The survey, called the Treasury of Extremely Metal-Poor O Stars, or TEMPOS, used ultraviolet observations from the Hubble Space Telescope’s Cosmic Origins Spectrograph to study massive stars in nearby dwarf galaxies that serve as the best available analogs for stars in the early universe. Published on September 21, 2026, in The Astrophysical Journal Supplement Series, the project assembled the largest dataset of its kind ever collected, covering 29 massive stars spread across six local dwarf galaxies, each with a metallicity below one-fifth of the sun’s. Astronomers use the term metallicity to describe the abundance of elements heavier than hydrogen and helium, and the universe’s first galaxies contained far fewer of these heavy elements than mature galaxies do today.

“Webb opened up a whole bunch of new questions about the evolution of these early galaxies—they’re weird,” said Grace Telford, assistant professor in the Department of Physics & Astronomy at the University of Utah and lead author of the study. “That’s the scientific motivation behind the TEMPOS program: to help understand what is going on in these early galaxies.” The survey’s unprecedented scale matters because individual massive stars in galaxies beyond the Milky Way are extraordinarily faint targets. Each star required up to 35 hours of precious Hubble observing time, making the construction of a statistically meaningful sample an expensive and painstaking undertaking that previous efforts had never achieved.

The scientific case for studying these stars rests on their outsized influence. Massive stars, defined as those with masses more than ten times greater than the sun, are rare but powerful engines of galactic change. They burn hot, bright and fast, flood their surroundings with intense radiation, shed material through stellar winds, and ultimately die as supernova explosions that deposit enormous amounts of energy and chemical enrichment into the surrounding gas. “They govern the evolution of their host galaxies by heating and essentially regulating the gas that’s then available to cool and form into new stars,” Telford explained. In a young galaxy, this feedback helps determine how efficiently the galaxy converts its gas reservoir into new generations of stars.

The stars at the center of the survey belong to a particularly consequential class. O-type stars are among the biggest and brightest stars in the universe, and they live relatively short lives by stellar standards. Near the end of their lifetimes, the most massive of them evolve into Wolf-Rayet stars, which release huge quantities of mass into space through powerful stellar winds that strip away their outer layers and expose their hot inner interiors. Understanding how these winds behave at low metallicity is therefore essential for modeling how the first generations of massive stars enriched the cosmos and sculpted the environments in which later stars formed.

The physics of stellar winds is intimately tied to metallicity. Metal ions within a star’s atmosphere couple the outgoing radiation to the surrounding material, effectively allowing light to push gas off the stellar surface. Astronomers have long expected that stars born with fewer heavy elements should drive weaker winds and lose less mass over their lifetimes. The TEMPOS observations confirmed the expected overall trend: as metallicity decreases, the maximum speed of the stellar winds decreases as well. But the data also delivered a surprise. At the lowest metallicities, in stars with metallicity below about ten percent of the sun’s, the wind speeds declined much more sharply than extrapolations from higher-metallicity trends would predict.

“There’s sort of a smooth trend and then suddenly for lowest-metallicity stars, the wind speed really drops off,” Telford said. “I was so excited to find that fun surprise in the data.” The discovery carries significant implications. If extremely metal-poor stars lose less mass through weaker winds, they retain more of their original mass as they age, which alters how they evolve, how they die, and how much energy and material they inject into their host galaxies. Because massive stars regulate the gas around them, changes in their mass-loss behavior could cascade outward, affecting star formation rates, gas temperatures and the ionizing radiation output that shapes a galaxy’s observable signature in the early universe.

Beyond wind speeds, the survey probed the chemistry of stellar atmospheres in a way no previous study had managed. Iron may be the single most important element in massive star physics: it plays a key role in launching stellar winds, determining how a star evolves throughout its lifetime, and triggering the supernova explosions that end massive stars’ lives. Yet iron abundance is notoriously difficult to measure in metal-poor environments. Instead, astronomers often use oxygen in a galaxy’s gas as a proxy for overall metallicity, because oxygen ions produce bright, easily observed emission lines when illuminated by massive stars. The convenience comes with an assumption—that iron and oxygen abundances track each other perfectly—which is not guaranteed to hold.

To test that assumption, the TEMPOS team measured the strengths of hard-to-detect iron absorption features in the ultraviolet spectra, assessing how much light the iron removed from what would otherwise be a flat level of ultraviolet emission. The results showed that massive stars in more oxygen-rich galaxies tend to have much stronger iron absorption in their UV spectra than stars in oxygen-poor galaxies, and the variation in absorption strengths across the sample suggests that these metal-poor stars span a wide range of iron abundances. “This is the first time we’ve had the statistical power to see that trend across a large sample of stars in six galaxies, all with different chemical compositions,” Telford said. “TEMPOS gives us the foundation for determining how massive-star physics changes as iron abundance changes in the very low-metallicity regime.”

