J1808-5104, the oldest star in our vicinity?

A star observed by Hubble would have 13.5 billion years

Credit: Ruffnax (Crew of STS-125) [Public domain], via Wikimedia Commons

The Hubble space telescope takes back in the history of the universe, looking ever further. Some of the galaxies captured by the space telescope emitted their light when the universe was barely 1 billion years old. These observations are valuable because they make it possible to draw up the history of the universe and to understand how it has evolved, and also to possibly predict its future transformations.

To study the youth of the universe is very interesting. It can also be interesting to study very old stars. J1808-5104 is part of the sun’s neighborhood since it is only two thousand light-years away from the sun. The team of astronomers thats discovered this star think it is 13.5 billion years old. In comparison, we estimate that the sun is only 4.6 billion years old.

How to estimate the age of such an old star ?

Credit: NASA / Walt Feimer [Public Domain], via Wikimedia Commons

Dating of J1808-5104 is possible thanks to the amount of metals it contains. Precisely, it contains almost none. In our models of the evolution of the universe, we suppose that the primordial universe consisted of hydrogen, helium and a tiny bit of lithium and beryllium. A first generation of stars was formed from these materials. Inside these stars, the light elements fused into increasingly heavy elements, this is what we call stellar nucleosynthesis.

These first stars would have had very short lives before exploding in supernovas and spreading in the universe the heavy materials they synthesized. According to this model, for a star to contain heavy elements, for example metals, it must be born after the death of the first stars. A star that is extremely poor in metal like J1808-5104 has a good chance to be part of the first generation of stars in the universe. J1808-5104 contains about ten thousand times less metal than the sun, which allows astronomers to estimate that this star was born when the universe was a few hundred million years old.

J1808-5104 challenges our models of universe formation

Credit: Kristian Pikner [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons

J1808-5104 is a red dwarf, meanwhile in our models we suppose that the very first stars of the universe were very massive and disappeared very quickly. Obviously, the primordial universe was also composed of some small, long-lived stars. We think that red dwarfs can live for thousands of billions of years, so the universe is still far too young for one of them to be already extincted. Although J1808-5104 is much older than the sun, it will continue to shine long after the sun is gone.

The discovery of this very old star encourages us to rethink the age of the Milky Way and of the various elements that compose it. Our region of the galaxy could be much older than we previously thought. Although J1808-5104 contains almost none metal, it contains some. Thats means that it may have had one or two ancestors. The hunt is still open to find a star of first generation that would have been formed from the material directly from the big bang.

Jonathan, From Space With Love

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