Primitive star could shed light on the dawn of the universe

On the fringes of the Milky Way, near a satellite galaxy called the Large Magellanic Cloud, researchers have discovered the most metal-poor, chemically primitive star ever found, according to new research.

Composed primarily of hydrogen and helium and containing less than 0.005% of the metals in the sun, the chemical makeup of the star SDSS J0715-7334 is the closest analog yet found to the first stars that formed in our universe.

“These pristine stars are windows into the dawn of stars and galaxies in the universe.”

Studying this low-mass, ultra-metal-poor star could help clarify astronomers’ ideas about this first generation of stars, called Population III stars, which astronomers cannot observe directly.

“No Population III stars have ever been observed, either because they were massive, lived fast and died young, or the lowest-mass Population III stars that could persist to the present day are extremely rare. Either way, the properties of this first stellar generation are some of the most important unknowns in modern astrophysics,” says coauthor Kevin Schlaufman, an associate professor of physics and astronomy at Johns Hopkins University.

Schlaufman originally identified SDSS J0715-7334 as a star of interest in 2014 for follow up as part of the current fifth generation of the Sloan Digital Sky Survey.

“While this star does not have a primordial composition itself, it is the closest astronomers have ever gotten to the Population III stellar generation on this particular metric,” he says.

SDSS J0715-7334 was formed from a gas cloud that had recently interacted with the material ejected by a Population III star’s supernova. Working backwards, astronomers can use the ratios of the elements in SDSS J0715-7334 to explore the mass of that Population III star and the energy of its supernova explosion.

“These pristine stars are windows into the dawn of stars and galaxies in the universe,” says first author Alexander Ji, an assistant professor of astronomy and astrophysics at University of Chicago.

A team of astronomers analyzed data gathered with the Magellan Clay Telescope and its high-resolution Magellan Inamori Kyocera Echelle spectrograph to determine that SDSS J0715-7334 is almost entirely hydrogen and helium with only trace amounts of carbon and iron.

The composition of SDSS J0715-7334 indicates that the Population III star that produced its carbon and iron was both unusually massive and exploded with uncommon vigor, the researchers say.

SDSS J0715-7334 is roughly 80,000 light years away in the vicinity of the Large Magellanic Cloud, the largest of the 100-200 small satellite galaxies that orbit the Milky Way. The Magellanic Clouds have only recently joined the Milky Way, and their long history of living alone has allowed them to ingest material from the cosmic web for a longer period than the Milky Way. Those conditions may have promoted the production of low-metallicity stars like SDSS J0715-7334.

“It’s possible that we’re going to find a relatively higher proportion of ultra-metal-poor stars in galaxies like the Magellanic Clouds than in our own Milky Way Galaxy,” Schlaufman says.

As part of the Sloan Digital Sky Survey, the researchers will continue to study the Milky Way’s formation and evolution, with Schlaufman leading an effort to study the oldest stars in the Milky Way.

“There is still lots to be done to understand what actually was going on in that era long, long ago when the Milky Way was young,” Schlaufman says. “We’ve only scratched the surface with this current phase of the Sloan Digital Sky Survey.”

Findings from the survey appear in the journal Nature Astronomy.

Additional coauthors are from the Harvard & Smithsonian Center for Astrophysics, the University of Chicago, the Max Planck Institute of Astronomy, Vanderbilt University, The Ohio State University, University of Florida, Monash University, Space Telescope Science Institute, Yale University, Universidad Católica del Norte, The University of Texas, Carnegie Institution for Science, Sorbonne Université, Heidelberg University, Eötvös Loránd University, Sean University of Illinois at Urbana-Champaign, Montana State University, the University of Colorado, and The Pennsylvania State University.

Support for the research came from from the National Science Foundation, Alfred P. Sloan Research Fellowship, Max Planck Society, European Research Council, NASA, Agence Nationale de la Recherche, Gruber Science Fellowship, ANID, Joint Committee ESO-Government of Chile, Hungarian Academy of Sciences, Kavli Institute for Cosmological Physics, UChicago Data Science Institute, and The University of Chicago.

Source: Johns Hopkins University

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