HE 1523-0901
The previous oldest star is the HE 1523-0901, which found in 2007 (Wiki Commons/ESO)

Astronomers believe they have found the oldest star in the universe, which formed at least 13.2 billion years ago, shortly after the Big Bang.

The star, which is just 186 light years away, has been named HD 140283 and has been studied by astronomers for over a century.

Researchers from Pennsylvania State University announced their finding at the American Astronomical Society, Nature reports.

"We believe this star is the oldest known in the universe with a well determined age," said astronomer Howard Bond.

The universe was formed around 13.75 billion years ago. It has long been known that the star is made of hydrogen and helium, which suggests it was formed in the early history of the universe; later stars are made from heavier elements.

The team worked out the star's age by using observations recorded with the Hubble Space Telescope between 2003 and 2011. The recordings were made by the telescope's guidance sensors that measure the position of stars in relation to reference stars.

Second generation star

After measuring the distance and the brightness of HD 140283, the scientists were able to determine the star's luminosity precisely.

HD 140283 has advanced to a phase in its life where it is exhausting hydrogen at its core, during which the star's luminosity is slowly dimming - a strong indicator if its age.

By using this knowledge, the team calculated that the star is 13.9 billion years old, give or take 700 million years, meaning it is 13.2 billion at its youngest and at least the same age as the previous oldest known star.

Volker Bromm, from the University of Texas, commented on the findings, saying HD 140283 is not one of the very first stars that formed as these did not contain significant amounts of elements heavier than helium.

The first generation of stars died in supernova explosions that heated surrounding gas a few hundred million of years after the big bang.

Before the second generation of stars formed, the gas had to cool down. The age of the HD 140283 star, Bromm says, suggests the cooling period was extremely short, lasting just a few tens of million years.

The age of the previous oldest star, HE 1523-0901, was worked out in 2007. It is located in the Milky Way in the constellation of Libra.