shows a star discovered by a team led by Dr. Stefan Keller with the SkyMapper telescope at the Siding Spring Observatory near Coonabarabran, New South Wales, Australia. The Australian astronomers said the star is 13.6 billion years old, making it the oldest identified to date. Agence France-Presse/Getty Images
CANBERRA, Australia—Astronomers in Australia have identified what they say is the oldest known star, estimating it was formed 13.6 billion years ago.
The astronomers said they found the star in the Milky Way galaxy, relatively close to the Earth, around 6,000 light years away. It is a few hundred million years younger than the universe and 400 million years older than a star identified in recent years by teams of astronomers in the U.S. and Europe.
Its age was determined by analyzing its iron content, the Australian team said in an article in the science journal Nature.
"The telltale sign that the star is so ancient is the complete absence of any detectable level of iron in the spectrum of light emerging from the star," said Stefan Keller, the operational scientist at Australian National University's Mount Stromlo SkyMapper Observatory.
The star—spotted on Jan. 2 during an operation to digitally map the Southern Hemisphere sky and dubbed SMSS J031300.36-670839.3—is believed to be part of a second generation of stars, Mr. Keller said. It was formed from debris thrown out by a massive star that died out quickly.

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"They were really massive objects that lived fast and died young. They had large explosions, and the shock wave from those first stars gave rise to this star that we are observing now," he said. "It unambiguously incorporates material from the first stellar generation."
The primordial star that gave rise to a second-generation star probably had a mass 60 times as large as our sun and exploded with less energy than might be expected, Dr. Keller said. That allowed a subsequent black hole to swallow most of its heavy elements before another star eventually formed nearby.
Iron and other relatively heavy elements gradually build up in successive stars born from supernovae, with the composition of our own sun indicating it was formed from hundreds of cycles of explosion, formation and re-formation.
All stars act like time capsules, Dr. Keller said, and SMSS J031300.36-670839.3 sheds light on the gas composition of the universe as it was 13.6 billion years ago.
"It tells us a lot about what those first stars were," he told Australian radio. "To form a star like the one we've located, there was likely only one supernova."
Professor Mike Bessell, who spotted the star while analyzing mapping data, said the light spectrum showed only a handful of elements, including hydrogen, carbon, magnesium, and calcium, which made its composition radically different from our sun's. The calcium, in particular, was what astronomers had expected from second-generation stars formed from progenitors consisting of hydrogen and helium.
SMSS J031300.36-670839.3 was one of 60 million photographed by the SkyMapper telescope in its first year of operation, Mr. Bessell said. Its discovery has since been confirmed by the team using the giant Magellan telescope in Chile.
"There's likely to be more stars like this, but they are very hard to find," he said. "We have to sort through vast amounts of data to find the very few of them that are out there. It's a needle in a haystack."
Write to Rob Taylor at rob.taylor@wsj.com