The oldest stars in our Milky Way galaxy have been discovered by an international team of researchers. These ancient stars could contain vital clues about how the first stars in the early universe died, and their discovery marks the first time that extremely metal-poor stars have been observed in the central region of the galaxy. The location of the stars suggests that they formed when the Milky Way underwent rapid chemical changes during the first 1–2 billion years of the universe.
After the Big Bang, only elements such as hydrogen, helium and some trace amounts of lithium existed in the universe. Heavier elements such as oxygen, nitrogen, carbon and iron – referred to as “metals” by astronomers – were forged in the extremely high-pressure centres of the first massive stars, which are predicted to have formed within 200 million years after the Big Bang. The metals were scattered across the cosmos when these first stars, known as “population III” stars, quickly burned out and exploded in supernovae. These explosions seeded the universe with the metals to form “population II” stars, which are still “metal-poor” compared with “population I” stars like the Sun.
Not the stars we are looking for?
A true first population-III star has not yet been discovered, although the best evidence for them was found earlier this year in an extremely bright and distant galaxy in the early universe. Astronomers believe that old metal-poor stars would have formed in the central regions or the “bulges” of galaxies, where the effects of gravity were the strongest. The Milky Way bulge underwent a rapid chemical enrichment in the early universe, and this should have created a host of metal-poor stars – indeed, we should find them there even today. However, metal-poor stars have only been found in the outer regions or the “halo” of the Milky Way and not at its centre.
Now, Louise Howes of the Australian National University in Canberra and an international team have used the SkyMapper telescope to identify nearly 500 extremely metal-poor stars in the Milky Way bulge. The team also confirmed that most of these old stars are in tight orbits around the galactic centre, rather than being halo stars passing through the bulge. The researchers also found that the chemical compositions of these stars are, for the most part, similar to typical halo stars of the same metal content (or metallicity). However, some unexpected differences exist when it comes to the amount of carbon in such stars.
Stars with a low metal content look slightly bluer than others, so the team could sift through the millions of stars at the centre and whittle the observations down to 14,000 promising candidates. From those, the researchers identified 500 stars that had less than 100th the amount of iron in the Sun, making it the first extensive catalogue of metal-poor stars in the bulge. Of these, Howse and colleagues focused on 23 candidates that were the most metal-poor, and from these data, they homed in on nine stars with a metal content less than 1000th of the amount seen in the Sun. This includes one star with an iron abundance 10,000 times lower than that of the Sun – now the record-breaker for the most metal-poor star in the centre of the galaxy.
To and fro
To ensure that these stars were truly old – and not those that had formed much later in other parts of the galaxy that were not as dense and are now merely passing through the centre – the researchers used precise measurements and computer simulations to plot the stars’ movement in the sky. This allowed them to predict where the stars came from and where they were moving to. The team found that while some stars were indeed just passing through, seven of them were formed in the bulge and had remained there since.
“These pristine stars are among the oldest surviving stars in the universe, and certainly the oldest stars we have ever seen,” says Howes. “These stars formed before the Milky Way, and the galaxy formed around them.” While it is currently not possible to directly determine the ages of these ancient stars, the researchers say that it could be inferred from data collected by the extended Kepler mission or its successors.
The team’s discovery also challenges current theories about the environment of the early universe from which these stars formed. “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,” says 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.” If true, such hypernovae would be one of the most energetic things in the universe, and very different from the kinds of stellar explosions that we see today.
The research is published in Nature.