Observation of a singular group of stars on the outskirts of the Milky Way revealed the remains of a star cluster dating back to the early ages of the Universe.
"To do galactic archeology, you have to understand what our galaxy has absorbed over time, know its foundations", explains Nicolas Martin, astrophysicist at the Strasbourg Astronomical Observatory and main author of the study published in the
Nature
journal
.
And for that, it is necessary to look very far in space and therefore in time, as the Hubble Space Telescope does and will soon do by James Webb.
Or else find the equivalent of "fossils" closer to us.
Stars formed over 12 billion years ago
The twenty stars identified by the international team led by Nicolas Martin are of this order. “They are among the very first to have formed in the Universe” “more than 12 billion years ago, maybe even more than 13”, that is to say a few hundred million years after the Big- bang. They extend in the shape of a "current" (called C19), a fragment of stars resulting from a stellar cluster "which passed too close to our galaxy and which it tore". This flap now stretches for thousands of light years, in the form of an appendage to the Milky Way's disk.
Astronomers have spotted these stars by their very low metallicity, a measure of the proportion of chemical elements heavier than hydrogen and helium contained in stars.
Because according to the theory of star formation, the first were only fueled by hydrogen and helium.
“As the successive generations of stars form, they create heavier chemical elements in their hearts,” explains the scientist.
When the star dies, these elements enrich the stellar gas, which will see the birth of other stars, enriched in turn with heavy elements.
An existence excluded by theory
Our sun, only 4.6 billion years old, is a case in point. It contains a little over 1.5% of these heavy elements, mainly carbon, oxygen and iron. The group of stars found by Nicolas Martin and his colleagues contains proportionately 2,500 times less. However, "current models of star formation do not seem to work at such low metallicities" for the creation of star clusters such as the one discovered by the team of scientists.
In other words, theory must now catch up with observation.
To date, only one other star cluster, with a metallicity much stronger than C19 but below the theoretical threshold, has been discovered in the Andromeda galaxy.
If the stars of C19 do not belong to the very first generation to appear in the Universe, they could "have been formed from gas contaminated by the very first".
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