Within our own Milky Way galaxy, an exceptionally pure ancient star has been identified, representing the most primordial stellar object discovered by astronomers to date.
This extraordinary celestial body is remarkably, though not completely, devoid of heavier elements, or “metals,” which are products of stellar evolution and subsequent supernovae. It stands as a relic from the nascent Universe, likely coalescing from primordial gas that had been enriched by one of the very first supernova events in cosmic history.
Once a low-mass star akin to our Sun, the object designated SDSS J0715-7334 has exhausted its main-sequence fuel. It is now a red giant nearing the conclusion of its existence, having persisted long enough to offer profound insights into epochs long past.
“These unadulterated stars serve as invaluable windows into the epoch when stars and galaxies first emerged in the Universe,” stated Alexander Ji, a cosmologist at the University of Chicago and lead researcher on this project.
Following the Universe’s genesis in the Big Bang, the cosmos was filled with a superheated, dense plasma composed of fundamental atomic nuclei and free electrons.
Any nascent light would have been unable to traverse this dense fog; photons would have been continuously scattered by the ambient electrons, effectively rendering the early Universe opaque.
However, by approximately 300,000 years post-Big Bang, the Universe had cooled sufficiently for protons and electrons to combine, forming neutral hydrogen and a minor quantity of helium. It was from dense concentrations within this pristine hydrogen and helium gas that the very first generation of stars, known as Population III stars, are believed to have originated.
Elements heavier than helium did not become widely disseminated throughout the cosmos until after these initial stars had completed their life cycles.
Stars generate energy through nuclear fusion, a process where atomic nuclei merge to create heavier elements, beginning with hydrogen fusing into helium, then helium into carbon, and so on. In astronomical terminology, elements heavier than helium are classified as metals.
The fusion process intrinsically culminates at iron, as fusing it demands more energy than is released. Nevertheless, even heavier elements are synthesized within the cataclysmic supernova explosions that signify the demise of massive stars. These explosive events disperse heavy elements into interstellar space, where they can then be incorporated into the formation of subsequent stellar generations.
Every star subjected to analysis has exhibited some degree of this metallic enrichment; however, the extent varies considerably. Those with the lowest metallicity, designated as Population II stars, possess such depleted chemical compositions that their enrichment can only be attributed to Population III stars.
“No Population III stars have ever been directly observed. This is possibly because they were massive, short-lived, and perished rapidly, or alternatively, the least massive Population III stars, which might have persisted to the present cosmic epoch, are exceedingly rare,” explained astronomer Kevin Schlaufman from Johns Hopkins University.
“Regardless of the reason, the characteristics of this inaugural stellar generation remain among the most significant unresolved questions in contemporary astrophysics.”
Consequently, Population II stars are highly prized by astronomers, who meticulously examine their chemical signatures to deduce information about the preceding stellar populations that synthesized them.
This brings us back to SDSS J0715-7334, discovered almost serendipitously by Ji and his students while they were engaged in identifying intriguing celestial objects using the Sloan Digital Sky Survey (SDSS) as part of their academic coursework.
During their inaugural observation session at the telescope, the second star they examined was SDSS J0715-7334. The initial plan was to observe it for a mere 10 minutes, but they ended up dedicating three hours to its study.
“I kept a close watch on the camera throughout the entire night to ensure its proper functioning,” recalled astronomer Natalie Orrantia, one of the students involved in the research.
Upon closer scrutiny, the star’s composition was revealed to be predominantly hydrogen and helium. Its metallicity measured at a mere 0.005 percent of that of the Sun, and it was nearly half the metallicity of the prior record-holder for low metal content.
A minuscule trace of iron was detectable in its spectrum – its total metallicity was 40 times lower than that of the next most iron-deficient star known. However, what truly astonished the research team was its astonishingly low concentration of carbon.
“The star contains such a negligible amount of carbon that it suggests an early deposition of cosmic dust played a role in its formation,” observed Ji. “This particular formation mechanism has only been documented once previously.”
Typically, the presence of specific elements like carbon or oxygen is essential for gas clouds to cool sufficiently to initiate star formation. The theorized formation pathway for Population III stars is thought to have relied on hydrogen molecules, which are less efficient cooling agents. However, once carbon emerged, it became the primary factor facilitating the cooling necessary for widespread star formation across the Universe.
The absence of significant carbon in SDSS J0715-7334’s spectral signature does not indicate cooling solely via pristine hydrogen, as was likely the case for the Universe’s very first stars.
Instead, its chemical makeup points towards formation within a peculiar intermediate regime, characterized by insufficient carbon for conventional cooling processes. In this scenario, minute quantities of cosmic dust, residual material from Population III supernovae, likely assisted the gas in collapsing.
“Nevertheless, a greater number of similarly metal-poor stars must be identified in diverse cosmic environments to rigorously test this hypothesis,” the research group, including Ji, detailed in their publication.
The star’s spatial coordinates and trajectory suggest it originated not from the Milky Way, but from the Large Magellanic Cloud, a satellite galaxy orbiting our own. This observation implies that the Large Magellanic Cloud might harbor a greater number of such undiscovered stars.
“It is plausible that we will detect a comparatively higher prevalence of ultra-metal-poor stars in galaxies like the Magellanic Clouds than within our own Milky Way Galaxy,” commented Schlaufman.
“There remains a substantial amount of work to be accomplished to fully comprehend the phenomena that transpired during that remote epoch when the Milky Way was in its infancy. We have merely begun to explore this fascinating area.”
This significant discovery has been formally announced in the esteemed journal Nature Astronomy.
