7 billion years ago, long before the birth of the Sun (appeared 4.6 billion years ago), stars were born. Two billion years later, they died, and dust from these stars, aggregated into a block, ended up falling fifty years ago in an Australian village.
Scientists at the Field Museum in Chicago have owned for five decades a piece of this big meteorite, fallen in September 1969 in Murchison. It is one of the most studied cosmic pebbles in the world by astrophysicists and cosmochemists, who analyze it from all angles as a historical capsule. In 1987, they discovered micrograins of an unprecedented type, presumably 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, made of silicon carbide. To distinguish the ancient grains from the young, the scientists first powdered a piece of the meteorite, then dissolved the fragments in acid, an operation that exposed the presolate grains. “It’s like burning the haystack to find the needle”, says in a pretty metaphor Philipp Heck, quoted in a press release accompanying the study, published Monday January 13 in the Proceedings of the American Academy of Sciences (PNAS).
Cosmic rays
The dating method starts from the following observation: when a dust is in space, it is exposed to cosmic rays. “Some of these cosmic rays interact with matter and form new elementssays Philipp Heck. And the longer it is exposed, the more these elements are formed. “I compare that to placing a bucket in a rainstorm. Assuming the rain is constant, the amount of water that collects in the bucket tells you how long it has been exposed. “, he adds. By measuring how many of these new elements are present in a presolar grain, we can tell how long it has been exposed to cosmic rays, which gives its age.
While only 20 grains of this meteorite had been dated by another method ten years ago, the researchers managed to date 40 micrograins, most of which were between 4.6 billion and 4.9 billion years old. These ages correspond to the moment when the dying stars began to release their material into space. This type of stars having a lifespan of about 2 billion to 2.5 billion years, we thus obtain a maximum age of 7 billion years for these dusts.
From these results, the researchers hypothesize that this period corresponds to an episode of increased stellar formation. This is a new element which could relaunch the debate between scientists on the question of whether new stars are born at a regular rate or whether there are peaks and troughs in the number of new stars.
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