The universe has not always been full of stars. One hundred and fifty million years after the Big Bang, there was no light, just clouds of gas in gigantic filaments that stretched throughout the expanding universe. Those clouds were the birthplace of the first stars, and observing them is one of the most active fields in astronomy today.
Now, a new discovery brings us closer to those stars and gives us important information on their nature. A Brazilian-American team has observed an “ultra metal-poor” star, a rare relic from the infancy of the Milky Way, itself about 13 billion years old. The star is also surprisingly bright, and the researchers are confident that further observations will provide important constraints on the chemical evolution of the early universe.
The star, named 2MASS J18082002–5104378, is a sub-giant star with a surface temperature of 5440 Kelvin, similar to the temperature of our Sun. It was studied using the ESO's New Technology Telescope. Interestingly, it shows an unbelievably low abundance of metals, elements heavier than hydrogen and helium – hence the label of ultra metal-poor. The first stars in the universe were made of hydrogen and helium; as they died, they enriched the universe with all the elements that make planets, comets and us.
The enrichment was gradual, so while some of the first stars were still shining, a new generation of stars began to form. 2MASS J18082002–5104378 is part of this generation. These stars were once ubiquitous in our galaxy, but they have become rarer and rarer as they gain more metals through supernovae explosions.
Elements up to iron are formed during nuclear fusion in the core of stars. Stars first burn hydrogen and form helium, then helium is used to form carbon and oxygen, and so on until iron is formed. Elements heavier than iron cannot be fused together without an energy deficit, so eventually the core of a star stops producing energy, and it collapses in on itself. The collapse is so powerful that the star is blown to bits: elements up to uranium form and are spread far and wide across the galaxy.
Increasing the number of observed ultra metal-poor stars will allow researchers to better understand the first stars, with the hope they can directly observe them when the James Webb Space Telescope goes online in 2018. 2MASS J18082002–5104378 could be an interesting candidate for further study.
In their research, published in Astronomy & Astrophysics, the team also highlights how their method used to identify this star, "which is based on the mismatch between spectral types derived from colors and observed spectral types," can be applied to find more stars of this type.
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