Supernovae jets might explain ancient stars rich in heavy elements

The explosive jets of supernovae might explain why some Milky Way stars are mysteriously rich in gold, platinum and uranium.

Researchers at the Niels Bohr Institute may have solved a mystery about ancient stars of the outer Milky Way galaxy. These stars are abnormally rich in heavy elements like gold, platinum and uranium – heavy elements normally seen in later generations of stars. The researchers believe the heavy elements in these very old stars originated in the explosive jets of supernovae. The supernovae jets might have enriched gas clouds with the heavy elements that later formed these stars.

NGC 4594, a disc-shaped spiral galaxy with around 200 billion stars. The Milky Way is a spiral galaxy, like NGC 4594. Above and below the galactic plane of both NGC 4594 and the Milky Way, there is a halo that includes older stars dating back to the galaxy’s childhood billions of years ago. In principle, the halo stars should all be primitive and poor in heavy elements like gold, platinum and uranium. But they are not. New research shows that the explanation might lie in violent jets from exploding giant stars. Image Credit: ESO

The research team observed 17 stars in the northern sky with European Southern Observatory (ESO) telescopes and with the Nordic Optical Telescope (NOT). Results of the study were published in The Astrophysical Journal Letters on November 14, 2011.

The 17 stars in the study are small, light stars, which live longer than large massive stars. They do not burn hydrogen longer but swell up into red giants that will later cool and become white dwarves. This image shows CS31082-001. Via Niels Bohr Institute

Shortly after the Big Bang, the universe is thought to have been dominated by mysterious dark matter along with the light elements hydrogen and helium. As the dark matter and gasses composed of hydrogen and helium clumped together via their own gravity, they formed the first stars.

In the scorching interior of these stars, thermonuclear fusion of hydrogen and helium formed the first heavier elements like carbon, nitrogen and oxygen. This process of fusion is what enables all the stars to shine, and the build-up of heavier elements from lighter ones is what gives us the diverse array of matter around us on Earth and in space today. Within a few hundred million years of the universe’s birth, all of the known elements are thought to have formed – but only in minute quantities. Thus the earliest stars should contain only a thousandth of the heavy elements seen today in later-generation stars, like our own sun.

Every time a massive star burns out and dies in a violent explosion known as a supernova, it spews newly formed heavy elements out into space. The heavy elements become part of vast clouds of gas, which ultimately contract and finally collapse to form new stars. In this way, the new generations of stars become richer in heavy elements.

Our Milky Way galaxy, seen from the inside. Image Credit: Steve Jurvetson

It is therefore surprising to find stars from the early universe that are relatively rich in the very heaviest elements. But they exist – even in our own galaxy, the Milky Way.

Terese Hansen, of the Niels Bohr Institute at the University of Copenhagen, said:

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. In a small percentage -– approximately 1-2 percent of these primitive stars — you find abnormal quantities of the heaviest elements relative to iron and other ‘normal’ heavy elements.

Hansen said 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 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.

She said:

My observations of the motions of the stars showed that the great majority of the 17 heavy-element rich stars are in fact single. Only three (20 percent) belong to binary star systems. This is completely normal; 20 percent of all stars belong to binary star systems. So the theory of the gold-plated neighboring star cannot be the general explanation. The reason why some of the old stars became abnormally rich in heavy elements must therefore be that exploding supernovae sent jets out into space. In the supernova explosion the heavy elements like gold, platinum and uranium are formed, and when the jets hit the surrounding gas clouds, they will be enriched with the elements and form stars that are incredibly rich in heavy elements.

Bottom line: A Niels Bohr Institute study published in The Astrophysical Journal Letters on November 14, 2011 reveals that ancient stars in the outer halo of our Milky Way galaxy – which are abnormally rich in heavy elements like gold, platinum and uranium – might have resulted from the explosive jets of supernovae. In this scenario, the supernovae jets would have enriched gas clouds with heavy elements that later formed these stars.

Read more at Niels Bohr Institute

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