How heavy elements formed in early history of Milky Way

Researchers at the Niels Bohr Institute have shed light on the mystery of the existence of large amounts of heavy elements in the ancient stars of the Milky Way.

Some of the oldest stars in the Milky Way—a kind of stellar fossils in the outer reaches of our galaxy, contain abnormally large amounts of heavy elements like gold, platinum and uranium.

The Niels Bohr researchers have been studying these ancient stars for several years with ESO’s giant telescopes in Chile in order to trace the origin of these heavy elements and with recent observations they have concluded how they could have been formed in the early history of the Milky Way.

They explained that the mysterious dark matter along with hydrogen and helium dominated the universe shortly after the Big Bang.

As the dark matter and gasses clumped together under their own gravity, they formed the first stars.

In the scorching interior of these stars, hydrogen and helium melted together and formed the first heavier elements like carbon, nitrogen and oxygen, and after a ‘short’ while (a few hundred million years) all of the known elements were in place.

Every time a massive star burns out and dies in a violent explosion known as a supernova, it releases clouds of gas and newly formed elements out into space, where the gas clouds contract again and finally collapse and form new stars.

In this way, the new generations of stars become richer and richer in heavy elements.

“In the outer parts of the Milky Way there are old ‘stellar fossils’ from our own galaxy’s childhood. These old stars lie in a halo above and below the galaxy’s flat disc,” said Terese Hansen, who is an astrophysicist in the research group Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen.   

“In a small percentage— approx. 1-2 per cent of these primitive stars, you find abnormal quantities of the heaviest elements relative to iron and other ‘normal’ heavy elements,” noted Hansen.   

She explained that there are two theories that can explain the early stars' overdose of heavy elements.

One theory is that these stars are all close binary star systems where one star has exploded as a supernova and has coated its companion star with a thin layer of freshly made gold, platinum, uranium and so on.

The other theory is that early supernovae (exploding giant stars) could shoot the heavy elements out in jets in different directions, so these elements would be built into some of the diffuse gas clouds that formed some of the stars we see today in the galaxy’s halo.

The results are published in prestigious Astrophysical Journal Letters.