The Murchison meteorite - MARY EVANS / SIPA

It is one of the most studied stones in the world by astrophysicists and "cosmochemists". In September 1969, a large meteorite fell in Murchison, Australia. This cosmic piece, which is in the Field Museum in Chicago, is actually composed of dust of stars born seven billion years ago - that is before the existence of the Sun - and died two billion years later, indicates a study published this Monday in the journal Pnas .

4.6-year-old micrograins

In 1987, the scientists had discovered in this piece micrograins of a new type, undoubtedly presolate, but which they could not date. Recently, the museum's meteorite curator, Philipp Heck, used a new method with colleagues to date these micrograins, formed from silicon carbide, the first mineral that forms when a star cools.

To distinguish old grains from young people, scientists ground a piece of the meteorite to powder, then dissolved the fragments in acid. This operation revealed presolar grains, most of which were between 4.6 and 4.9 billion years old.

A baby boom of stars before our Sun

These ages correspond to the moment when the stars started to fall apart. This type of stars having a lifespan of about 2 to 2.5 billion years, we go back to seven billion years. The new dating determined by this team thus confirms an astronomical theory which predicted a "baby boom" of stars before the formation of our Sun, instead of a constant rate of stellar births.

"At one time, more stars formed than normal, and at the end of their life, they started to produce dust," says Philipp Heck. Now charge to use the same method on other meteorites. But according to the scientist, there are less than five known and large enough in collections to reveal such secrets.


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