An international team of astronomers has discovered some of the oldest stars in the Milky Way. In the process, they were able to learn a lot about the abundance of different elements in the very early universe.
These stars, which are part of the second generation of stars to form in the universe, were discovered near the very center of the Milky Way and they are believed to have started shining 200 million years after the Big Bang. The scientists think that their findings, published in Nature, give an indication of the life and death of the very first stars.
The first stars are believed to have been huge, having up to 1,000 times the mass of the Sun. It is thought that when these objects reached the end of their lives, they exploded in hypernova explosions, tens of times stronger than the supernovae we see in the universe today. The first generation have not been directly observed yet, but astronomers hope to see them when the James Webb Space Telescope (JWST) starts operation in 2018.
The second generation, including the stars from this study, are metal-poor: They are made almost exclusively of helium and hydrogen, with only traces of heavier elements (the “metals”). The more metal a star has, the quicker it forms and the smaller it is.
After the Big Bang, the universe was composed of just hydrogen and helium. This is why the first-generation stars were so big. The carbon in our bodies, the oxygen in the air and all the other heavy elements we find in the universe were formed by the first two generations of stars.
The discovery didn’t come easy. There are millions of stars in the Milky Way’s bulge, so the team had to develop a strategy to make the gargantuan task of observing them in detail more manageable.
Since very metal-poor stars are slightly bluer than other stars, the researchers selected 14,000 promising stars from the ANU SkyMapper telescope in Australia. Only 23 of them were then studied in more detail in follow-up observations.
“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 co-author Dr Andrew Casey of Cambridge’s Institute of Astronomy in a statement. “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 elements released by the first stars act as a chemical signature that is still present in the stars observed in the study.
“This work confirms that there are ancient stars in the centre of our galaxy. The chemical signature imprinted on those stars tells us about an epoch in the universe that’s otherwise completely inaccessible,” said Casey. “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.”