Stars are the original nuclear fusion reactors. As it evolves, a star continuously fuses hydrogen, helium, and—if it becomes big enough—heavier elements, releasing the energy created into space as light and heat. The earliest stars, however, were a little different. Astronomers believed that the universe’s first stars were more “pristine,” meaning that they consisted of only the lightest of elements.
For years, scientists thought it unlikely that such primordial stars still existed—until now. A team of astronomers led by Alexander Ji at the University of Chicago recently revealed that they had spotted a star, SDSS J0715-7334, that appears to have an unprecedentedly low concentration of metals (astronomy shorthand for elements other than hydrogen and helium). The star seems to be sitting in the halo of the Large Magellanic Cloud, a dwarf galaxy about 163,000 light-years from the Milky Way. The astronomers detailed the discovery in a paper uploaded to arXiv, which has yet to be peer-reviewed.
Tracing stellar ‘genetics’
Carl Sagan is famous for saying that we are made of “star stuff,” and at a glance, our evolution and that of stars’ appears surprisingly similar. Just like us, stars, too, belong to distinct “generations” that have slightly different elemental compositions. Scientific consensus has long held that the earliest stars, mainly composed of hydrogen and helium, eventually exploded into supernovae. These cataclysms essentially seeded our universe with element-rich gases that helped to birth the next generation of stars.
Yet astronomers believed we could still get a peek at the direct descendants of the very first stars—a group referred to as Population III—by searching for low-mass stars with low metallicity.
‘Pristine’ outlier
The team first detected SDSS J0715-7334 using data from the Sloan Digital Sky Survey, and then used the Magellan telescope in Chile to further observe the star. To be clear, this isn’t the first “pristine” star astronomers have found—the James Webb Space Telescope has proven particularly adept at picking out candidates for early-generation stars—but SDSS J0715-7334 sets itself apart from the rest, the researchers noted.
It is “over ten times more metal-poor” than previous Webb discoveries, “some of which have been claimed to be potentially metal-free,” the astronomers wrote. Not only that, but the star has extremely low levels of carbon, which may be a first for similar discoveries. Together, these features make it “the most pristine composition of any object known in the universe,” the researchers argued.
The low carbon levels are particularly interesting, as previously discovered “pristine” or metal-poor stars generally displayed high carbon levels. Astronomers believed that relatively heavier elements like carbon allow these stars to cool down and not explode.
The lack of carbon on SDSS J0715-7334 suggests that “different environments in different places in the universe cool their gas differently at early times,” Anna Frebel, an astronomer at MIT who was not involved in the new paper, told New Scientist. “We can ask the question, why do they cool it differently, but I don’t think we have a good answer to that,” she added.
To find out, the search for these kinds of primordial stars must continue, the researchers said.