Astronomers have discovered the oldest known stars in our galaxy, and believe they could give vital insights into the early Universe, including how the first stars died.
The international teams of astronomers from the University of Cambridge and the Australian National University have found that swirling around the center of the Milky Way are stars that had formed not long after the Universe began.
“These stars live very close to the center of the galaxy and have probably been there for almost the entire age of the universe,” Louise Howes, from The Australian National University (ANU), lead author of the study, said.
Stars this old have been seen around the edges of the Milky Way, but it is a first for them being detected in the galaxy’s central bulge. Scientists believe the stars date back to almost 13.6 billion years, which is 200 million years after the birth of the Universe.
Howes said: “These pristine stars are among the oldest surviving stars in the Universe, and certainly the oldest stars we have ever seen.”
‘These stars formed before the Milky Way, and the galaxy formed around them.’
Watch ANU TV explain oldest stars found near the center of the Milky Way:
The stars contain extremely low amounts of metal, making them surprisingly pure. One of the stars discovered, called SMSS J181609.62-333218.7, is the most metal-poor star to be found in the center of our galaxy to date, and is about 25,000 light-years away.
“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,” Howes explained.
‘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,” she added.
The discovery of the nine stars now challenges the current theories on the environment in which these stars were formed. The astronomers have also found the stars contain chemical fingerprints, which point toward the conclusion that the very first stars may have died in deaths known as hypernovae.
“The stars have surprisingly low levels of carbon, iron, and other heavy elements, which suggests the first stars might not have exploded as normal supernovae,” said Howes, “Perhaps they ended their lives as hypernovae — poorly understood explosions of probably rapidly rotating stars producing 10 times as much energy as normal supernovae.”
The astronomers used images taken from the SkyMapper Telescope to identify 14,000 promising stars with the potential of being metal-poor. Then by using a spectrograph on a bigger telescope, they looked at them in more detail. A spectrograph breaks up the light of the star, similar to a prism, which allows astronomers to make more detailed measurements.
Project leader Professor Martin Asplund, from ANU Research School of Astronomy and Astrophysics, said: “The ANU SkyMapper telescope has a unique ability to detect the distinct colors of anemic stars — stars with little iron — which has been vital for this search.”
After selecting 23 of the most metal-poor stars, the researchers then used the larger telescope in the Atacama Desert in Chile to identify nine stars with a metal content less than one-thousandth of the amount seen in the Sun. One of these was one-ten-thousandth the amount, making it the most metal-poor star in the center of the galaxy.
Dr. Andrew Casey of Cambridge’s Institute of Astronomy, one of the study’s co-authors, said in a statement: “If you could compress all the iron in the Sun to the size of your fist, some of these stars would contain just a tiny pebble by comparison.”
‘They’re very, very different kinds of stars.’
To be certain the stars weren’t just traveling through the center, researchers used precise measurements of the stars’ movement in the sky. This enabled them to predict how the stars were moving, and where they had been.
While some stars were found to be passing through, seven of them were found to have spent their entire lives in the center of our galaxy. Computer simulations have suggested that the seven stars would have formed in the very early Universe.
“There are so many stars in the center of our Galaxy — finding these rare stars is really like looking for a needle in a haystack,” said Casey. “But if we select these stars in the right way, it’s like burning down the farm and sweeping up the needles with a magnet.”
“This work confirms that there are ancient stars in the center of our Galaxy. The chemical signature imprinted on those stars tells us about an epoch in the Universe that’s otherwise completely inaccessible,” Casey added. “The Universe was probably very different early on, but to know by how much, we’ve really just got to find more of these stars: more needles in bigger haystacks.”