Washington (AFP)

Seven billion years ago, before the existence of the Sun, stars were born. Two billion years later, they died, and dust from these stars, aggregated into a block, ended up falling 50 years ago in an Australian village.

Scientists at the Field Museum in Chicago have owned a piece of this large meteorite that fell in Murchison in September 1969 for five decades. 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 had discovered micrograins of an unprecedented type, presumably presolate, but which they had not been able to 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 the old grains from the young (everything is relative), the scientists reduced to a powder a piece of the meteorite, then they dissolved the fragments in acid, an operation which made appear the presolar grains, moreover of 4.6 billion years.

It's like burning the haystack to find the needle, explains in a pretty metaphor Jennika Greer, the student co-author of the study, published Monday in the journal PNAS.

When a dust is in space, it is exposed to cosmic rays, which slowly changes its composition. The more cosmic rays the grain has received, the more the elements that compose it change ... which makes it possible to date it. 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 and 4.9 billion years old.

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 7 billion years.

The new dating 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 producing dust," said Philipp Heck.

Now charge to use the same method on other meteorites, but according to Philipp Heck there are less than five known and large enough in collections to deliver such secrets.

© 2020 AFP