Cloud debris came from distant stars

We are stardust, according to hippy-era songsmith Joni Mitchell - and now scientists have a better idea where all that cosmic dust comes from.

It was forged by distant stars six times larger than the Sun that end their lives by blowing off their outer layers.

Scientists had always suspected that the Solar System's building blocks came from such stars, known as Asymptotic Giant Branch (AGB) stars, but proof was lacking.

They were able to stand up the theory by examining tiny amounts of the primordial dust grains still surviving in meteorites.

The chemical composition of the grains revealed important clues about the nuclear processes inside stars that led to their formation.

The Sun and its family of planets are thought to have formed from an interstellar cloud of dust and gas some 4.6 billion years ago.

While most of the original dust grains vanished in the course of creating the Solar System, a few remain embedded in meteorites to this day.

A first inspection seemed to suggest that the grains recovered from meteorites did not match those expected from AGB stars.

But this discrepancy was solved by taking into account the effect of nuclear reactions within AGB stars, which left behind a tell-tale chemical signal.

The Luna Collaboration study involved around 40 scientists from the UK, Italy, Germany and Hungary.

British researcher Professor Marialuisa Aliotta, from the University of Edinburgh, said: "It is a great satisfaction to know that we have helped to solve a long-standing puzzle on the origin of these key stardust grains.

"Our study proves once again the importance of precise and accurate measurements of the nuclear reactions that take place inside stars."

Project leader Dr Maria Lugaro, from Konkoly Observatory in Hungary, said: "The long-standing question of the missing dust was making us very uncomfortable: it undermined what we know about the origin and evolution of dust in the galaxy."

The findings are published in the journal Nature Astronomy.