Scientists find one of the most ancient stars that formed in another galaxy

The first generation of stars transformed the universe. Inside their cores, simple hydrogen and helium fused into a rainbow of elements. When these stars died, they exploded and sent these new elements across the universe. The iron running in your veins and the calcium in your teeth and the sodium powering your thoughts were all born in the heart of a long-dead star.

No one has been able to find one of those first generation of stars, but scientists have announced a unique finding: a star from the second generation that originally formed in a different galaxy from ours.

"This star provides a unique window into the very early element-forming process in galaxies other than our own," said Anirudh Chiti, a University of Chicago postdoctoral fellow and first author on a paper announcing the findings. "We have built up an idea of the how these stars that were chemically enriched by the first stars look like in the Milky Way, but we don't yet know if some of these signatures are unique, or if things happened similarly across other galaxies."

The paper was published March 20 in Nature Astronomy.

'Fishing needles out of haystacks'

Chiti specializes in what is called stellar archaeology: Reconstructing how the earliest generations of stars changed the universe. "We want to understand what the properties of those first stars were and what were the elements they produced," said Chiti.

But no one has yet managed to directly see these first-generation stars, if any remain in the universe. Instead, Chiti and his colleagues look for stars that formed from the ashes of that first generation.

It's hard work, because even the second generation of stars is now incredibly ancient and rare. Most stars in the universe, including our own sun, are the result of tens to thousands of generations, building up more and more heavy elements each time. "Maybe fewer than 1 in 100,000 stars in the Milky Way is one of these second-gen stars," he said. "You really are fishing needles out of haystacks."

But it's worth it to get snapshots of what the universe looked like back in time. "In their outer layers, these stars preserve the elements near where they formed," he explained. "If you can find a very old star and get its chemical composition, you can understand what the chemical composition of the universe was like where that star formed, billions of years ago."

An intriguing oddity

For this study, Chiti and his colleagues aimed their telescopes at an unusual target: the stars that make up the Large Magellanic Cloud.

The Large Magellanic Cloud is a bright swath of stars visible to the naked eye in the Southern Hemisphere. We now think it was once a separate galaxy that was captured by the Milky Way's gravity just a few billion years ago. This makes it particularly interesting because its oldest stars were formed outside the Milky Way -- giving astronomers a chance to learn about whether conditions in the early universe all looked the same, or were different in other places.

The scientists searched for evidence of these particularly ancient stars in the Large Magellanic Cloud and catalogued ten of them, first with the European Space Agency's Gaia satellite and then with the Magellan Telescope in Chile.

One of these stars immediately jumped out as an oddity. It had much, much less of the heavier elements in it than any other star yet seen in the Large Magellanic Cloud. This means it was probably formed in the wake of the first generation of stars -- so it had not yet built up heavier elements over the course of repeated star births and deaths.

Mapping out its elements, the scientists were surprised to see that it had a lot less carbon than iron compared to what we see in Milky Way stars.

"That was very intriguing, and it suggests that perhaps carbon enhancement of the earliest generation, as we see in the Milky Way, was not universal," Chiti said. "We'll have to do further studies, but it suggests there are differences from place to place.

"I think we're filling out the picture of what the early element enrichment process looked like in different environments," he said.

Their findings also corroborated other studies that have suggested that the Large Magellanic Cloud made much fewer stars early on compared to the Milky Way.

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Ancient stars made extraordinarily heavy elements

How heavy can an element be? An international team of researchers has found that ancient stars were capable of producing elements with atomic masses greater than 260, heavier than any element on the periodic table found naturally on Earth. The finding deepens our understanding of element formation in stars.

We are, literally, made of star stuff. Stars are element factories, where elements constantly fuse or break apart to create other lighter or heavier elements.

When we refer to light or heavy elements, we're talking about their atomic mass.

Broadly speaking, atomic mass is based on the number of protons and neutrons in the nucleus of one atom of that element.

The heaviest elements are only known to be created in neutron stars via the rapid neutron capture process, or r-process.

Picture a single atomic nucleus floating in a soup of neutrons.

