Rare Earth Element Discovered In Ancient Stars

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In three stars far, far away, one of the rarest elements known to man has been spotted, improving our understanding about how heavy elements are created, bolstering evidence that a rare type of supernova may have been responsible for their creation.

Tellurium — a brittle and toxic semiconducting metal — has for the first time been discovered in the atmospheres of three stars that are nearly 12 billion years old. The stars, all a few thousand light-years from Earth, live inside the Milky Way.

With the help of the Hubble Space Telescope, astronomers from MIT and other institutions were able to “see” tellurium by the light it absorbs.

The wavelength of starlight tellurium absorbs is in the ultraviolet part of the electromagnetic spectrum — the kind of light that is readily absorbed by our atmosphere. Therefore, ground-based observatories couldn’t detect tellurium even if they wanted to; Hubble, orbiting high above our atmosphere, was essential for this study.

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“We want to understand the evolution of tellurium — and by extension any other element — from the Big Bang to today,” said Anna Frebel, an assistant professor of astrophysics at MIT. “Here on Earth, everything’s made from carbon and various other elements, and we want to understand how tellurium on Earth came about.”

Immediately after the Big Bang, some 13.75 billion years ago, only hydrogen, helium and lithium existed.

As the Universe cooled and clouds of gas collapsed under gravity, heavier elements than lithium flashed into existence inside the cores of baby stars around 300 million years after the Big Bang. But the heaviest elements can only be created by the violent death of stars: supernovae.

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But if tellurium is present in these three ancient stars, the rare earth element was created 12 billion years ago, meaning it must have been produced by a rapid and rare kind of supernova. Tellurium was then flung into space and eventually went on to form other stars, laced with other star-forming elements.

“You can make iron and nickel in any ordinary supernova, anywhere in the universe,” said Frebel in an MIT press release. “But these heavy elements seem to only be made in specialized supernovas. Adding more elements to the observed elemental patterns will help us understand the astrophysical and environmental conditions needed for this process to operate.”

Astrophysicists theorize that any elements heavier than iron were created via core-collapse supernovae. Such supernovae create the extreme conditions necessary for rapid neutron capture — commonly known as the “r-process” (“r” is for “rapid”) — a process thought to explain neutron-rich elements in the periodic table.

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Discovering rare, heavy elements in ancient stars therefore provides evidence that rapid core collapse supernovae happened in the very early Universe, seeding space with rare earth elements. These elements were then laced with ancient stars (as observed by Frebel’s team) and can be found in small quantities in planets such as ours.

However, as tellurium has been so hard to detect elsewhere in the galaxy, its abundance (and therefore formation process) has been a mystery until now and more work is needed to detect other — undiscovered — elements in the Milky Way. For example, tin is a hard element to spot and selenium (an element very similar to tellurium) has yet to be found anywhere in the Universe beyond the solar system.

“If you look at the periodic table, tellurium is right in the middle of these elements that are hard for us to measure,” said Jennifer Johnson, an associate professor of astronomy at Ohio State University. “If we need to understand how (the r-process) works in the universe, we really have to measure this part of the periodic table. It’s really cool that they got this element (tellurium) in this sea of unknown-ness.”

Studies such as this are fascinating in that they are finding clues not only to the formation processes surrounding rare earth elements in our Universe, they are adding a new thread to the fabric of our understanding of how everything we know and love came into being.

To quote Carl Sagan, “We’re made of star-stuff.”

Image: Supernovae remnants are laced with heavy elements. Credit: X-ray (NASA/CXC/MIT/D.Dewey et al. & NASA/CXC/SAO/J.DePasquale); Optical (NASA/STScI)

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