In the exurbs 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 from the Sloan Digital Sky Survey.
Findings from the survey are published in the journal Nature Astronomy.
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 the universe. Studying this low-mass, ultra-metal-poor star could help clarify astronomers' ideas about the 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," said co-author 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."
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," said 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 said.
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," said Schlaufman.
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 said. "We've only scratched the surface with this current phase of the Sloan Digital Sky Survey."
Authors include Vedant Chandra from the Harvard & Smithsonian Center for Astrophysics; Selenna Mejias-Torres, Zhongyuan Zhang, Hillary Diane Andales, Ha Do, Natalie Orrantia, Rithika Tudmilla, Pierre N. Thibodeaux, and Guilherme Limberg from the University of Chicago; Philipp Eitner, and Maria Bergemann from the Max Planck Institute of Astronomy; Keivan Stassun from Vanderbilt University; Madeline Howell, and Jennifer Johnson from The Ohio State University; Jamie Tayar from University of Florida; Andrew Casey and Riley Thai from Monash University; Joleen K. Carlberg from Space Telescope Science Institute; William Cerny from Yale University; José Fernández-Trincado from Universidad Católica del Norte; Keith Hawkins from The University of Texas; Juna Kollmeier from Carnegie Institution for Science; Chervin Laporte from Sorbonne Université; Tadafumi Matsuno from Heidelberg University; Szabolcs Mészáros from Eötvös Loránd University; Sean Morrison from University of Illinois at Urbana-Champaign; David Nidever from Montana State University; Guy Stringfellow from the University of Colorado; and Donald Schneider from The Pennsylvania State University.
