The oldest stars ever seen have been discovered near the centre of the Milky Way Galaxy, astronomers say.

The newly identified stars reported in the journal Nature, date back to almost 13.6 billion years, just 200 million years after the birth of the universe.

"The stars we're looking at would have formed only a couple of million years after the very first stars began to shine," said the study's lead author Louise Howes, who carried out the research as part of her PhD at the Australian National University.

"These stars live very close to the centre of the galaxy and have probably been there for almost the entire age of the universe."

The oldest of these stars is a red giant named SMSS J181609.62 -333218.7 which was discovered in the galactic bulge, about 25,000 light-years away.

It is thought to have started its life as a fairly ordinary orange dwarf star of about 0.8 solar masses, making it slightly smaller and cooler than the Sun.

"Because these stars have been around for so long they retain the information in their atmospheres about what the universe was like when it formed," Ms Howes said.

"So, by discovering these stars we can work out details about what elements were around in the early universe. It will tell us how the very first stars formed and most crucially how they died, which will give us an idea of their size and the time scales involved."

Ms Howes and her colleagues made the discovery using the Australian National University's skymapper telescope at the Siding Spring Observatory to search the Milky Way's galactic bulge for stars with very ancient chemical compositions.

"There are a lot of theoretical models of the formation of the Milky Way which predict that the oldest stars — the ones that have been around the longest — should now be lying close to the centre of the galaxy rather than the outskirts, and they will have been orbiting close to the centre for many billions of years," said Howes.

Everything is made of star stuff

The Big Bang gave rise to a universe filled only with hydrogen, helium, and trace levels of lithium.

The first stars were uniquely created out of just these three elements. All the heavier elements on the periodic table, including the iron in your blood and the calcium in your bones, were first produced in the cauldrons of these primordial stars.

The unique chemical composition of these first stars, meant they were all giants, with masses over 40 times that of the Sun. And when they died, they underwent spectacular explosions called hypernovae, which spread these heavier elements throughout the cosmos, seeding future generations of stars.

"A hypernova is a special kind of supernova which produces about ten times as much energy as a regular supernova," said Howes.

Consequently, the levels of heavier elements gradually increased across the universe s successive generations of stars were born, created more heavy elements, and eventually died.

Astronomers use the ratio of these heavier elements— such as iron or carbon — with hydrogen to determine the age of a star. The higher the ratio the more recently the star formed.

Out of an initial 14,000 stars, Howes and colleagues identified a hand-full with the right chemical composition and orbit to indicate they were formed directly out of the material blasted into space through the hypernovae explosions.

"One star — SMSS J181609.62 -333218.7 — has an iron abundance 10,000 times lower than that of the Sun, and no detectable carbon signature," Ms Howes said.

According to Howes, that makes this star older than the previous record holder, an orange dwarf star named SMSS J031300.36 -670839.3 which was detected in the galactic halo last year.

"The fact that we haven't found any carbon in this star indicates that it was probably formed from a hypernova, which is quite different to the supernovae which formed the halo star," said Howes.

"It's amazing that we discovered something that's really so old and has seen the entire history of the universe unfold around it."



Loading...