ESO finds 'impossible' star in Leo

The European Space Agency announced a team using the Very Large Telescope (VLT) recently zoomed in on a faint Milky Way star in the constellation Leo that has so few elements it falls into the "forbidden zone" of a widely accepted star formation theory.

Composed mostly of hydrogen and helium, with only the tiniest traces of heavier elements, the star SDSS J102915+172927 is probably more than 13 billion years old, yet would have been thought impossible by many scientists, according to the ESO astronomers, whose findings appeared September 1 in the journal Nature. The researchers found this visually unremarkable galactic halo star, glowing faintly in the Milky Way's Lion, chemically bizarre, because it contains the lowest amounts of metals of any star studied to date. The astronomers used the VLT's X-shooter and UVES instruments to analyze the unusual chemical composition of SDSS J102915+172927, and found the proportion of metals in the star was at least 20,000 times lower than the Sun's. According to the researchers, most cosmologists believe the lightest elements, hydrogen and helium, formed immediately after the Big Bang along with some lithium, while most of the heavier elements were generated by stars later, then dispersed by supernova explosions, so that newer stars formed within the element-enriched interstellar medium since then have higher proportions of metals. ESO astronomer Lorenzo Monaco explained, “The star we have studied is extremely metal-poor, meaning it is very primitive. It could be one of the oldest stars ever found.” The star's extremely low lithium content also surprised the team, because scientists expect the oldest stars to be composed mostly of hydrogen, helium and lithium, plus a few metals, matching the widely theorized composition of the earliest Universe. “It is a mystery how the lithium that formed just after the beginning of the Universe was destroyed in this star, " project supervisor Piercarlo Bonifacio remarked. But the team concluded there may be many "freakish" stars like this out there. Lead author Elisabetta Caffau stated, “We have identified several more candidate stars that might have metal levels similar to, or even lower than, those in SDSS J102915+172927. We are now planning to observe them with the VLT to see if this is the case.”