Exceptional star is the most pristine object known in the universe

A star located fairly close to us, which appears to have almost zero heavy elements created by supernovae, might be a descendant of the universe’s earliest stars.

Astronomers propose that the first stars were primarily composed of the hydrogen and helium present right after the Big Bang. Only after these stars depleted their fuel and exploded in supernovae were heavier elements than helium dispersed. The gas enriched with elements from these explosions subsequently formed new generations of stars, continuing a cycle that eventually led to the creation of all the elements found in contemporary stars and planets.

While most stars in our galaxy are far removed from the original generation of stars, some astronomers, referred to as ““stellar archaeologists”, have located stars that are nearly spotless. These stars are believed to be “second generation,” originating from the remnants of the very first stars.

Recently, Alexander Ji and his team at the University of Chicago identified a star that holds the record for the lowest metal content – which astronomers define as all elements besides hydrogen or helium – known in the universe. Named SDSS J0715-7334, this star is situated in the Large Magellanic Cloud, a neighboring satellite galaxy of the Milky Way, and possesses a metal content of approximately 0.8 parts per million, about 20,000 times lower than that of our sun.

After detecting the star’s exceptionally low metallicity in data from the Sloan Digital Sky Survey, Ji’s team subsequently observed it with the Magellan telescope at Las Campanas Observatory in Chile. Their findings revealed very low iron levels, similar to those observed in other almost pristine stars. Remarkably, they also detected extremely low carbon levels, nearly absent compared to stars within the Milky Way.

“This is an exciting discovery, but [regarding iron levels] it’s just slightly more extreme than other examples we’ve encountered,” remarks Anke Ardern-Arentsen at the University of Cambridge. “However, what’s particularly intriguing is that most known nearly pristine stars possess a significant amount of carbon, yet this one does not.”

This observation raises questions about how it formed, possibly indicating a distinct formation process compared to near-pristine stars found in the Milky Way, suggests Anna Frebel from MIT.

Creating a star as large as SDSS J0715-7334 generally requires a relatively cool and small gas clump, often needing heavier elements with high-energy electrons, like carbon, to facilitate cooling. However, the star’s minimal carbon presence complicates this cooling process.

One alternative possibility involves a cloud of cosmic dust composed of heavier elements that aided in its cooling, a scenario not typically observed at such an early stage in the universe’s development, at least not within our galaxy.

“This leads to the question: do different regions of the universe cool their gases in varied manners during early epochs?” Frebel notes. “We might ponder why these environments differ in their cooling processes, but we currently lack a satisfactory explanation.”