The work marks a decisive break from a long-standing bottleneck in the field. Telford had previously modeled three of the stars in the TEMPOS sample in detail, but such small numbers could never reveal population-level trends. “With only three, you don’t see these trends,” she said. “We’ve always just been stuck in this low number statistics regime, so this is our very best attempt to build a big enough sample to do something more useful.” The researchers are now combining the Hubble ultraviolet spectra with visible-light observations from the Keck Observatory in Hawaii, a pairing that will allow them to model the stars in greater detail and measure properties such as chemical abundances and wind-driven mass-loss rates. Meanwhile, the science-ready ultraviolet spectra will be made publicly available through the Mikulski Archive for Space Telescopes, giving the broader research community a resource for pursuing additional questions about massive stars and their role in galaxy evolution. As Webb continues to deliver puzzling portraits of the universe’s first galaxies, TEMPOS offers astronomers the stellar physics they will need to finally interpret what they are seeing.

Subject of Research: Ultraviolet spectroscopy of extremely metal-poor massive stars to model stellar winds and early galaxy evolution

Article Title: Galaxies in the early universe are weird. These stars may explain why

Article References: Galaxies in the early universe are weird. These stars may explain why. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: massive stars, stellar winds, metallicity, early galaxies, Hubble Space Telescope, James Webb Space Telescope, O-type stars, Wolf-Rayet stars, dwarf galaxies, iron abundance, supernovae, galaxy evolution

For decades, astrophysicists have explored the connection between LGRBs and broad-lined type Ic supernovae – a subclass specifically tied to stripped-envelope Wolf–Rayet progenitors. The most energetic explosions launch highly relativistic jets that break through the stellar envelope, emitting intense gamma-ray radiation observable across cosmological distances. Yet not all jets succeed in emerging; some are “choked” within their host stars, yielding softer transients such as X-ray flashes or weaker, low-luminosity gamma-ray bursts. This gradation in jet success hints at a more nuanced interplay of progenitor properties, jet dynamics, and circumstellar environments than previously considered, motivating ongoing investigations into the continuum of relativistic outflows and transient behaviors.

Amid these efforts, a mysterious class of extragalactic fast X-ray transients has emerged, perplexing researchers because their rapid, luminous flares span timescales from mere seconds up to thousands of seconds. Their origins remain enigmatic, straddling theoretical models of jet physics and explosion mechanisms. Recent observations have failed to fit these phenomena neatly into existing schemes describing LGRBs or standard X-ray flashes, prompting proposals of alternative physical channels or progenitor conditions responsible for such transients.

A breakthrough in this field has now come with the detection of a notably bright X-ray transient, designated EP240414a, by the Einstein Probe—an advanced space observatory specialized in all-sky monitoring of X-ray emissions. Significantly, this transient coincides spatially and temporally with the type Ic broad-lined supernova SN 2024gsa, located at a cosmological redshift of 0.401. The association between the transient and the supernova provides a critical observational cornerstone to deepen our understanding of the diverse explosion scenarios linked to massive star deaths.

EP240414a’s X-ray light curve reveals an energy spectrum sharply distinct from classical LGRBs or their softer, low-luminosity cousins. The emission is extremely soft, peaking at energies less than 1.3 keV, which situates it in a spectral regime atypical for known relativistic jet-powered explosions. This softness coupled with the transient’s evolution rules out conventional high-energy jet breakout models, suggesting either substantially different jet properties or additional environmental interactions influencing the emission characteristics.

In a coordinated multiwavelength campaign following the initial X-ray detection, astronomers employed optical and radio telescopes to scrutinize the aftermath of the explosion. Observations uncovered the presence of a weak relativistic jet interacting with an extended circumstellar shell enveloping the progenitor star. This scenario contrasts with classical LGRBs where ultra-powerful jets penetrate the star’s envelope, but instead evokes a picture of a less powerful engine driving a successful, albeit relatively weak relativistic outflow that energizes the surrounding material.

The progenitor star implicated in SN 2024gsa and its transient, EP240414a, is believed to be a Wolf–Rayet star with notably reduced core angular momentum compared to traditional LGRB progenitors. This deficiency in rotation could account for the jet’s diminished power and the resulting observational signatures. Stellar rotation is a crucial parameter in magnetorotational core-collapse models that generate the conditions necessary for ultra-relativistic jets, meaning that even moderate variations can dramatically alter the explosion’s nature and the observable transient’s characteristics.