Suddenly, a bunch of those neutrons get stuck to the nucleus in a very short time period -- usually in less than one second -- then undergo some internal neutron-to-proton changes, and voila!

A heavy element, such as gold, platinum or uranium, forms.

The heaviest elements are unstable or radioactive, meaning they decay over time.

One way that they do this is by splitting, a process called fission.

"The r-process is necessary if you want to make elements that are heavier than, say, lead and bismuth," says Ian Roederer, associate professor of physics at North Carolina State University and lead author of the research.

Roederer was previously at the University of Michigan.

"You have to add many neutrons very quickly, but the catch is that you need a lot of energy and a lot of neutrons to do so," Roederer says.

"We have a general idea of how the r-process works, but the conditions of the process are quite extreme," Roederer says.

"We don't have a good sense of how many different kinds of sites in the universe can generate the r-process, we don't know how the r-process ends, and we can't answer questions like, how many neutrons can you add? Or, how heavy can an element be? So we decided to look at elements that could be made by fission in some well-studied old stars to see if we could start to answer some of these questions."

The team took a fresh look at the amounts of heavy elements in 42 well-studied stars in the Milky Way.

The stars were known to have heavy elements formed by the r-process in earlier generations of stars.

By taking a broader view of the amounts of each heavy element found in these stars collectively, rather than individually as is more common, they identified previously unrecognized patterns.

Those patterns signaled that some elements listed near the middle of the periodic table -- such as silver and rhodium -- were likely the remnants of heavy element fission.

The team was able to determine that the r-process can produce atoms with an atomic mass of at least 260 before they fission.

"That 260 is interesting because we haven't previously detected anything that heavy in space or naturally on Earth, even in nuclear weapon tests," Roederer says.

"But seeing them in space gives us guidance for how to think about models and fission -- and could give us insight into how the rich diversity of elements came to be."

Read more at Science Daily

Ancient star explosions revealed in deep-sea sediments

 A mystery surrounding the space around our solar system is unfolding thanks to evidence of supernovae found in deep-sea sediments.

Professor Anton Wallner, a nuclear physicist at ANU, led the study which shows Earth has been travelling for the last 33,000 years through a cloud of faintly radioactive dust.

"These clouds could be remnants of previous supernova explosions, a powerful and super bright explosion of a star," Professor Wallner said.

Professor Wallner conducted the research at the ANU Heavy Ion Accelerator Facility (HIAF). He also holds joint positions at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and Technical University Dresden (TUD) in Germany.

The researchers searched through several deep-sea sediments from two different locations that date back 33,000 years using the extreme sensitivity of HIAF's mass spectrometer. They found clear traces of the isotope iron-60, which is formed when stars die in supernova explosions.

Iron-60 is radioactive and completely decays away within 15 million years, which means any iron-60 found on Earth must have been formed much later than the rest of the 4.6-billion-year old earth and arrived here from nearby supernovae before settling on the ocean floor.

Professor Wallner previously found traces of iron-60 at about 2.6 million years ago, and possibly another at around 6 million years ago, suggesting earth had travelled through fallout clouds from nearby supernovae.

For the last few thousand years the solar system has been moving through a denser cloud of gas and dust, known as the local interstellar cloud, (LIC), whose origins are unclear. If this cloud had originated during the past few million years from a supernova, it would contain iron-60, and so the team decided to search more recent sediment to find out.

Sure enough, there was iron-60 in the sediment at extremely low levels -- equating to radioactivity levels in space far below Earth's natural background levels -- and the distribution of the iron-60 matched earth's recent travel through the local interstellar cloud. But the iron-60 extended further back and was spread throughout the entire 33,000 year measurement period.

The lack of correlation with the solar system's time in the current local interstellar cloud seems to pose more questions than it answers. Firstly, if the cloud was not formed by a supernova, where did it come from? And secondly, why is there iron-60 so evenly spread throughout space?

"There are recent papers that suggest iron-60 trapped in dust particles might bounce around in the interstellar medium," Professor Wallner said.

"So the iron-60 could originate from even older supernovae explosions, and what we measure is some kind of echo.

Read more at Science Daily