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Scientists have discovered the remains of a star cluster dating back to the early ages of the universe, on the outskirts of the Milky Way, the existence of which was theoretically unlikely, according to the authors of a study published in the journal Nature.
“Studying the archeology of galaxies requires understanding what our galaxy has absorbed through time, and knowing its foundations,” said Nicolas Martin, astrophysicist at the Strasbourg Astronomical Observatory.
This entails looking very far into space and therefore in time, as the Hubble Space Telescope does and what the James Webb Telescope will soon do, or being able to find the equivalent of “fossils” at closer distances to Earth.
And the group of twenty stars spotted by the multinational team led by Nicolas Martin belongs to this category. The researcher explained that it is “among the first stars that were formed in the universe” “more than 12 billion years ago, and perhaps even more than 13 billion,” that is, hundreds of millions of years after the Big Bang.
These stars extend in the form of a “stream” (named C19), which is a strip of stars produced by a star cluster that “passed close to our galaxy and was torn apart,” the researcher said.
Today, this band extends over thousands of light years, in the form of an appendix to the disk of the Milky Way.
Astronomers discovered these stars thanks to their extremely low metallicity, which is a measure of the proportion of chemical elements heavier than hydrogen and helium in stars.
According to the theory of star formation, hydrogen and helium alone were feeding the first.
“Successive generations of stars produce during their formation heavier chemical elements in their cores,” said Martin, the lead author of the study, which was published Wednesday in the journal Nature. When the star dies, these elements enrich the stellar gas that witnesses the birth of other stars, which is also enriched with heavy elements.
Our relatively young Sun, which is 4.6 billion years old, is a good example, as it contains just over 1.5 percent of these heavy elements, most notably carbon, oxygen and iron.
The cluster of stars observed by Nicolas Martin and colleagues contains about 2,500 times less of these elements, noting that “current models of star formation do not seem to work with such low minerals” to form star clusters as those discovered by the team of scientists.
This means that theory has to keep up with observation now. To date, only one star cluster has been detected in the Andromeda galaxy, with a mineral much stronger than C19 but below the theoretical threshold.
Astronomers looked to determine C19 in data from the “Gala” satellite, which has mapped more than 1.5 billion stars in the galaxy to date.
They then combined stellar currents identified thanks to the Pristine program, which uses the Canada-France-Hawaii Telescope to measure the metallicity of the stars. The effort became international, with a series of observations to improve measurements using the two large telescopes Gemini North in Hawaii and GTC in the Canary Islands.
The team will now follow two leads, studying other previously observed stellar streams and refining measurements of C19’s chemical analysis, pending the launch of the European Very Large Telescope (ELT) in the future (within five years).
And if the C19 stars were not from the first generation that appeared in the universe, they could have been “formed from gas contaminated by the first stars”.
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