DOI: 10.1038/s41550-024-02223-w
No one has been able to find any 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'
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 one 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
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
One of these stars immediately umped 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.
Chiti is currently leading an imaging program to map out a large
portion of the southern sky to find the earliest stars possible. "This
discovery suggests there should be many of these stars in the Large
Magellanic Cloud if we look closely," he said. "It's really exciting to
be opening up stellar archaeology of the Large Magellanic Cloud, and to
be able to map out in such detail how the first stars chemically enriched the universe in different regions."
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