Not all archaeologists study ancient pottery and arrowheads. If you’re a stellar archeologist, you seek the oldest stars in the universe—those born long before our own sun and planet came into being.
A group led by University of Chicago scientists has discovered a star that appears to date back to the second generation of stars ever formed. Still inside the tiny primordial galaxy where it was first born, it has a unique elemental makeup that can tell us about these early days of the universe—as well as how most of our periodic table came to exist.
“This is the first really clear detection of which elements are initially produced in primordial galaxies,” said Anirudh Chiti, the first author of the study, a postdoctoral researcher at UChicago at the time of the work and now at Stanford University. “It’s a nice missing piece of the puzzle about how elements were formed back in those early days.”
The study is published March 16 in Nature Astronomy.
In the hearts of these massive stars, atoms were fusing to become increasingly heavy elements. Hunting for the most ancient stars In those early days after the Big Bang, the universe was a lot less interesting than it is now.
There were stars, but they were all the same kind of massive star made of three elements—hydrogen, helium and lithium—because those were the only elements that existed. You wouldn’t be able to find any of the calcium, gold or other elements that make up our world today, because those elements first had to be forged inside the stars themselves.
In the hearts of these massive stars, atoms were fusing to become increasingly heavy elements. When those stars exploded at the end of their lives, new stars formed from the debris, and the process happened over and over until we got the full range of elements that we know and love today.
Scientists have pieced together this broad outline from other evidence. But no one had yet been able to find a star from this period still in its original galaxy, which would help untangle remaining questions about this process of element formation.
Hunting for such ancient stars is the specialty of Alexander Ji, a UChicago assistant professor of astronomy and astrophysics.
“To find them, what you want to do is look for the stars with the lowest amount of heavy elements,” he explained, because the heavier elements only built up with time.
Today’s telescopes allow scientists to not only spot a star, but also measure its elemental makeup by separating out the wavelengths of light coming from it.
Chiti had led a star survey with the Dark Energy Camera, so they combed the map for potential candidates. Then they used the Magellan Telescopes at Las Campanas Observatory and the European Southern Observatory’s Very Large Telescope to delve deeper into the makeup of a handful of the most promising candidates.
One stood out.
The star is in a tiny, 10-billion-year galaxy, located in the Pictor constellation and known as Pictor II. This star, called PicIII-503, has a very distinct makeup compared to modern stars; for example, it contains about 100,000 times less iron than our sun does.
This rare finding is exciting, the scientists said, but also sheds light on a long-standing stellar mystery about how these early stars formed.A supernova, but weak As astronomers began looking for ancient stars, they noticed older stars tended to have far more carbon compared to their modern counterparts.
Scientists had theories about how this might have happened, but most of these stars were spotted after they had already been moved from where they were born—our Milky Way galaxy has been sucking up smaller stars and galaxies for billions of years.
That made it hard to confirm or rule out these theories. It’s a bit like Earth archaeologists looking at a piece of ancient pottery that had already been dug up at some point by graverobbers and sold as a curio; it still gives you some information, but without the original context, you can’t say much about the people who made it.
But because PicIII-503 is still in its original tiny, primordial galaxy, scientists could see that its composition gave weight to one particular formation theory—which has to do with how the parent star explodes.
At the end of a really massive star’s life, Ji explained, “it has this onion-skin structure, with the lighter elements like carbon in the outer layers, and the heavier ones inside. Then when the star dies, it might be a very weak explosion where only the lightest outer layers get ejected.”
A highly powerful explosion would have flung the star’s guts far away, out of the bounds of the small galaxies that populated the universe back then. But a weaker explosion could mean the debris stuck around to become part of the next generation of stars.
“It’s a really nice finding because we have seen a lot of these carbon-rich stars in our own Milky Way galaxy, and now we can see how these stars likely originated,” said Chiti.
The team is now mapping other tiny primordial galaxies, hoping to find more of these ancient stars and learn how the process might have varied across different environments.
Other UChicago authors on the study included postdoctoral researcher Guilherme Limberg and Assoc. Prof. Alex Drlica-Wagner. The study also used data taken by the Dark Energy Survey and the European Space Agency’s Gaia mission.
