One of the most pressing questions dogging astronomers and cosmologists is how the universe began to take shape after the Big Bang. In their quest to understand the formation of the early universe, many turn outward and study stars, supervoids, and other cosmic objects that are billions of light years away. But, recently, an international team of astronomers led by researchers from the University of Cambridge and the Australian National University, turned inward to focus on the center of our own Milky Way.
Here they found the oldest stars ever seen in our galaxy: metal-deficient giants serving as cosmic obituaries detailing the lives and deaths of the first stars in the universe.
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According to the team, which published its findings in Nature last week, the stars under study have been at the center of the Milky Way for billions of years and contain “chemical fingerprints” indicating that the universe’s first stars died in massive explosions called hypernovae, which are about ten times as energetic as regular supernovae. Moreover, one of the stars is the most metal-deficient star to have yet been discovered in the center of the galaxy.
This last discovery is of particular importance because the chemical composition of stars can tell astronomers a good deal about their formation. In the immediate aftermath of the Big Bang, the universe only contained hydrogen, helium, and trace amounts of lithium. So, to find the oldest stars, astronomers look for those containing large amounts of hydrogen with few other elements.
Eventually, the group found nine stars all of which had at less than one-thousandth of the metal content of our Sun. The most metal-deficient star of the nine consisted of less than one ten-thousandth parts metal and is now the candidate for the most metal-poor star in the galaxy.
“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,” said Dr. Andrew Casey of Cambridge’s Institute of Astronomy in a statement. “They’re very, very different kinds of stars.”
Astronomers have long thought that the first stars must have formed in the center of the galaxy, where gravity is the strongest. Yet decades of observation seemed to fly in the face of these assumptions, with most of the stars in the center of the Milky Way possessing a metal content similar to that of stars much closer to our Sun. In light of these observations, the researchers couldn’t count on the low metal content of these stars alone to guarantee that they were around in the early days of the universe. Rather, they may have formed much later in less dense parts of the galaxy and just now happened to be passing through the center.
To check its observations, the team used precise measurements of the stars’ movements to predict where they were heading and where they had been in the past. What the astronomers found is that seven of the stars had spent their entire lives in the center of the galaxy, which according to computer simulations run by the team, means they must’ve been around in the universe’s infancy.
“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,” Casey said. “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.”
The stars discovered by the team date back to before the Milky Way was even formed, when the universe was only about 300 million years old. Yet what makes these stars so rare is that they contain a chemical fingerprint left on them from even older stars, a fingerprint which suggest that this first generation of stars died in brilliant hypernovae, one of the most energetic occurrences in the universe.
“This work confirms that there are ancient stars in the center of our galaxy,” Casey said. “The chemical signature imprinted on those stars tells us about an epoch of the universe that’s otherwise completely inaccessible. The universe was probably very different early on, but to know how much, we’ve really got to find more of these stars: more needles in bigger haystacks.”