The supernova itself was located on the outskirts of a massive galaxy, a position suggestive of progenitor formation and evolution pathways differing from those in the star-forming regions typically producing classical LGRB progenitors. Environmental factors such as metallicity, binarity, and stellar feedback may have influenced the evolution of this Wolf–Rayet star and its final collapse. Thus, EP240414a and its supernova challenge astronomers to reconsider the diversity of explosion engines active in the universe and their dependence on progenitor and galactic environments.

From a theoretical perspective, the existence of such weak relativistic jets with successful but diminished breakout capabilities broadens the landscape of core-collapse end states. It indicates that there may be a continuum of jet powers governed by progenitor core spin and magnetic field properties rather than a binary classification of successful versus failed jets. Consequently, the gamma-ray and X-ray transient zoo may be more diverse and nuanced, including fast X-ray transients powered by weak jets rather than the high-luminosity events dominating the classical picture.

Moreover, the discovery of EP240414a highlights the essential role of all-sky monitoring instruments like the Einstein Probe in uncovering new transient populations. The ability to detect soft X-ray transients and coordinate multiwavelength follow-up observations is crucial for piecing together the complex interplay of jet physics, explosion dynamics, and circumstellar interactions. Such instruments open new discovery space by capturing events that would otherwise escape detection due to their intermediate luminosities and unusual spectral properties.

In terms of astrophysical implications, understanding weak relativistic jets bears significance beyond stellar death. These jets may contribute to cosmic ray acceleration, enrichment of the interstellar medium, and feedback processes that regulate star formation. Their observed interactions with circumstellar shells also shed light on mass-loss histories of massive stars, an area critical for reconstructing the final stages of stellar evolution.

Furthermore, the connection between weak jets and properties of progenitor angular momentum poses stringent tests for models of angular momentum transport and loss in massive stars. It underscores the importance of magnetohydrodynamic simulations and stellar evolution calculations that incorporate rotation, magnetic fields, and binary interactions to predict explosion outcomes and transient classifications accurately.

EP240414a thereby serves as a crucial piece in the puzzle of massive star explosions, opening pathways for future surveys to identify similar weak relativistic jet events. With improved observational capabilities, this new class of transients may become critical benchmarks for understanding jet launching mechanisms, progenitor diversity, and explosion energetics.

Scientifically, these findings demonstrate how nuanced the classification of cosmic transients has become, signaling a shift from broad categorizations toward a multidimensional parameter space capturing variations in jet power, progenitor structure, and environmental context. The binary distinction of LGRBs and failed jets is softened by discoveries like EP240414a, encouraging refinement of theoretical frameworks to incorporate intermediate cases.

Looking ahead, the synergy between transient detection facilities, wide-band follow-ups, and theoretical advances will illuminate whether weak relativistic jets are common endpoints for a significant fraction of Wolf–Rayet stars. Such understanding may bridge the gap between high-energy astrophysics, stellar evolution, and cosmology, enriching our knowledge of how massive stars influence and illuminate the universe.

The study of EP240414a and SN 2024gsa also exemplifies how serendipitous discoveries can reshape astrophysical paradigms. It is an invitation to remain vigilant for unconventional signatures that challenge current models and expand the landscape of known cosmic explosions. In this spirit, continued investment in sensitive all-sky X-ray monitors, rapid-response multiwavelength instrumentation, and theoretical modeling will drive the next leaps in revealing the lifecycle of the most massive stars.

In conclusion, the discovery of the fast X-ray transient EP240414a associated with the type Ic-BL supernova SN 2024gsa reveals a hidden population of Wolf–Rayet star explosions powered by weak yet successful relativistic jets. This new class of transients with softer X-ray spectra and intermediate jet powers challenges the classical understanding of LGRB progenitors and explosion mechanisms and highlights the complex interdependence of progenitor core rotation, jet dynamics, and circumstellar environments. Such advances promise to deepen our grasp of the most violent stellar deaths in the cosmos while unveiling new astrophysical processes shaping the universe.

Subject of Research:
Fast X-ray transients and weak relativistic jets associated with broad-lined type Ic supernovae originating from Wolf–Rayet stars.

Article Title:
A fast X-ray transient from a weak relativistic jet associated with a type Ic-BL supernova.

Article References:
Sun, H., Li, WX., Liu, LD. et al. A fast X-ray transient from a weak relativistic jet associated with a type Ic-BL supernova. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02571-1

Image Credits:
AI Generated